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Effect of Photoperiod and Quantitative Feed Restriction in a Broiler Strain on Onset of Lay in Females and Onset of Semen Production in Males: a Genetic Hypothesis
BREEDING AND GENETICS Effect of Photoperiod and Quantitative Feed Restriction in a Broiler Strain on Onset of Lay in Females and Onset of Semen Production in Males: A Genetic Hypothesis Y. Eitan and M. Soller1 Department of Genetics, The Silberman Life Sciences Institute, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel toperiod treatments were additive. Relative to QR, the AL feeding treatment increased WTSXM by 832 g for males and 1,089 g for females; ASXM was decreased by 15.8 d for males and 9.3 d for females. Relative to OS, the DR photoperiod increased WTSXM by 591 g for males and 513 g for females; ASXM was increased by 17.5 d for males and 26.8 d for females. The DR-OS photoperiod increased WTSXM by 86 and 169 g for males and females, respectively; ASXM was increased by 6.1 d and 4.9 d, respectively. Under DR, the delay in onset of sexual maturity caused by QR was not due to failure to reach threshold body weight or age and, hence, appears to have resulted from the feed restriction itself. A similar delay for QR was found under OS but might have been due to failure to reach threshold body weight. It is speculated that the delay in onset of sexual maturity caused by feed restriction may be an ecological adaptation or, alternatively, a result of nutritional imbalance.
(Key words: broiler, photoperiod, feed restriction, sexual maturity) 2001 Poultry Science 80:1397–1405
INTRODUCTION Following the attainment of minimum age and body weight thresholds, entry into lay ensues promptly in female chickens provided with a photoperiod exceeding a threshold value termed “critical day length” (CDL). Photoperiod information is transduced via an extra-retinal photoreceptor that provides inputs to hypothalamic neurons secreting gonadotropin-releasing hormones I and II (Sharp et al., 1990). These releasing hormones, in turn, cause the release of pituitary gonadotrophins, particularly luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which stimulate gonadal development, eventually resulting in onset of lay (Dunn and Sharp, 1990; Lewis et al., 1999). The response of female chickens to a specific photoperiod on entering lay is
2001 Poultry Science Association, Inc. Received for publication June 26, 2000. Accepted for publication May 26, 2001. 1 To whom correspondence should be addressed: [email protected]. ac.il.
strongly dependent upon the photoperiod of the growing house (Morris, 1967; Ernst et al., 1987; Lewis et al., 1997). Intense selection of broiler stocks for rapid juvenile growth rate has resulted in two distinct episodes of reproductive collapse. The first, occurring in the 1960s in females and in the 1980s in males, was due to problems associated with overconsumption of food and resultant obesity. This phenomenon is now controlled by implementation of feed restriction from an early age, with males reared separately from females. The second, occurring in the 1980s in females and 1990s in males, was first noted as reduced lay and fertility of out-of-season broiler breeders (i.e., birds hatched January to May) when reared under natural lighting. This is now controlled by implementation of an 8 h light (L):16 h darkness (D) photoperiod
Abbreviation Key: AL = ad libitum; ASXM = age at first egg (females) or first mature semen production (males); CDL = critical daylength; D = h of darkness; DR = controlled-light dark room; DR-OS = controlledlight dark room, shifted to open shed; GLM = general linear models; L = h of light; O = open shed; QR = quantitative feed restriction; WTSXM = body weight at first egg (females) or first mature semen production (males).
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ABSTRACT The effects of feed restriction and photoperiod on body weight (WTSXM) and age (ASXM) at onset of mature semen production in males and onset of lay in females of a broiler female line were examined. Feeding treatments were as follows: ad libitum (AL) and quantitative feed restriction (QR). Photoperiod treatments were as follows: open shed (OS), in which the chicks were reared under naturally increasing daylight with supplemental light; dark room (DR), in which chicks were reared under short days, gradually increasing from 6 h of light (L) to 11.5L; and dark room to open shed (DR-OS), in which chicks reared under short days in the dark room were transferred to the open shed at 149 d. Treatment effects were similar in direction in males and females, suggesting similar control of entry into reproduction of the two sexes, although within a given treatment, males matured earlier than females. Effects of feeding and pho-
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MATERIALS AND METHODS Stocks and Rearing Procedures Our experiment included male and female chicks of the Ross female broiler breeder line #308.2 The chicks were hatched on September 20, 1998, and were reared at the North Talpiot Experimental Farm of the Hebrew University in Jerusalem. The chicks were vaccinated at hatch against Marek’s disease, with the Rispens vaccine, and against infectious bronchitis and Newcastle disease virus with live vaccine. At 14 d they were vaccinated against infectious bursal disease and Newcastle disease virus with an inactivated vaccine. A proprietary starter
2
Efrat Hatchery, Efrat 90435, Israel.
mash (210 g protein and 3,150 cal ME/g) was fed until 14 d, a broiler mash (170 g protein and 3,250 cal ME/g) until 42 d, a grower mash (140 g protein and 2,850 cal ME/g) until 157 d, and a layer mash (150 g protein and 3,100 cal ME/g) thereafter. All chicks were brooded for 7 d in electrically heated batteries. Following the brooding period, the chicks were transferred to floor pens until 44 d of age. At 44 d, 30 males and 30 females were transferred to individual cages with individual feeding troughs, in an open shed. At the same time, 65 males and 63 females were transferred to individual cages with individual feeding troughs in a controlled-light dark room.
Experimental Treatments Feeding and Photoperiod Treatments. There were two feeding treatments: AL and QR (under QR, feed quantities in the dark room and open shed were adjusted so as to maintain similar growth curves in the two locations). The feeding schedule for the QR birds is given in Table 1, according to sex and photoperiod treatment. Growth curves were according to the Ross Management Guide for the male line. There were three photoperiod treatments: open shed (OS), controlled-light dark room (DR), and controlled-light dark room shifted to open shed (DR-OS). OS chicks were reared under naturally decreasing daylight until mid-December and increasing natural light thereafter. The detailed photoperiod schedules for the various treatments are also given in Table 1. Any particular photoperiod treatment was the same for males and females and for AL and QR birds. Artificial light was provided by an incandescent light source. DR-OS chicks were reared in the dark room as above until 149 d, at which age they were transferred from DR to OS but otherwise were maintained on their previous feeding schedule, AL or QR as the case might be. Photoperiod treatment of the QR-OS birds was identical to that of the DR birds, when they were in the DR, and was identical to that of the OS birds when they were in the OS. The gradual increase in light intensity of the DR treatment, starting from a low level, was intended to provide minimal photostimulation consistent with achieving CDL. Previous work has shown that differences in photoperiod responses between genetic types are magnified under conditions of low photostimulation (Eitan and Soller, 1991, 1994, 1996). Thus, we anticipated that the gradual increase would provide maximum expression of differences in photoperiod responses. In all, there were six experimental treatments: AL-OS, AL-DR, AL-DR-OS, QR-AL, QR-DR, and QR-DR-OS.
Data Collection Chicks were weighed every 2 wk. Weighing was by a digital scale accurate to 1 g until 41 d and by a spring scale accurate to 20 g, thereafter. Comb height was measured by a straight-edge ruler, scored at 1-mm intervals. Height was measured as the distance from the base of the skull
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during rearing (Timmons et al., 1983), followed by sharp photostimulation with a single increase in daylength between 20 and 22 wk of age. The positive response to photoperiod manipulation implies that the selection for rapid juvenile growth rate has affected photoresponsiveness. A number of studies show that genetic factors affect photoresponsiveness. In a comparison of layer and dwarf broiler stocks, it was noted that baseline LH levels were greater, entry into lay following photostimulation was earlier, and critical and saturation daylengths were lower for the layer as compared to broiler stocks (Dunn and Sharp, 1990). A lower critical daylength for layers, as compared to broiler stocks, was also noted by Eitan et al. (1998). Other studies showed that suboptimal photoperiods had a strong delaying effect on onset of lay in broiler strain as compared to layer strain females (Eitan and Soller, 1994). Effects of selection for juvenile growth rate on reproductive performance via obesity can be rationalized as mediated by the ability of fatty tissue to convert estradiol to estriol (Fishman et al., 1975) and to reduce levels of sex hormone-binding globulin (Botwood et al., 1995). Effects of selection for rapid juvenile growth rate on reproductive performance via photoresponsiveness are more difficult to explain. In this context, it was unexpected, but intriguing, to find that in broiler breeder females, quantitative feed restriction (QR) as compared to ad libitum (AL) feeding apparently delays the onset of lay following photostimulation (Robinson, 1994; Eitan et al., 1998). Indirect evidence has indicated that the effect is not a result of failure to attain the well-known body weight thresholds for entry into lay (Brody et al., 1980, 1984; Bornstein et al., 1984). This evidence implied that QR per se may affect the response of broiler breeder females to photoperiod, in this way providing an explanation for the association of continued selection for rapid juvenile growth rate with current problems in broiler reproductive performance. The present experiment was carried out to confirm the effect of feed restriction on response to photostimulation, and extend the study to males, with appropriate controls for threshold weight effects.
