Selection for Body Weight at Eight Weeks of Age.

BREEDING AND GENETICS Selection for Body Weight at Eight Weeks of Age. 19. Influences of Heterozygosity and Dwarfism on Early Egg Production and Associated Traits D. J. ZELENKA and P. B. SIEGEL1 Poultry Science Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (Received for publication December 30, 1986) ABSTRACT Body weight, body components, and reproductive traits were compared during the first 60 days after onset of lay in pullets from lines of White Plymouth Rocks selected for high or low juvenile body weight, their reciprocal F, crosses, an F 2 cross, and dwarfs from the parental lines. Absolute and relative body weight gains were less for dwarf than for nondwarf pullets in the high weight line. In the low weight line, there were differences in absolute but not relative weight gains. Among nondwarfs, both relative and absolute changes in body weight during the first 60 days of lay were similar for crosses and the high weight parental line, and considerably less for the low weight parental line. Numbers of ovulation were the same for the crosses and the high weight parental line, however, more normal eggs were produced by F, crosses than either parental line. Rate of production was influenced mainly by variation in body weight at onset of lay in the high weight nondwarfs and low weight dwarfs, and by age at first egg in low weight nondwarf pullets. None of the independent variables had a significant association with normal egg production in crosses and high weight dwarf populations. (Key words: body weight, dwarfism, reproduction, heterosis) 1987 Poultry Science 66:915-920

INTRODUCTION

Age, body weight, and carcass composition necessary for the onset of egg production in chickens are population dependent and subject to environmental influences including feed consumption (e.g., Solleretal., 1982, Dunnington etal., 1983, Leeson and Summers, 1983,Bornstein et al., 1984; Brody et al., 1984; Renden and Marple, 1986). The requirements of these factors for onset of lay, however, may differ from those for maintenance of part-year egg production (Renden and Marple, 1986) and persistence of lay (Gyles et al, 1982, 1984). This paper addresses the association between early egg production and changes in carcass traits in dwarf and normal chickens from White Plymouth Rock populations known to differ in feed consumption, body weight, and age at sexual maturity. MATERIALS AND METHODS

Stocks. Pullets from two dwarf and five normal White Plymouth Rock populations were studied in this experiment. Lines high weight

'To whom correspondence should be addressed.

normal (HN) and low weight normal (LN) had undergone 25 generations of selection for high and low 56-day body weight, respectively (Dunnington and Siegel, 1985). They were mated to produce reciprocal Fj crosses (HL and LH); the F 2 cross was obtained from HL x HL matings (the sire line was identified first and the dam line second in all crosses). Populations high weight dwarf (HD) and low weight dwarf (LD) were developed via introduction of the sexlinked dwarfing allele (dw) into samples of the HN and LN lines (Reddy and Siegel, 1977; Zelenkaef al., 1987). Husbandry. All chicks were produced from age-contemporary parents. Chicks were removed from the hatcher on April 6th, vaccinated for Marek's disease, vent sexed, wingbanded, and the females placed on litter in light-controlled pens. Lighting was continuous to 14 days of age, and from 0600 to 1800 hr to 57 days, when chicks were moved to a windowed house and exposed to natural lighting. At 127 days of age, pullets were transferred to single-bird cages with wire floors in a windowless room, with artificial lighting from 0600 to 2000 hr. Starter, developer, and breeder diets (Siegel, 1962) in mash form and water were provided ad libitum. A sample of 10 pullets was removed from each

915

ZELENKA AND SIEGEL

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population for a companion experiment (Zelenka et al., 1987); this should not create a bias in these data. Traits Measured and Statistical Analyses. Individual body weights were obtained on the day of first oviposition and 60 days later. All oviposits during this 60-day period were classified as either normal or defective (van Middelkoop and Siegel, 1976) and expressed as number of yolks, normal eggs, and defective eggs. Papers located under the cages were changed on alternate days; this facilitated accuracy in classification. Each pullet was killed by cervical dislocation 60 days after its first oviposit; measurements were obtained for tarsus-metatarsus length, weight of the breast, liver and abdominal fat pad, and the number of developing yolks 3= .3 g. Breast, liver, and abdominal fat pad weights were expressed as a percentage of body weight; the denominator was calculated as [body weight - (ovary weight + oviduct weight)]. Populations were compared by one-way analysis of variance with percentages transformed to arc sine square roots and absolute weights to natural logarithms prior to analysis. When F values were significant (P=s.05), differences among means were tested by Duncan's multiple range test. In addition, nonorthogonal linear contrasts (Scheffe, 1970) were used to make comparisons among groups of means to test for heterosis [(HL + LH) - (HN + LN)], for recombination [4(HLHL) - (2HL + HN + LN)], and for reciprocal effects [HL - LH]. Stepwise multiple regressions were cal-