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TABLE 1. Feeding and photoperiod treatments
Feeding treatment: Quantitative restriction Males Age (d) 0 50 102 142 156 164
− − − − − +
49 101 141 155 163
Females
DR and DR-OS (g/d)
OS (g/d)
Age (d)
AL 60 75 85 95 110
AL 60 80 90 100 110
0 57 86 133 142 156 164
− − − − − − +
56 85 132 141 155 163
DR and DR-OS (g/d)
OS (g/d)
AL 70 65 70 70 80 90
AL 70 70 70 80 90 95
Photoperiod treatment DR Age (d) 0 − 43 44 − 149
223 +
NL 6L:18D (5 − 10 lx) + 0.5L/wk (increasing lx)
Age (d) 0 − 43 44 − 149 150 − 167
OS
Photoperiod NL 6L:18D (5 − 10 lx) to OS
Age (d) 0 − 43 44 − 167 168 +
Photoperiod NL NL 14L:10D (35 − 50 lx)
11.4L:12.5D (30 to 50 lx)
1 DR = light-controlled dark room; DR-OS = light-controlled dark room then shifted to open shed (OS); NL = natural light; L:D = hours light:hours darkness in a photoperiod.
to the highest point on the second blade from the rear. Comb measurements were taken at approximately 2-wk intervals from 14 wk to the end of the experiment. Comb size is commonly used as a surrogate measure for development of reproductive function in chickens (Lowry, 1958; Snapir et al., 1969; Rozenboim et al., 1986; Jacoby et al., 1992; Rozenboim et al.,1993; Eitan et al., 1998). Body weight and comb height measurements were taken only until the bird entered lay (females) or reached fully mature semen production (males). Starting at 16 wk, each male was individually checked at intervals of 1 or 2 wk for onset of semen production, by manual massage. Following procedures developed by Pirzak (1976), onset of mature semen production was evaluated by eye, and scored on a scale of 6 to 1, as follows: where 6 = shrunken cloacal exit; 5 = extrusion of rudimentary penis, without semen production; 4 = production of seminal fluid; 3 = production of yellow semen; 2 = some indication of white semen; 1 = production of white semen. A score of 1 was taken to represent achievement of fully mature semen production on the part of the male, and semen testing was discontinued for that male. At this time, age and body weight were obtained for the bird. For females, age and body weight at first egg were recorded, as was weight of first egg.
Statistical Analyses Data was analyzed, separately for males and females by the SAS general linear models (GLM) procedure (SAS Institute, 1985), according to the following model: Yijk = m + bi + tj + btij + eijk
where, Yijk is the variable being analyzed of individual k of photoperiod treatment i and feeding treatment j, m is the overall mean, bi is the effect of photoperiod treatment i (i = 1, 2 or 3), tj is the effect of feeding treatment j (j = 1 or 2), btij is the interaction between photoperiod and feeding treatment, and eijk is the residual error term. The model provided estimates of treatment contrasts. Statistical significance of differences in means and variances among treatments were determined by t-test and by Ftest, respectively (Walpole and Myers, 1978). In calculating mean treatment-by-sex trait values against chronological age, the value for a particular bird at a given chronological age was as measured for that age, until the bird reached reproductive maturity. From that point on, the value for the bird at onset of reproductive maturity was used for that bird without further change in calculating mean treatment trait value. Because at a given chronological age different birds will be at different physiological states with respect to onset of sexual maturity, changes in mean trait values of an experimental group with age primarily reflect the changing proportions of birds that are in each state. In order to account for these differences, the data were plotted against physiological age. For this plot, the date at which each bird individually entered lay (females) or mature semen production (males) was taken as the zero point for that bird. Measurement dates for each bird were then assigned physiological age values, measured in number of weeks preceding the zero point. For each variable, the mean value for the birds of each treatment at each physiological age was calculated and plotted against physiological age.
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150 − 223
Photoperiod
DR-OS
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EITAN AND SOLLER TABLE 2. Mean age (ASXM) and body weight (WTSXM) at first egg (females) or at first mature semen production (males), according to feeding and photoperiod treatment, with the standard deviation of ASXM in parentheses Males
Females
Treatment1
No.
ASXM (d)
WTSXM (g)
No.
ASXM (d)
WTSXM (g)
AL-DR AL-DR-OS AL-OS QR-DR
17 11 14 14
175.0bc (15.4)ab 170.1c (6.4)c 154.0d (7.7)bc 187.6a (9.9)bc
4,711a 4,345b 4,244bc 4,004c
12 13 15 20
193.0b 176.8d 169.0e 205.1a
QR-DR-OS QR-OS
10 15
176.4b (4.4)c 170.1c (9.8)bc
3,494d 3,412d
18 15
183.2c (7.5)bc 178.3d (5.6)bc
(13.5)ab (8.2)bc (5.1)c (12.1)ab
4,748a 4,167b 4,274b 3,698c 3,354d 3,185e
a–d
Values in the same column with no common superscript differ significantly by t-test (means) or F-test (standard deviations) (P < 0.05). Standard deviations of body weight did not differ significantly among treatments and, hence, were pooled, yielding a standard deviation of 407 g for males and 201 g for females. 1 AL = feeding ad libitum; QR = quantitative feed restriction; DR = light-controlled dark room; DR-OS = lightcontrolled dark room then shifted to open shed (OS).
RESULTS Treatment Effects on Mean Age and Body Weight at Sexual Maturity Table 2 shows mean age (ASXM) and body weight (WTSXM) at first egg (females) or at first mature semen production (males), according to feeding and photoperiod treatments. For both traits in males and females, GLM analysis showed significant differences between the two feeding treatments and among the three photoperiod treatments. In all cases, however, the feeding treatment × photoperiod treatment interaction within sexes was not statistically significant. Table 3 shows GLM estimates of the treatment effects. Considering feeding treatments, mean WTSXM within sexes was greater for the AL feeding treatment than for the QR treatment, irrespective of photoperiod. The differences were statistically significant in all instances, except for AL-OS males as compared to QR-DR males. The GLM estimates of the AL feeding treatment effect on WTSXM were significantly less for males than for females (Table 3). For ASXM, there was considerable overlap of treatment means between AL and QR birds, depending on photoperiod. Nevertheless, at each comparable photoperiod treatment, ASXM of the AL birds was less than that for the
corresponding QR birds (Table 2). The difference was statistically significant in all instances, except for DR-OS males. The GLM estimates of the AL feeding treatment effect on ASXM were significantly greater for males than for females (Table 3). For photoperiod treatments of males and females, irrespective of feeding treatment, WTSXM and ASXM of the DR birds was greater than that of the DR-OS or OS birds (Table 2). The differences were statistically significant, except for ASXM of AL-DR-OS males. Compared to OS, the GLM estimates of the DR photoperiod effects on WTSXM did not differ significantly for males and females; ASXM, effects, however, were significantly less for males than for females (Table 3). For males and females, irrespective of feeding treatment, WTSXM and ASXM of the DR-OS birds were greater than of the OS birds, except for WTSXM of ALOS females. In only two instances, however, were the differences statistically significant [WTSXM of AL-OS males and QR-OS females (Table 2)]. Compared to OS, the GLM estimates of the DR-OS photoperiod effect on WTSXM and ASXM did not differ significantly for males and females, respectively (Table 3). Considering all three photoperiod treatments of WTSXM and ASXM, the difference between DR and DR-OS treatments was generally much larger than that between the DR-OS and OS treatments (Table 2) . A major exception, however, was the
TABLE 3. General linear models estimates of treatment effects on mean age (ASXM) and body weight (WTSXM) at first egg (females) or at first mature semen production (males), according to sex WTSXM (g)
ASXM (d)
Treatment contrast1
Males
Females
Males
Females
AL vs. QR DR vs. OS DR-OS vs. OS
+ 832** + 591** + 86
+ 1,089** + 513** + 169
− 15.8** + 17.5** + 6.1*
− 9.3** + 26.8** + 4.9*
1 AL = feeding ad libitum; QR = quantitative feed restriction; DR = light-controlled dark room; DR-OS = light-controlled dark room then shifted to open shed (OS). *P < 0.05; **P < 0.01.
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In interpreting the physiological age curves, it should be noted that data points were taken at somewhat irregular intervals; consequently, not all birds are represented at each physiological age. For example, with body weight measurements taken at 2-wk intervals, only half of the birds will have body weight measurements at any given physiological age. This procedure introduces a degree of experimental sampling in moving from point to point along the physiological age curves. Also, photoperiod treatments were applied according to chronological age. Therefore, when compared at a given physiological age, the birds can be at very different photoperiods, depending on the effects of the photoperiod and feeding treatments on their progress toward sexual maturity.
PHOTOPERIOD AND FEED RESTRICTION
Treatment Effects on Kinetics of Body Weight, Comb Size, and Semen Maturity Index According to Chronological and Physiological Age When body weight was plotted against physiological age (data not shown), males and females showed strictly linear growth, from −19 wk (males) and −21 wk (females), until onset of sexual maturity (0 wk). This result was true for AL and QR feeding treatments. As expected, at comparable physiological age, body weight under AL was always greater than under QR. Under AL and QR, weight gains for DR were greater than for DR-OS or OS; gains under these two treatments were similar. In both sexes, comb height plotted against physiological age (Figure 1) was generally greater for AL as compared to QR birds and for the DR as compared to the other photoperiod treatments, which would indicate that DR may delay egg production rather than sexual development. The shape of the comb height curve differed between males and females. Males (Figure 1A) showed a slow increase −12 to −6 wk, with a distinct increment
FIGURE 1. Comb size plotted against physiological age (age in wk, counting back from onset of lay in females or mature semen production in males; R15 to R0), according to treatment. 1A, males; 1B, females. Feeding treatments: AL = feeding ad libitum (filled symbols); QR = quantitative feed restriction (open symbols). Photoperiod treatments: DR = light-controlled dark room (diamonds); DR-OS = light-controlled dark room then shifted to open shed (circles); OS = open shed (triangles).
from −3 to −1 wk, and a very sharp increase in the last week prior to onset of sexual maturity (from −1 to 0 wk). Females (Figure 1B) showed a gradually accelerating increase starting at −15 wk and continuing until 0 wk. Similar results were reported previously (Eitan et al., 1998). Within sexes, the shape of the curve was similar over all treatments. Figure 2 shows mean comb height plotted against chronological age. The shapes of the curves are very different than the physiological age curves, and as noted in methods, and primarily represent differences in the proportion of birds at different stages of sexual maturity. For males, the semen maturity curve (Figure 3), when plotted against physiological age, closely paralleled the comb-height curve. There was a very slow increment in semen maturity score from −6 until −3 wk, a distinct increment from −3 to −2 wk, and a very sharp increment in the last 2 wk prior to onset of sexual maturity (from −2 to 0 wk). The shape of the curve was similar over all treatments. In detail, however, the curves for semen
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AL males, for which the difference between DR-OS and OS (16.1 d) was three times that between DR and DR-OS (4.9 d). The treatment effects were additive and similar for males and females. The largest expected differences for WTSXM were between the AL-DR and QR-OS treatments (+1,423 g for males, +1,602 g for females). The observed differences were slightly less than expected under additivity (1,299 and 1,563 g for males and females, respectively). For ASXM, the largest expected differences were between the AL-OS and QR-DR treatments (−33.3 d for males and −36.1 d for females). The observed differences were exactly equal (−33.3 vs. −33.6) to those expected under additivity. Under all treatments, males entered sexual maturity before the females. The difference was greatest for the DR treatment, under AL and QR feeding treatments (18.0 and 17.5 d for AL and QR, respectively); was least for the DR-OS treatment (6.7 and 6.8 d for AL and QR, respectively); and was intermediate for the OS treatment (15.0 and 8.2 d for AL and QR, respectively). The within-treatment standard deviation of WTSXM did not differ significantly among treatment combinations (data not shown), but in all instances standard deviation of the DR treatment combinations was distinctly greater than for the other treatment combinations, and those for the DR-OS and OS treatment combinations were very similar. The within-treatment standard deviation of ASXM differed significantly among treatment combinations, with that for DR being greater and for DR-OS being generally less than for the other treatments (Table 2), which was anticipated. The DR treatment was designed to exaggerate differences in photoresponsiveness among the birds; the DR-OS treatment was expected to bring the birds into sexual maturity in a more synchronized manner.
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FIGURE 2. Comb size plotted against chronological age, according to treatment. 2A, males; 2B, females. Feeding treatments: AL = feeding ad libitum (filled symbols); QR = quantitative feed restriction (open symbols). Photoperiod treatments: DR = light-controlled dark room (diamonds); DR-OS = light-controlled dark room then shifted to open shed (circles); OS = open shed (triangles).
maturity and comb height differed somewhat. Namely, at −1 wk, scores for semen maturity tended to group according to photoperiod treatment, with OS treatment being the most advanced, DR-OS being the least advanced, and DR, intermediate. Scores for comb-height tended to group according to feeding treatment, with the AL treatments as a group tending to somewhat larger values than the QR treatments. Here, too, sexual maturity scores plotted against chronological age (Figure 4) show a very different pattern than the physiological age pattern. Again, this finding is primarily a reflection of the proportion of birds at the different stages of sexual maturity for a given chronological age. At onset of sexual maturity, average comb height in males and females under AL was significantly greater than under QR (Table 4). However, the ratio of comb heights (AL/QR) was less than the ratio body weight (1.22 and 1.25, respectively). Average comb size of females under DR was significantly greater than under under OS, but the ratio (DR/OS) (1.14) was the same as for body weight (1.15). For males, average comb size was virtually the same under DR and OS, although the body weight TABLE 4. Average comb height at onset of sexual maturity according to sex and to feeding and photoperiod treatments
FIGURE 3. Semen maturity score plotted against physiological age (age in wk, counting back from onset of lay in females or mature semen production in males; R5 to R0), according to treatment. Feeding treatments: AL = feeding ad libitum (filled symbols); QR = quantitative feed restriction (open symbols). Photoperiod treatments: DR = lightcontrolled dark room (diamonds); DR-OS = light-controlled dark room then shifted to open shed (circles); OS = open shed (triangles).
Treatment1
Males (mm)
Females (mm)
AL QR Ratio: AL/QR DR OS Ratio DR/OS
48.6a 41.8b 1.16 46.3a 45.7a 1.01
29.9a 26.8b 1.11 31.2a 27.3b 1.14
a,b Values in the same column and same treatment type (feeding or photoperiod) with no common superscript differ significantly by t-test. 1 AL = feeding ad libitum; QR = quantitative feed restriction; DR = light-controlled dark room; OS = open shed.
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FIGURE 4. Semen maturity score plotted against chronological age, according to treatment. Feeding treatments: AL = feeding ad libitum (filled symbols); QR = quantitative feed restriction (open symbols). Photoperiod treatments: DR = light-controlled dark room (diamonds); DROS = light-controlled dark room then shifted to open shed (circles); OS = open shed (triangles).
PHOTOPERIOD AND FEED RESTRICTION
ratio was 1.14. Thus, in females comb height appears to be a function of body size alone, but in males other factors may also be involved.
DISCUSSION
strongly controlled by body weight than by photoperiod; whereas in females, the onset of sexual maturity is more strongly controlled by photoperiod than by body weight. Effects of feeding and photoperiod treatments were roughly additive, leading to very great differences in body weight and age between the extreme treatment combinations. AL gave greater body weight for age as compared to QR, and DR gave a longer growing period until sexual maturity as compared to OS. Thus, the contrast of AL-DR vs. QR-OS gave maximum difference in WTSXM: 1,299 and 1,563 g for males and females, respectively. Similarly, the contrast of AL-OS vs. QR-DR gave maximum difference in ASXM: 33.6 and 36.1 d for males and females, respectively. Interestingly, the treatment giving maximum difference in ASXM gave minimal or close to minimal differences in WTSXM and vice versa. This contrast is plausible, because a maximum difference in ASXM gave more opportunity for the QR treatment groups to catch up in body weight to the AL groups, whereas a minimum difference in ASXM gave maximum opportunity for the greater rate of gain for AL, as compared to QR, be expressed. In the DR-OS treatment, birds were transferred from a dark room and a photoperiod of 6L:18D to the OS and a natural photoperiod of 13L:11D. It was anticipated that the sudden transfer to a longer photoperiod would have a stimulatory effect and bring the birds into sexual maturity earlier than the OS birds maintained throughout on natural light. However, this stimulation did not occur. In males and females, and under AL and QR feeding treatments, ASXM of the DR-OS birds was delayed relative to the OS birds. It may be that the OS birds had already initiated the processes leading to sexual maturity with the beginning of longer daylengths in January, before the DR birds were transferred to the OS. The delay of the AL-DR-OS birds was not due to failure to attain threshold body weight, because the comparable QR-OS birds entered lay at very similar age but at much lower body weight. This result implies that the DR photoperiod delayed onset of sexual maturity in the DR-OS birds. On this assumption, processes leading toward onset of sexual maturity would have been initiated immediately on transfer of the AL birds from DR to OS conditions. Following transfer from DR to OS, AL-DR-OS birds came into sexual maturity 20 and 26 d later, for males and females, respectively. Thus, there would appear to be a minimum requirement of 3 wk for males, and of 4 wk for females, from initiation of steps leading to sexual maturity to the achievement of mature semen production, or onset of lay, even when the birds are well prepared with respect to age and body weight. The more rapid attainment of sexual maturity by male birds as compared to females under comparable feeding and photoperiod regimes has been reported previously (Meier and MacGregor, 1972; Brake, 1990). For both sexes, onset of sexual maturity as measured by ASXM was delayed in the QRDR-OS and QR-OS treatments as compared to the corresponding AL treatments. However, in these instances body weight of the QR birds was low, and it is possible to