culated within populations to evaluate the effects of age and body weight at onset of lay and change in body weight during the laying period on hen-day normal egg and yolk production. Effects of age and body weight at onset of lay and hen-day yolk production on breast and abdominal fat pad weights after 60 days of lay were also assessed by regression. RESULTS

Body Weight and Carcass Traits. High weight line females matured at earlier ages and with heavier body weights than those from the low line (Table 1). Although the F! crosses (LH and HL) did not mature at significantly earlier ages than parental line HN, heterosis was highly significant (-21%) and recombination loss was - 5 % . There was no evidence of heterosis (1%) or recombination loss (-3%) for body weight at onset of lay. Comparison of dwarfs and nondwarfs show that HN pullets matured at higher weights and earlier ages than did HD pullets. Although the weight pattern in the low line was similar to that observed for the high line, LN pullets matured at later ages than did LD pullets. During the 60-day laying period, absolute and relative changes in body weight were considerably greater for HN than for HD pullets (Table 1). In the low line, this pattern was less dramatic with respect to absolute weight and not evident for relative weight. Weight changes among nondwarfs, both absolute and relative,

TABLE 1. Means and standard errors for age and body weight at first oviposition and changes in body weight during the first 60 days of lay, by population (White Plymouth Rock pullets)

Population'

n

At first egg Body weight

Age (Days)

HN HL LH LN F* HD LD

16 17 28 13 15 15 26

171 ± 164 ± 164 ± 245 ± 177 ± 196 ± 219±

Change in body weight (g)

a

4 3a 2a 12 d 3a 7b 5C

2,602 1,966 1,948 1,283 1,887 2,199 860

e

± 64 ± 27 c ± 29 c ± 46b + 66 c ±75d + 21a

(%)2

• c

499 ± 5 5 349 ± 2 1 c 392 ± 26 c 112±48b 326 ± 4 6 e 71 + 2 7 a b 68 ± 1 3 a

19 ± 2C 18 ± l c 20 ± l c 9±3b 17 + 2C 3± l a 8±2ab

Means in a column with different superscripts are significantly different (P<.05). 1

HN = High weight normal; HL = high low (F, cross); LH = low high (F, cross); LN = low weight normal; = HL X HL; HD = high weight dwarf; LD = low weight dwarf. 2

(Weight after 60 days lay — weight at first egg)/weight at first egg.

HETEROZYGOSITY, DWARFISM, AND EGG PRODUCTION

917

TABLE 2. Means and standard errors for length of tarsus-metarsus (TM) and weight of breast, abdominal fat pad (AFP), and liver after 60 days of lay, by population (White Plymouth Rock pullets) Population'

HN HL LH LN F2 HD LD

TM

Breast

AFP

Liver

(mm)

(g)

(%)

(g)

(%)

(g)

<%)

84 ± 1*78 ± l d 79 ± l d 70 ± 1 ' 79 ± l d 61 ± 2 b 45±la

502 ± 1 5 d 366± 8 C 351 ± 7 C 180± 7 b 340 ± 15 c 362 ± 8 C 137 ± 4 a

16.4 ± .3 C 16.2 ± .2 C 15.4 ± . 2 b 13.3 ± 15.8 ± . 3 b c 16.5 ± 3 •f 15.3 ± .2 b

117 ± 9 e 95 + 4 d e 108 ± 5^

3.8 ± 3 C • < 4.2 ± 2 cd 4.7 ± . 2 d 2.9 ± . 2 b 3.7 + Ac 2.9 + .2 b 2.0 ± . 2 a

58 ± 3 d 43±lc 47 ± 2 C 30 ± 2 b 43±3C 40 ± 3 C 18 + 2 a

1.9 ± 1.9 ± 2.1 ± 2.2 + 2.0 ± 1.8 ± 2.0 +

K

3

9

*

4

j

80 ± 9 c d 65 ±6C 18 ± l a

la la la la la la la

Means in a column with different superscripts are significantly different (P<.05). 1