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The AL feeding treatment and the DR photoperiod treatment were associated with higher WTSXM. Their effects on ASXM, however, were opposed: AL decreased ASXM, whereas DR increased ASXM. This response is due to a different direction of causality for the two treatments. The primary effect of the AL feeding treatment within a given photoperiod treatment is to increase body weight for age, which, in turn, brings the birds to earlier sexual maturity through earlier attainment of threshold weight for sexual maturity (Brody et al., 1980, 1984; Bornstein et al., 1984), A direct positive effect on photoresponsiveness may also be involved (see later discussion), causing the bird to respond to a lower level of photostimulation achieved earlier in the treatment period. The primary effect of the DR photoperiod treatment within a given feeding treatment is to delay onset of sexual maturity, which, in turn, brings the birds to a higher body weight at sexual maturity, through continued accumulation of daily gain over the additional days of growth. GLM estimates of the effects of feeding treatment (AL vs. QR) on WTSXM were strikingly similar in males and females (832 and 1,089 g, respectively), as were the effects of photoperiod (DR vs. OS, 591 and 513 g, respectively). For both sexes, effects on WTSXM were twice as strong for feeding treatment as for photoperiod treatment. In contrast, for ASXM, although direction of effects was the same, magnitude differed according to treatment type and sex. In particular, comparing sexes within treatments, the effect of feeding treatment was almost twice as strong for males as for females (−15.8 and −9.3 d, respectively); the reverse was true for the effect of photoperiod treatment (+17.5 and +26.8 d, respectively). Similarly, when comparing treatments within sexes, for males, feeding and photoperiod treatments had roughly equal magnitude of effects on ASXM (−15.8 d and +17.5 d, respectively), whereas in females, effects of feeding treatment on ASXM were one-third that of photoperiod treatment (−9.3 d and +26.8 d, respectively). The similarity in direction of treatment effects on ASXM for males and females suggests a generally similar physiological basis for control of onset of sexual maturity in the two sexes. That similar genetic factors may be active in the two sexes is supported by the observation that suboptimal photoperiods had a strong delaying effect on onset of sexual maturity and mature semen production in broiler strain males as compared to layer strain males (Eitan and Soller, 1996), similar to the effect observed in females (Eitan and Soller, 1994). Also, selection for photoresponsiveness in males had correlated effects on photoresponsiveness in the females of the selection lines (Eitan and Soller, 2000 ). The differences in quantitative response between the sexes in treatment effects on ASXM appear to imply that in males the onset of sexual maturity is more
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feed restriction to maximize fertility and reproductive performance in broilers (Wilson et al., 1965; Blair et al., 1976; Jones et al., 1977; Lilburn et al., 1992; Yu et al., 1992; Lopez and Leeson, 1995; Walsh and Brake, 1997). The critical experiment will be a comparison of AL and QR feeding treatments under conventional and high protein diets. Levels of feed restriction (relative to consumption AL) have been continually increasing in heavy breed birds as a consequence of the continued increase in juvenile growth rate while keeping mature body weight constant. Thus, in the absence of genetic or nutritional ameliorative measures, both hypotheses predict a continued decrease in photoperiodic responsiveness in heavy breed birds with consequent deleterious effects on reproductive performance.
ACKNOWLEDGMENTS This research was supported by a grant from the Israel Poultry Council and The Israel Science Fund. We thank Tikva Geno for dedicated technical assistance.
REFERENCES Blair, R., M. M. MacCowan, and W. Bolton, 1976. Effects of food regulation during the growing and laying stages on the productivity of broiler breeders. Br. Poult. Sci. 17:215–223. Bornstein, S., I. Plavnik, and Y. Lev, 1984. Body weight and/or fatness as potential determinants of the onset of egg production in broiler breeder hens. Br. Poult. Sci. 25:323–341. Botwood, N., D. Hamilton-Fairley, D. Kiddy, S. Robinson, and S. Frank, 1995. Sex hormone-binding globulin and female reproductive function. J. Steroid Biochem. Mol. Biol. 53:529–531. Brake, J., 1990. The effect of a 2-hour increase in photoperiod at 18 weeks of age on broiler breeder performance. Poultry Sci. 69:910–914. Brody, T., Y. Eitan, M. Soller, I. Nir, and Z. Nitsan, 1980. Compensatory growth and sexual maturity in broiler females reared under severe food restriction from day of hatching. Br. Poult. Sci. 21:437–446. Brody, T. B., P. B. Siegel, and J. A. Cherry, 1984. Age, body weight and body composition requirements for the onset of sexual maturity of dwarf and normal chickens. Br. Poult. Sci. 25:245–252. Dunn, I. C., and P. J. Sharp, 1990. Photoperiodic requirements for LH release in juvenile broiler and egg-laying strains of domestic chickens fed ad libitum or restricted diets. J. Reprod. Fertil. 90:329–335. Eitan, Y., and M. Soller, 1991. Two way selection for threshold BW at first egg in broiler strain females. 2. Effect of supplemental light on weight and age at first egg. Poultry Sci. 70:2017–2022. Eitan, Y., and M. Soller, 1994. Selection for high and low threshold BW at first egg in broiler strain females. 4. Photoperiodic drive in the selection lines and in commercial layers and broiler breeders. Poultry Sci. 73:769–780. Eitan, Y., and M. Soller, 1996. Selection for high and low threshold BW at first egg in broiler strain females. 7. Effect of photoperiod on BW and age at onset of mature semen production in males of the selection lines, and in commercial broiler and layer males. Poultry Sci. 75:828–832. Eitan, Y., and M. Soller, 2000. Two-way selection for high and low responsiveness to photostimulation in broiler strain males. Poultry Sci. 79:1227–1232.
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interpret the delay as due to a longer time required by the QR birds to achieve threshold body weight for onset of sexual maturity. The delay of onset of sexual maturity in the QR-DR treatment, as compared to the AL-DR treatment (12.6 and 12.1 d for males and females, respectively), however, cannot be attributed to this cause, as body weight of the QR-DR birds at onset of maturity was well above that of the QR-DR-OS or QR-OS birds when they reached maturity. Similarly, the delay in onset of sexual maturity in the QR-DR treatment cannot be attributed to failure to achieve threshold age for sexual maturity, because in both sexes the QR-DR birds entered sexual maturity later than all other birds. We conclude, therefore, that feed restriction per se, delays the onset of sexual maturity in males and females under conditions of suboptimal light and, possibly, under conditions of optimal lighting as well. Differential response to photoperiod due to management or genetic factors apparently includes two components, namely differences in the strength and rate of development of the hormonal response to photostimulation (termed photoperiodic drive; Follet and Nicholls, 1984) and differences in critical daylength (Dunn and Sharp, 1990). It can be difficult to distinguish which of the two factors may be operative in a given situation (Eitan et al., 1998). However, in the present experiment, as shown by the physiological age curves for comb height (Figure 1A) and semen score (Figure 2), once the processes leading to sexual maturity were initiated, the rate of approach to sexual maturity was the same for AL-DR and QR-DR birds, regardless of sex. This finding agrees with results by Renema et al (1999) showing that once pubertal ovary development commences, it proceeds at a similar rate under different feed and photoperiod treatments. Thus, the effect of feed restriction may be to increase the critical day length required for initiation of the process leading to onset of sexual maturity. Results that can be interpreted in a similar manner have been reported previously (Robinson, 1994; Eitan et al., 1998). We have previously speculated that an effect of limited feed on CDL makes ecological sense. In the event of food shortages in early spring, it would be adaptive to delay the onset of lay in anticipation of better conditions later in the season. On this hypothesis, the underlying causality is endo-physiological in nature, and hence genetic approaches may provide a solution to the decreased photoresponsiveness induced by feed restriction. Indeed, selection for increased photoresponsiveness in males has been found to have a positive effect on onset of sexual maturity and rate of lay in females (Eitan and Soller, 2000). Alternatively, the effect of feed restriction on photoperiod responsiveness could be a secondary result of nutritional deficit introduced by feed restriction (J. Brake, 1998, Dept. of Poultry Science, North Carolina State University, Raleigh, NC, personal communication). On the nutritional hypothesis, higher feed protein content during growth should have a positive effect on subsequent performance. Indeed, many studies show the importance of maintaining high protein intake along with
PHOTOPERIOD AND FEED RESTRICTION
brew University of Jerusalem, Jerusalem, Israel (in Hebrew, English summary). Renema, R. A., F. E. Robinson, J. A. Proudman, M. Newcombe, and R. I. McKay, 1999. Effects of body weight and feed allocation during sexual maturation in broiler breeder hens. 2. Ovarian morphology and plasma hormone profiles. Poultry Sci. 78:629–639. Robinson, F. E., 1994. Ovarian form and function in chickens of varying reproductive status. Final Report, Alberta Agricultural Research Institute Project Number #AAR1920202. University of Alberta, Edmonton, AB Canada. Rozenboim, I., G. Gvaryahu, B. Robinzon, N. Sayag, and N. Snapir, 1986. Induction of precocious development of reproductive function in cockerels by Tamoxifen administration. Poultry Sci. 65:1980–1983. Rozenboim, I., N. Snapir, E. Arnon, R. Ben-Aryeh, W. H. Burke, P. J. Sharp, Y. Koch, and B. Robinzon, 1993. Precocious puberty in tamoxifen treated cockerels: hypothalamic gonadotropin-releasing hormone-I and plasma luteinizing hormone, prolactin, growth hormone and testosterone. Br. Poult. Sci. 34:533–542. SAS Institute, 1985. SAS User’s Guide: Statistics. Version 5 Edition. SAS Institute Inc., Cary, NC. Sharp, P. J., R. T. Talbot, G. M. Main, I. C. Dunn, H. M. Fraser, and N. S. Huskisson, 1990. Physiological roles of chicken LHRH-I and II in the control of gonadotropin release in the domestic chicken. J. Endocrinol. 124:291–299. Snapir, N., I. Nir, F. Furuta, and S. Lepkovsky, 1969. Effect of administered testosterone proprionate on cocks functionally castrated by hypothalamic lesions. Endocrinology 84:611– 618. Timmons, M. B., T. D. Siopes, G. A. Martin, and G. R. Baughman, 1983. Darkout housing and lighting programs for broiler breeders. ASAE, Tech. Paper No. 83-4520, 1983 Winter Meeting. ASAE, Chicago, IL. Walpole, R. E., and R. H. Myers, 1978. Probability and Statistics for Engineers and Scientists. 2nd ed. Macmillan Publishing Co., Inc., New York, NY. Walsh, T. J., and J. Brake, 1997. The effect of nutrient intake during rearing of broiler breeder females on subsequent fertility. Poultry Sci. 76:297–305. Wilson, H. R., P. W. Waldroup, J. E. Jones, D. J. Duerre, and R. H. Harms, 1965. Protein levels in growing diets and reproductive performance of cockerels. Poultry Sci. 44:29–37. Yu, M. W., F. E. Robinson, R. G. Charles, and R. Weingardt, 1992. Effect of feed allowance during rearing and breeding on female broiler breeders. 2. Ovarian morphology and production. Poultry Sci. 71:1750–1761.
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Eitan, Y., M. Soller, and I. Rozenboim, 1998. Comb size and estrogen levels toward the onset of lay in broiler and layer strain females under ad libitum and restricted feeding. Poultry Sci. 77:1593–1600. Ernst, R. A., J. R. Milliam, and F. B. Mather, 1987. Review of life history lighting programs for commercial laying fowls. World’s Poultry Sci. J. 43:45–55. Fishman, J., R. M. Boyer, and L. Hellman, 1975. Influence of body weight on estradiol metabolism in young women. J. Clin. Endocrinol. Metab. 41:989–991. Follet, B. K., and T. J. Nicholls, 1984. Photorefractoriness in Japanese quail: possible involvement of the thyroid gland. J. Exp. Zool. 232:577–580. Jacoby, S., N. Snapir, I. Rozenboim, E. Arnon, R. Meidan, R., and B. Robinzon, 1992. Tamoxifen advances puberty in the White Leghorn hen. Br. Poult. Sci. 33:101–111. Jones, J. E., B. L. Hughes, and K. A. Wall, 1977. Effect of light intensity and source on tom reproduction. Poultry Sci. 56:1417–1420. Lewis, P. D., G. C. Perry, and T. R. Morris, 1997. Effect of size and timing of photoperiod increase on age at first egg and subsequent performance of two breeds of laying hen. Br. Poult. Sci. 38:142–150. Lewis, P. D., G. C. Perry, T. R. Morris, and J. A. Douthwaite, 1999. Effect of timing and size of photoperiod change on plasma FSH concentration and the correlation between FSH and age at first egg in pullets. Br. Poult. Sci. 40:380–384. Lilburn, M. S., J. W. Steigner, and K. E. Nestor, 1992. The influence of dietary protein on carcass composition and sexual maturity in a random population of Japanese quail (R1) and subline of R1 selected for increased body weight. Comp. Biochem. Physiol. 102A:385–388. Lopez, G., and S. Leeson, 1995. Response of broiler breeders to low protein diets. 1. Adult breeder performance. Poultry Sci. 74:685–695. Lowry, M. M., 1958. Compensatory testicular adjustments following the injection of small quantities of androgen into unilaterally castrated White Leghorn cockerels. Poultry Sci. 37:1129–1136. Meier, A. H., and R. MacGregor, 1972. Temporal organization in avian reproduction. Am. Zool. 12:257–271. Morris, T. R., 1967. Light requirements of the fowl. Pages 15– 39 in: Environmental Control in Poultry Production. T. C. Carter, ed. Oliver and Boyd, Edinburgh, UK. Pirzak, R., 1976. Effect of Light Spectrum and Vitamin C on Sexual Development of Male Turkeys. M.S. Thesis. The He-
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(Key words: broiler, photoperiod, feed restriction, sexual maturity) 2001 Poultry Science 80:1397–1405
INTRODUCTION Following the attainment of minimum age and body weight thresholds, entry into lay ensues promptly in female chickens provided with a photoperiod exceeding a threshold value termed “critical day length” (CDL). Photoperiod information is transduced via an extra-retinal photoreceptor that provides inputs to hypothalamic neurons secreting gonadotropin-releasing hormones I and II (Sharp et al., 1990). These releasing hormones, in turn, cause the release of pituitary gonadotrophins, particularly luteinizing hormone (LH) and follicle-stimulating hormone (FSH), which stimulate gonadal development, eventually resulting in onset of lay (Dunn and Sharp, 1990; Lewis et al., 1999). The response of female chickens to a specific photoperiod on entering lay is
2001 Poultry Science Association, Inc. Received for publication June 26, 2000. Accepted for publication May 26, 2001. 1 To whom correspondence should be addressed: [email protected]. ac.il.
strongly dependent upon the photoperiod of the growing house (Morris, 1967; Ernst et al., 1987; Lewis et al., 1997). Intense selection of broiler stocks for rapid juvenile growth rate has resulted in two distinct episodes of reproductive collapse. The first, occurring in the 1960s in females and in the 1980s in males, was due to problems associated with overconsumption of food and resultant obesity. This phenomenon is now controlled by implementation of feed restriction from an early age, with males reared separately from females. The second, occurring in the 1980s in females and 1990s in males, was first noted as reduced lay and fertility of out-of-season broiler breeders (i.e., birds hatched January to May) when reared under natural lighting. This is now controlled by implementation of an 8 h light (L):16 h darkness (D) photoperiod
Abbreviation Key: AL = ad libitum; ASXM = age at first egg (females) or first mature semen production (males); CDL = critical daylength; D = h of darkness; DR = controlled-light dark room; DR-OS = controlledlight dark room, shifted to open shed; GLM = general linear models; L = h of light; O = open shed; QR = quantitative feed restriction; WTSXM = body weight at first egg (females) or first mature semen production (males).
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ABSTRACT The effects of feed restriction and photoperiod on body weight (WTSXM) and age (ASXM) at onset of mature semen production in males and onset of lay in females of a broiler female line were examined. Feeding treatments were as follows: ad libitum (AL) and quantitative feed restriction (QR). Photoperiod treatments were as follows: open shed (OS), in which the chicks were reared under naturally increasing daylight with supplemental light; dark room (DR), in which chicks were reared under short days, gradually increasing from 6 h of light (L) to 11.5L; and dark room to open shed (DR-OS), in which chicks reared under short days in the dark room were transferred to the open shed at 149 d. Treatment effects were similar in direction in males and females, suggesting similar control of entry into reproduction of the two sexes, although within a given treatment, males matured earlier than females. Effects of feeding and pho-
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MATERIALS AND METHODS Stocks and Rearing Procedures Our experiment included male and female chicks of the Ross female broiler breeder line #308.2 The chicks were hatched on September 20, 1998, and were reared at the North Talpiot Experimental Farm of the Hebrew University in Jerusalem. The chicks were vaccinated at hatch against Marek’s disease, with the Rispens vaccine, and against infectious bronchitis and Newcastle disease virus with live vaccine. At 14 d they were vaccinated against infectious bursal disease and Newcastle disease virus with an inactivated vaccine. A proprietary starter
2
Efrat Hatchery, Efrat 90435, Israel.
mash (210 g protein and 3,150 cal ME/g) was fed until 14 d, a broiler mash (170 g protein and 3,250 cal ME/g) until 42 d, a grower mash (140 g protein and 2,850 cal ME/g) until 157 d, and a layer mash (150 g protein and 3,100 cal ME/g) thereafter. All chicks were brooded for 7 d in electrically heated batteries. Following the brooding period, the chicks were transferred to floor pens until 44 d of age. At 44 d, 30 males and 30 females were transferred to individual cages with individual feeding troughs, in an open shed. At the same time, 65 males and 63 females were transferred to individual cages with individual feeding troughs in a controlled-light dark room.