HN = High weight normal; HL = high low (F, cross); LH = low high (F, cross); LN •• low weight normal; = HL X HL; HD = high weight dwarf; LD = low weight dwarf.

were greater for HN than for LN pullets. Crosses did not differ significantly from the HN parental population; however, heterosis was 21 and 35% for absolute and relative weight changes, respectively. Respective values for recombination effects'were 0% and 5%. Length of tarsus-metatarsus and weights of breast, abdominal fat pad, and liver were larger for nondwarf than dwarf pullets within each line (Table 2). Breast and liver weights of HN and HD pullets relative to body weight were similar, while abdominal fat pads were larger for HN than HD pullets. In the low weight line, relative breast weight was larger and abdominal fat pad weight was smaller for dwarf than normal pullets. There were no differences in relative liver weights.

The F, crosses (HL and LH) were intermediate and different from parental lines for tarsus-metatarsus length and breast and liver weights (Table 2). Heterotic and recombination effects were small suggesting additive genetic variation for these traits. Differences for liver weight disappeared when expressed relative to body weight. Relative breast weights of HN, HL, LH, and F 2 pullets were greater than those for LN, and the mean for LH pullets was less than that of HN and HL but not F 2 . Heterosis for breast weight was 5 and 7% on an absolute and relative body weight basis, respectively. Recombination effects were minor in both cases. Among nondwarfs, LN pullets had the smallest abdominal fat pads (Table 2). Heterosis was 30 and 33% for abdominal fat on an absolute

TABLE 3. Means and standard errors for the number of yolks, normal eggs, and defective eggs during the first 60 days of lay and for number ofdeveloping yoiks at autopsy, by population (White Plymouth Rock pullets)

Population 1

Normal eggs

Developing yolks

Defective

Yolks

HN HL LH LN F2 HD LD

49 ± 2° 49±2d 51±ld 39 ± 2 b 45 ±2:cd 40 ± 2be 21 ± 2 a

38±2b 45 + 2 C 47 + l c 38 ± 2 b 43 ± 2 b c 39 ± 2 b 21 ± 2 a

8.4: . 5 " 6.3 : .3 C 6.0 ± .2 . C 4.5 +. . 2 b 5.7 ± .4 C 6.3 ± . 3 C 2.5 ± . 2 a

11 ± 2 C 4± lb 4± lb 1 + <1ab 2 + 1ab 1 + < 1ab <1 ± < l a

a—d Means in a column with different superscripts are significantly different (P<.05). HN = High weight normal; HL = high low (F[ cross); LH = low high (F, cross); LN = low weight normal; F 2 = HL X HL; HD = high weight dwarf; LD = low weight dwarf. 1

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ZELENKA AND SIEGEL

and relative body weight basis, respectively; both were highly significant. Recombination loss was - 8 % for the former and -2% for the latter. Egg Production Traits. The HN and LN pullets produced more yolks and had more developing yolks than their respective dwarf counterparts (Table 3). Normal egg production was the same for HN and HD pullets but greater for LN than LD pullets. This dichotomy occurred because while both LN and LD pullets laid few defective eggs, the frequency of such eggs was considerably greater for HN than for HD pullets. Although they had fewer developing yolks at autopsy, neither the reciprocal crosses (HL and LH) nor the F 2 differed from line HN in number of yolks (Table 3). Numbers of yolks and developing yolks were greater for F, and F 2 crosses than for LN pullets. Normal egg production was higher for F, crosses than for either parental population with the F 2 intermediate to the F, and the parental lines. There was no evidence of reciprocal effects for this trait.

Heterosis was 14% for number of yolks, 5% for the number of developing yolks, and 21% for the number of normal eggs. Recombination losses were 4, - 1 1 , and - 3 % , respectively. The 33% heterosis for reducing defective eggs reflected a correction via crossing for the high incidence of defective eggs produced by HN pullets. Regression Analyses. Influences of independent variables on percent hen-day yolk and normal egg production were population dependent. For example, the association of body weight at onset of lay on normal egg production was negative for HN pullets, positive for LD pullets, and not significant for the other stocks (Table 4). Age at the onset of lay negatively influenced normal egg production of LN pullets, while no single variable had a major influence on F, (HL and LH), F 2 , and HD pullets. Similarly, weight at onset of lay was positively associated with yolk production in LD pullets whereas age at onset was negatively associated with this trait in LN pullets, a pattern consistent with that ob-

TABLE 4. Percent of total variation of dependent variable accounted for by independent variables within populations (White Plymouth Rock pullets)1 Population 2

Variable Dependent

Hen-day normal egg production Hen-day yolk production

Breast weight Abdominal fat pad weight

1

Independent 3

HN

1 2 3

<1 428* 4

HL

LH

3 1 <1

5 1 9

<1 2 1

2 <1 4

LN

4.43** 1 <1

HD

F2

LD

1 6 5

<1 12 4

<1 433** 4

437* 2 <1

<1 8 1

6 11 9

<1 420* 9

1 2 3

5 1 3

1 2 4

<1 480** <1

3 t39** t24»*

<1 445** 4

452** <1 <1

<1 470** 1

3 455** 12

1 460** 2

1 2 4

3 455** 2

2 6 5

5 421* <1

10 1 t57**

3 427* <1

1 474** 3

9 441** <1

4 = Positive slope; 4 = negative slope.

2

HD= High weight normal; HL = high low (F, cross); LH = low high (F, cross); LN = low weight normal; F 2 = HL X HL; HD = high weight dwarf; LD = low weight dwarf. 3 1 = Age at onset; 2 = body weight at onset; 3 = change in body weight during the first 60 days of lay; 4 = hen-day yolk production.

*P<.05.

**P<.01.

HETEROZYGOSITY, DWARFISM, AND EGG PRODUCTION

served for normal egg production. No single variable was significantly associated with ovulation rates in the remaining stocks. Body weight was positively associated with breast weight in all populations except LN, and with abdominal fat pad weight in all but LN and HL (Table 4).In LN pullets, age at onset of lay was negatively associated with breast weight whereas both body weight and yolk production were positively associated with breast weight in HL pullets. There was a positive association between yolk production and abdominal fat pad weight in line LN, although no one variable was associated with this trait in the HL cross.

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nington (1985) was also observed in this experiment and appears to be associated with the number of developing yolks. In HN pullets, 78% (38/49) of the ovulations yielded normal eggs, whereas values ranged from 92% for reciprocal F, pullets to 100% for LD pullets. It may be hypothesized that heterosis for normal egg production is due more to regularity of ova development and synchrony in ovulation than to the production of yolks. A similar picture emerges for the mode of action of the dwarfing allele, as HD pullets ovulated less and produced fewer defective eggs than their HN counterparts. ACKNOWLEDGMENT

DISCUSSION

The heterosis for age but not weight at sexual maturity observed in this experiment was consistent with previous data for these populations (Zelenka et al, 1986, 1987) and with data related to the concept of onset of lay in avian populations (Murphy, 1986). Considerable heterosis was found for absolute and for relative changes in body weight during the 60-day laying period as well as for abdominal fat pad weight. Crosses exhibited a tendency to perform nearer the level of the high than the low parental line. Although F, crosses had the highest normal egg production rate, there were no differences in yolk number for Fj crosses and the HN parental line. However, the HN pullets may have produced more yolks than the crosses, with some ovulations being undetected due to internal ovulation and reabsorption of yolk material. Evidence for this hypothesis is the disproportionate number of developing yolks observed at autopsy in HN pullets. Frequencies of defective eggs were greater in nondwarfs than dwarfs in the H line. The low incidence of defective eggs in the L line precluded dwarf-nondwarf comparisons within that line. Results obtained in this experiment were consistent with the hypothesis that crossing (King and Bruckner, 1952) and the dwarf allele (Jaap, 1969; Reddy and Siegel, 1977, Abplanalp, 1984; Leenstra et al. 1986) enhance regularity in ovulation patterns of chickens. The data also suggest that, on a relative basis, dwarfing is more effective than heterosis in reducing the incidence of defective eggs. A negative relationship between body weight and normal egg production similar to that reported by Jaap and Muir (1968), Kinney (1969), Nestor and Bacon (1972), and Siegel and Dun-

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