Experimental Treatments Feeding and Photoperiod Treatments. There were two feeding treatments: AL and QR (under QR, feed quantities in the dark room and open shed were adjusted so as to maintain similar growth curves in the two locations). The feeding schedule for the QR birds is given in Table 1, according to sex and photoperiod treatment. Growth curves were according to the Ross Management Guide for the male line. There were three photoperiod treatments: open shed (OS), controlled-light dark room (DR), and controlled-light dark room shifted to open shed (DR-OS). OS chicks were reared under naturally decreasing daylight until mid-December and increasing natural light thereafter. The detailed photoperiod schedules for the various treatments are also given in Table 1. Any particular photoperiod treatment was the same for males and females and for AL and QR birds. Artificial light was provided by an incandescent light source. DR-OS chicks were reared in the dark room as above until 149 d, at which age they were transferred from DR to OS but otherwise were maintained on their previous feeding schedule, AL or QR as the case might be. Photoperiod treatment of the QR-OS birds was identical to that of the DR birds, when they were in the DR, and was identical to that of the OS birds when they were in the OS. The gradual increase in light intensity of the DR treatment, starting from a low level, was intended to provide minimal photostimulation consistent with achieving CDL. Previous work has shown that differences in photoperiod responses between genetic types are magnified under conditions of low photostimulation (Eitan and Soller, 1991, 1994, 1996). Thus, we anticipated that the gradual increase would provide maximum expression of differences in photoperiod responses. In all, there were six experimental treatments: AL-OS, AL-DR, AL-DR-OS, QR-AL, QR-DR, and QR-DR-OS.
Data Collection Chicks were weighed every 2 wk. Weighing was by a digital scale accurate to 1 g until 41 d and by a spring scale accurate to 20 g, thereafter. Comb height was measured by a straight-edge ruler, scored at 1-mm intervals. Height was measured as the distance from the base of the skull
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during rearing (Timmons et al., 1983), followed by sharp photostimulation with a single increase in daylength between 20 and 22 wk of age. The positive response to photoperiod manipulation implies that the selection for rapid juvenile growth rate has affected photoresponsiveness. A number of studies show that genetic factors affect photoresponsiveness. In a comparison of layer and dwarf broiler stocks, it was noted that baseline LH levels were greater, entry into lay following photostimulation was earlier, and critical and saturation daylengths were lower for the layer as compared to broiler stocks (Dunn and Sharp, 1990). A lower critical daylength for layers, as compared to broiler stocks, was also noted by Eitan et al. (1998). Other studies showed that suboptimal photoperiods had a strong delaying effect on onset of lay in broiler strain as compared to layer strain females (Eitan and Soller, 1994). Effects of selection for juvenile growth rate on reproductive performance via obesity can be rationalized as mediated by the ability of fatty tissue to convert estradiol to estriol (Fishman et al., 1975) and to reduce levels of sex hormone-binding globulin (Botwood et al., 1995). Effects of selection for rapid juvenile growth rate on reproductive performance via photoresponsiveness are more difficult to explain. In this context, it was unexpected, but intriguing, to find that in broiler breeder females, quantitative feed restriction (QR) as compared to ad libitum (AL) feeding apparently delays the onset of lay following photostimulation (Robinson, 1994; Eitan et al., 1998). Indirect evidence has indicated that the effect is not a result of failure to attain the well-known body weight thresholds for entry into lay (Brody et al., 1980, 1984; Bornstein et al., 1984). This evidence implied that QR per se may affect the response of broiler breeder females to photoperiod, in this way providing an explanation for the association of continued selection for rapid juvenile growth rate with current problems in broiler reproductive performance. The present experiment was carried out to confirm the effect of feed restriction on response to photostimulation, and extend the study to males, with appropriate controls for threshold weight effects.
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TABLE 1. Feeding and photoperiod treatments
Feeding treatment: Quantitative restriction Males Age (d) 0 50 102 142 156 164
− − − − − +
49 101 141 155 163
Females
DR and DR-OS (g/d)
OS (g/d)
Age (d)
AL 60 75 85 95 110
AL 60 80 90 100 110
0 57 86 133 142 156 164
− − − − − − +
56 85 132 141 155 163
DR and DR-OS (g/d)
OS (g/d)
AL 70 65 70 70 80 90
AL 70 70 70 80 90 95
Photoperiod treatment DR Age (d) 0 − 43 44 − 149
223 +
NL 6L:18D (5 − 10 lx) + 0.5L/wk (increasing lx)
Age (d) 0 − 43 44 − 149 150 − 167
OS
Photoperiod NL 6L:18D (5 − 10 lx) to OS
Age (d) 0 − 43 44 − 167 168 +
Photoperiod NL NL 14L:10D (35 − 50 lx)
11.4L:12.5D (30 to 50 lx)
1 DR = light-controlled dark room; DR-OS = light-controlled dark room then shifted to open shed (OS); NL = natural light; L:D = hours light:hours darkness in a photoperiod.
to the highest point on the second blade from the rear. Comb measurements were taken at approximately 2-wk intervals from 14 wk to the end of the experiment. Comb size is commonly used as a surrogate measure for development of reproductive function in chickens (Lowry, 1958; Snapir et al., 1969; Rozenboim et al., 1986; Jacoby et al., 1992; Rozenboim et al.,1993; Eitan et al., 1998). Body weight and comb height measurements were taken only until the bird entered lay (females) or reached fully mature semen production (males). Starting at 16 wk, each male was individually checked at intervals of 1 or 2 wk for onset of semen production, by manual massage. Following procedures developed by Pirzak (1976), onset of mature semen production was evaluated by eye, and scored on a scale of 6 to 1, as follows: where 6 = shrunken cloacal exit; 5 = extrusion of rudimentary penis, without semen production; 4 = production of seminal fluid; 3 = production of yellow semen; 2 = some indication of white semen; 1 = production of white semen. A score of 1 was taken to represent achievement of fully mature semen production on the part of the male, and semen testing was discontinued for that male. At this time, age and body weight were obtained for the bird. For females, age and body weight at first egg were recorded, as was weight of first egg.
Statistical Analyses Data was analyzed, separately for males and females by the SAS general linear models (GLM) procedure (SAS Institute, 1985), according to the following model: Yijk = m + bi + tj + btij + eijk
where, Yijk is the variable being analyzed of individual k of photoperiod treatment i and feeding treatment j, m is the overall mean, bi is the effect of photoperiod treatment i (i = 1, 2 or 3), tj is the effect of feeding treatment j (j = 1 or 2), btij is the interaction between photoperiod and feeding treatment, and eijk is the residual error term. The model provided estimates of treatment contrasts. Statistical significance of differences in means and variances among treatments were determined by t-test and by Ftest, respectively (Walpole and Myers, 1978). In calculating mean treatment-by-sex trait values against chronological age, the value for a particular bird at a given chronological age was as measured for that age, until the bird reached reproductive maturity. From that point on, the value for the bird at onset of reproductive maturity was used for that bird without further change in calculating mean treatment trait value. Because at a given chronological age different birds will be at different physiological states with respect to onset of sexual maturity, changes in mean trait values of an experimental group with age primarily reflect the changing proportions of birds that are in each state. In order to account for these differences, the data were plotted against physiological age. For this plot, the date at which each bird individually entered lay (females) or mature semen production (males) was taken as the zero point for that bird. Measurement dates for each bird were then assigned physiological age values, measured in number of weeks preceding the zero point. For each variable, the mean value for the birds of each treatment at each physiological age was calculated and plotted against physiological age.
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150 − 223
Photoperiod
DR-OS
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EITAN AND SOLLER TABLE 2. Mean age (ASXM) and body weight (WTSXM) at first egg (females) or at first mature semen production (males), according to feeding and photoperiod treatment, with the standard deviation of ASXM in parentheses Males
Females
Treatment1
No.
ASXM (d)
WTSXM (g)
No.
ASXM (d)
WTSXM (g)
AL-DR AL-DR-OS AL-OS QR-DR
17 11 14 14
175.0bc (15.4)ab 170.1c (6.4)c 154.0d (7.7)bc 187.6a (9.9)bc
4,711a 4,345b 4,244bc 4,004c
12 13 15 20
193.0b 176.8d 169.0e 205.1a
QR-DR-OS QR-OS
10 15
176.4b (4.4)c 170.1c (9.8)bc
3,494d 3,412d
18 15
183.2c (7.5)bc 178.3d (5.6)bc
(13.5)ab (8.2)bc (5.1)c (12.1)ab
4,748a 4,167b 4,274b 3,698c 3,354d 3,185e
a–d
Values in the same column with no common superscript differ significantly by t-test (means) or F-test (standard deviations) (P < 0.05). Standard deviations of body weight did not differ significantly among treatments and, hence, were pooled, yielding a standard deviation of 407 g for males and 201 g for females. 1 AL = feeding ad libitum; QR = quantitative feed restriction; DR = light-controlled dark room; DR-OS = lightcontrolled dark room then shifted to open shed (OS).
RESULTS Treatment Effects on Mean Age and Body Weight at Sexual Maturity Table 2 shows mean age (ASXM) and body weight (WTSXM) at first egg (females) or at first mature semen production (males), according to feeding and photoperiod treatments. For both traits in males and females, GLM analysis showed significant differences between the two feeding treatments and among the three photoperiod treatments. In all cases, however, the feeding treatment × photoperiod treatment interaction within sexes was not statistically significant. Table 3 shows GLM estimates of the treatment effects. Considering feeding treatments, mean WTSXM within sexes was greater for the AL feeding treatment than for the QR treatment, irrespective of photoperiod. The differences were statistically significant in all instances, except for AL-OS males as compared to QR-DR males. The GLM estimates of the AL feeding treatment effect on WTSXM were significantly less for males than for females (Table 3). For ASXM, there was considerable overlap of treatment means between AL and QR birds, depending on photoperiod. Nevertheless, at each comparable photoperiod treatment, ASXM of the AL birds was less than that for the
corresponding QR birds (Table 2). The difference was statistically significant in all instances, except for DR-OS males. The GLM estimates of the AL feeding treatment effect on ASXM were significantly greater for males than for females (Table 3). For photoperiod treatments of males and females, irrespective of feeding treatment, WTSXM and ASXM of the DR birds was greater than that of the DR-OS or OS birds (Table 2). The differences were statistically significant, except for ASXM of AL-DR-OS males. Compared to OS, the GLM estimates of the DR photoperiod effects on WTSXM did not differ significantly for males and females; ASXM, effects, however, were significantly less for males than for females (Table 3). For males and females, irrespective of feeding treatment, WTSXM and ASXM of the DR-OS birds were greater than of the OS birds, except for WTSXM of ALOS females. In only two instances, however, were the differences statistically significant [WTSXM of AL-OS males and QR-OS females (Table 2)]. Compared to OS, the GLM estimates of the DR-OS photoperiod effect on WTSXM and ASXM did not differ significantly for males and females, respectively (Table 3). Considering all three photoperiod treatments of WTSXM and ASXM, the difference between DR and DR-OS treatments was generally much larger than that between the DR-OS and OS treatments (Table 2) . A major exception, however, was the
TABLE 3. General linear models estimates of treatment effects on mean age (ASXM) and body weight (WTSXM) at first egg (females) or at first mature semen production (males), according to sex WTSXM (g)
ASXM (d)
Treatment contrast1
Males
Females
Males
Females
AL vs. QR DR vs. OS DR-OS vs. OS
+ 832** + 591** + 86
+ 1,089** + 513** + 169
− 15.8** + 17.5** + 6.1*
− 9.3** + 26.8** + 4.9*
1 AL = feeding ad libitum; QR = quantitative feed restriction; DR = light-controlled dark room; DR-OS = light-controlled dark room then shifted to open shed (OS). *P < 0.05; **P < 0.01.
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In interpreting the physiological age curves, it should be noted that data points were taken at somewhat irregular intervals; consequently, not all birds are represented at each physiological age. For example, with body weight measurements taken at 2-wk intervals, only half of the birds will have body weight measurements at any given physiological age. This procedure introduces a degree of experimental sampling in moving from point to point along the physiological age curves. Also, photoperiod treatments were applied according to chronological age. Therefore, when compared at a given physiological age, the birds can be at very different photoperiods, depending on the effects of the photoperiod and feeding treatments on their progress toward sexual maturity.
PHOTOPERIOD AND FEED RESTRICTION
Treatment Effects on Kinetics of Body Weight, Comb Size, and Semen Maturity Index According to Chronological and Physiological Age When body weight was plotted against physiological age (data not shown), males and females showed strictly linear growth, from −19 wk (males) and −21 wk (females), until onset of sexual maturity (0 wk). This result was true for AL and QR feeding treatments. As expected, at comparable physiological age, body weight under AL was always greater than under QR. Under AL and QR, weight gains for DR were greater than for DR-OS or OS; gains under these two treatments were similar. In both sexes, comb height plotted against physiological age (Figure 1) was generally greater for AL as compared to QR birds and for the DR as compared to the other photoperiod treatments, which would indicate that DR may delay egg production rather than sexual development. The shape of the comb height curve differed between males and females. Males (Figure 1A) showed a slow increase −12 to −6 wk, with a distinct increment
FIGURE 1. Comb size plotted against physiological age (age in wk, counting back from onset of lay in females or mature semen production in males; R15 to R0), according to treatment. 1A, males; 1B, females. Feeding treatments: AL = feeding ad libitum (filled symbols); QR = quantitative feed restriction (open symbols). Photoperiod treatments: DR = light-controlled dark room (diamonds); DR-OS = light-controlled dark room then shifted to open shed (circles); OS = open shed (triangles).
from −3 to −1 wk, and a very sharp increase in the last week prior to onset of sexual maturity (from −1 to 0 wk). Females (Figure 1B) showed a gradually accelerating increase starting at −15 wk and continuing until 0 wk. Similar results were reported previously (Eitan et al., 1998). Within sexes, the shape of the curve was similar over all treatments. Figure 2 shows mean comb height plotted against chronological age. The shapes of the curves are very different than the physiological age curves, and as noted in methods, and primarily represent differences in the proportion of birds at different stages of sexual maturity. For males, the semen maturity curve (Figure 3), when plotted against physiological age, closely paralleled the comb-height curve. There was a very slow increment in semen maturity score from −6 until −3 wk, a distinct increment from −3 to −2 wk, and a very sharp increment in the last 2 wk prior to onset of sexual maturity (from −2 to 0 wk). The shape of the curve was similar over all treatments. In detail, however, the curves for semen
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AL males, for which the difference between DR-OS and OS (16.1 d) was three times that between DR and DR-OS (4.9 d). The treatment effects were additive and similar for males and females. The largest expected differences for WTSXM were between the AL-DR and QR-OS treatments (+1,423 g for males, +1,602 g for females). The observed differences were slightly less than expected under additivity (1,299 and 1,563 g for males and females, respectively). For ASXM, the largest expected differences were between the AL-OS and QR-DR treatments (−33.3 d for males and −36.1 d for females). The observed differences were exactly equal (−33.3 vs. −33.6) to those expected under additivity. Under all treatments, males entered sexual maturity before the females. The difference was greatest for the DR treatment, under AL and QR feeding treatments (18.0 and 17.5 d for AL and QR, respectively); was least for the DR-OS treatment (6.7 and 6.8 d for AL and QR, respectively); and was intermediate for the OS treatment (15.0 and 8.2 d for AL and QR, respectively). The within-treatment standard deviation of WTSXM did not differ significantly among treatment combinations (data not shown), but in all instances standard deviation of the DR treatment combinations was distinctly greater than for the other treatment combinations, and those for the DR-OS and OS treatment combinations were very similar. The within-treatment standard deviation of ASXM differed significantly among treatment combinations, with that for DR being greater and for DR-OS being generally less than for the other treatments (Table 2), which was anticipated. The DR treatment was designed to exaggerate differences in photoresponsiveness among the birds; the DR-OS treatment was expected to bring the birds into sexual maturity in a more synchronized manner.
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FIGURE 2. Comb size plotted against chronological age, according to treatment. 2A, males; 2B, females. Feeding treatments: AL = feeding ad libitum (filled symbols); QR = quantitative feed restriction (open symbols). Photoperiod treatments: DR = light-controlled dark room (diamonds); DR-OS = light-controlled dark room then shifted to open shed (circles); OS = open shed (triangles).
maturity and comb height differed somewhat. Namely, at −1 wk, scores for semen maturity tended to group according to photoperiod treatment, with OS treatment being the most advanced, DR-OS being the least advanced, and DR, intermediate. Scores for comb-height tended to group according to feeding treatment, with the AL treatments as a group tending to somewhat larger values than the QR treatments. Here, too, sexual maturity scores plotted against chronological age (Figure 4) show a very different pattern than the physiological age pattern. Again, this finding is primarily a reflection of the proportion of birds at the different stages of sexual maturity for a given chronological age. At onset of sexual maturity, average comb height in males and females under AL was significantly greater than under QR (Table 4). However, the ratio of comb heights (AL/QR) was less than the ratio body weight (1.22 and 1.25, respectively). Average comb size of females under DR was significantly greater than under under OS, but the ratio (DR/OS) (1.14) was the same as for body weight (1.15). For males, average comb size was virtually the same under DR and OS, although the body weight TABLE 4. Average comb height at onset of sexual maturity according to sex and to feeding and photoperiod treatments
FIGURE 3. Semen maturity score plotted against physiological age (age in wk, counting back from onset of lay in females or mature semen production in males; R5 to R0), according to treatment. Feeding treatments: AL = feeding ad libitum (filled symbols); QR = quantitative feed restriction (open symbols). Photoperiod treatments: DR = lightcontrolled dark room (diamonds); DR-OS = light-controlled dark room then shifted to open shed (circles); OS = open shed (triangles).
Treatment1
Males (mm)
Females (mm)
AL QR Ratio: AL/QR DR OS Ratio DR/OS
48.6a 41.8b 1.16 46.3a 45.7a 1.01
29.9a 26.8b 1.11 31.2a 27.3b 1.14
a,b Values in the same column and same treatment type (feeding or photoperiod) with no common superscript differ significantly by t-test. 1 AL = feeding ad libitum; QR = quantitative feed restriction; DR = light-controlled dark room; OS = open shed.
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FIGURE 4. Semen maturity score plotted against chronological age, according to treatment. Feeding treatments: AL = feeding ad libitum (filled symbols); QR = quantitative feed restriction (open symbols). Photoperiod treatments: DR = light-controlled dark room (diamonds); DROS = light-controlled dark room then shifted to open shed (circles); OS = open shed (triangles).
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ratio was 1.14. Thus, in females comb height appears to be a function of body size alone, but in males other factors may also be involved.
DISCUSSION
strongly controlled by body weight than by photoperiod; whereas in females, the onset of sexual maturity is more strongly controlled by photoperiod than by body weight. Effects of feeding and photoperiod treatments were roughly additive, leading to very great differences in body weight and age between the extreme treatment combinations. AL gave greater body weight for age as compared to QR, and DR gave a longer growing period until sexual maturity as compared to OS. Thus, the contrast of AL-DR vs. QR-OS gave maximum difference in WTSXM: 1,299 and 1,563 g for males and females, respectively. Similarly, the contrast of AL-OS vs. QR-DR gave maximum difference in ASXM: 33.6 and 36.1 d for males and females, respectively. Interestingly, the treatment giving maximum difference in ASXM gave minimal or close to minimal differences in WTSXM and vice versa. This contrast is plausible, because a maximum difference in ASXM gave more opportunity for the QR treatment groups to catch up in body weight to the AL groups, whereas a minimum difference in ASXM gave maximum opportunity for the greater rate of gain for AL, as compared to QR, be expressed. In the DR-OS treatment, birds were transferred from a dark room and a photoperiod of 6L:18D to the OS and a natural photoperiod of 13L:11D. It was anticipated that the sudden transfer to a longer photoperiod would have a stimulatory effect and bring the birds into sexual maturity earlier than the OS birds maintained throughout on natural light. However, this stimulation did not occur. In males and females, and under AL and QR feeding treatments, ASXM of the DR-OS birds was delayed relative to the OS birds. It may be that the OS birds had already initiated the processes leading to sexual maturity with the beginning of longer daylengths in January, before the DR birds were transferred to the OS. The delay of the AL-DR-OS birds was not due to failure to attain threshold body weight, because the comparable QR-OS birds entered lay at very similar age but at much lower body weight. This result implies that the DR photoperiod delayed onset of sexual maturity in the DR-OS birds. On this assumption, processes leading toward onset of sexual maturity would have been initiated immediately on transfer of the AL birds from DR to OS conditions. Following transfer from DR to OS, AL-DR-OS birds came into sexual maturity 20 and 26 d later, for males and females, respectively. Thus, there would appear to be a minimum requirement of 3 wk for males, and of 4 wk for females, from initiation of steps leading to sexual maturity to the achievement of mature semen production, or onset of lay, even when the birds are well prepared with respect to age and body weight. The more rapid attainment of sexual maturity by male birds as compared to females under comparable feeding and photoperiod regimes has been reported previously (Meier and MacGregor, 1972; Brake, 1990). For both sexes, onset of sexual maturity as measured by ASXM was delayed in the QRDR-OS and QR-OS treatments as compared to the corresponding AL treatments. However, in these instances body weight of the QR birds was low, and it is possible to
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The AL feeding treatment and the DR photoperiod treatment were associated with higher WTSXM. Their effects on ASXM, however, were opposed: AL decreased ASXM, whereas DR increased ASXM. This response is due to a different direction of causality for the two treatments. The primary effect of the AL feeding treatment within a given photoperiod treatment is to increase body weight for age, which, in turn, brings the birds to earlier sexual maturity through earlier attainment of threshold weight for sexual maturity (Brody et al., 1980, 1984; Bornstein et al., 1984), A direct positive effect on photoresponsiveness may also be involved (see later discussion), causing the bird to respond to a lower level of photostimulation achieved earlier in the treatment period. The primary effect of the DR photoperiod treatment within a given feeding treatment is to delay onset of sexual maturity, which, in turn, brings the birds to a higher body weight at sexual maturity, through continued accumulation of daily gain over the additional days of growth. GLM estimates of the effects of feeding treatment (AL vs. QR) on WTSXM were strikingly similar in males and females (832 and 1,089 g, respectively), as were the effects of photoperiod (DR vs. OS, 591 and 513 g, respectively). For both sexes, effects on WTSXM were twice as strong for feeding treatment as for photoperiod treatment. In contrast, for ASXM, although direction of effects was the same, magnitude differed according to treatment type and sex. In particular, comparing sexes within treatments, the effect of feeding treatment was almost twice as strong for males as for females (−15.8 and −9.3 d, respectively); the reverse was true for the effect of photoperiod treatment (+17.5 and +26.8 d, respectively). Similarly, when comparing treatments within sexes, for males, feeding and photoperiod treatments had roughly equal magnitude of effects on ASXM (−15.8 d and +17.5 d, respectively), whereas in females, effects of feeding treatment on ASXM were one-third that of photoperiod treatment (−9.3 d and +26.8 d, respectively). The similarity in direction of treatment effects on ASXM for males and females suggests a generally similar physiological basis for control of onset of sexual maturity in the two sexes. That similar genetic factors may be active in the two sexes is supported by the observation that suboptimal photoperiods had a strong delaying effect on onset of sexual maturity and mature semen production in broiler strain males as compared to layer strain males (Eitan and Soller, 1996), similar to the effect observed in females (Eitan and Soller, 1994). Also, selection for photoresponsiveness in males had correlated effects on photoresponsiveness in the females of the selection lines (Eitan and Soller, 2000 ). The differences in quantitative response between the sexes in treatment effects on ASXM appear to imply that in males the onset of sexual maturity is more
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feed restriction to maximize fertility and reproductive performance in broilers (Wilson et al., 1965; Blair et al., 1976; Jones et al., 1977; Lilburn et al., 1992; Yu et al., 1992; Lopez and Leeson, 1995; Walsh and Brake, 1997). The critical experiment will be a comparison of AL and QR feeding treatments under conventional and high protein diets. Levels of feed restriction (relative to consumption AL) have been continually increasing in heavy breed birds as a consequence of the continued increase in juvenile growth rate while keeping mature body weight constant. Thus, in the absence of genetic or nutritional ameliorative measures, both hypotheses predict a continued decrease in photoperiodic responsiveness in heavy breed birds with consequent deleterious effects on reproductive performance.
ACKNOWLEDGMENTS This research was supported by a grant from the Israel Poultry Council and The Israel Science Fund. We thank Tikva Geno for dedicated technical assistance.
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interpret the delay as due to a longer time required by the QR birds to achieve threshold body weight for onset of sexual maturity. The delay of onset of sexual maturity in the QR-DR treatment, as compared to the AL-DR treatment (12.6 and 12.1 d for males and females, respectively), however, cannot be attributed to this cause, as body weight of the QR-DR birds at onset of maturity was well above that of the QR-DR-OS or QR-OS birds when they reached maturity. Similarly, the delay in onset of sexual maturity in the QR-DR treatment cannot be attributed to failure to achieve threshold age for sexual maturity, because in both sexes the QR-DR birds entered sexual maturity later than all other birds. We conclude, therefore, that feed restriction per se, delays the onset of sexual maturity in males and females under conditions of suboptimal light and, possibly, under conditions of optimal lighting as well. Differential response to photoperiod due to management or genetic factors apparently includes two components, namely differences in the strength and rate of development of the hormonal response to photostimulation (termed photoperiodic drive; Follet and Nicholls, 1984) and differences in critical daylength (Dunn and Sharp, 1990). It can be difficult to distinguish which of the two factors may be operative in a given situation (Eitan et al., 1998). However, in the present experiment, as shown by the physiological age curves for comb height (Figure 1A) and semen score (Figure 2), once the processes leading to sexual maturity were initiated, the rate of approach to sexual maturity was the same for AL-DR and QR-DR birds, regardless of sex. This finding agrees with results by Renema et al (1999) showing that once pubertal ovary development commences, it proceeds at a similar rate under different feed and photoperiod treatments. Thus, the effect of feed restriction may be to increase the critical day length required for initiation of the process leading to onset of sexual maturity. Results that can be interpreted in a similar manner have been reported previously (Robinson, 1994; Eitan et al., 1998). We have previously speculated that an effect of limited feed on CDL makes ecological sense. In the event of food shortages in early spring, it would be adaptive to delay the onset of lay in anticipation of better conditions later in the season. On this hypothesis, the underlying causality is endo-physiological in nature, and hence genetic approaches may provide a solution to the decreased photoresponsiveness induced by feed restriction. Indeed, selection for increased photoresponsiveness in males has been found to have a positive effect on onset of sexual maturity and rate of lay in females (Eitan and Soller, 2000). Alternatively, the effect of feed restriction on photoperiod responsiveness could be a secondary result of nutritional deficit introduced by feed restriction (J. Brake, 1998, Dept. of Poultry Science, North Carolina State University, Raleigh, NC, personal communication). On the nutritional hypothesis, higher feed protein content during growth should have a positive effect on subsequent performance. Indeed, many studies show the importance of maintaining high protein intake along with
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