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Response of Leghorn Pullets to Protein and Energy in the Diet when Reared in Regular or Hot-Cyclic Environments
Response of Leghorn Pullets to Protein and Energy in the Diet when Reared in Regular or Hot-Cyclic Environments STEVE LEESON and J. D. SUMMERS Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, Canada NIG 2WJ (Received for publication November 19, 1987)
1989 Poultry Science 68:546-557 INTRODUCTION
Pullets that mature earlier dictate the need for growing programs that maximize both growth rate and optimum development of associated body components (Summers and Leeson, 1983). Nutrient intake plays a major role in dictating growth rate; the major nutrients, energy and protein (amino acids), are of economic importance and thus can be manipulated to control growth according to commercial requirements. Qualitative and quantitative nutrient restriction invariably result in reduced body weight (Wells, 1980) and most often uneconomical adult performance (Leeson, 1986a). It is generally assumed that birds will regulate their energy intake when offered a diet providing a range of energy concentrations. At moderate environmental temperatures, the mechanism is quite precise (Payne, 1967). However, there are indications that the energy intake of pullets can be maximized through use
of low energy diets. Leeson and Summers (1984a) show comparable or improved growth rates of pullets reared on diets with 2,085 vs. 3,050 kcal ME/kg. Similarly, Cunningham and Morrison (1976) give evidence for increased energy intake of birds fed diets with lower energy content. Under hot weather conditions, however, optimum pullet growth cannot be achieved with low energy diets (Leeson, 1986a), and invariably high energy diets containing fat are essential. These observations suggest that pullets react differently to energy concentration of the diet depending upon environmental temperature. Protein intake is also a factor influencing growth and development of pullets, and again environmental temperature may have a confounding effect on requirements. Under moderate environmental conditions, birds show little response to elevated protein intake (Leeson and Summers, 1981); although under hot weather conditions, increased dietary protein
546
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ABSTRACT Four experiments were conducted to determine the growing pullet's response to graded levels of dietary ME (2.650 to 3,150 kcal ME/kg) or CP (15 to 20% CP) when reared in conventional (constant 22 C) or hot-cyclic temperatures (22 to 32 C). Each diet was tested with six replicate groups of eight caged birds from 1 day to 140 days of age. Body weight and feed intake were monitored periodically. Increasing the energy concentration of the diet resulted in increased body weight, with this effect being most pronounced in the hot environment. At 140 days of age, birds fed diets providing 2,650 or 2,750 kcal ME/kg were smaller than birds fed diets providing 2,950 or more kcal ME/kg. Increasing the dietary energy concentration resulted in reduced feed intake and a general tendency to increase energy intake. In other experiments, increasing the dietary protein level had an initial effect of increasing body weights, although by 140 days of age, differential dietary protein levels of 15 to 20% had no effect on body weight. Increasing the protein level of the diet resulted in increased protein intake but had no effect on energy intake. It is concluded that given adequate protein intake, pullet growth is most responsive to energy intake. Pullet growth to 140 days of age will be maximized with cumulative nutrient intakes of 21 Meal ME and 1,200 g CP. It appears that such intakes can be achieved with mean dietary specifications during rearing of 2,900 kcal ME/kg and 18% CP. However, with multidiet programs, as commonly used in the industry, it appears that pullet growth is initially most sensitive to dietary protein level, whereas energy intake becomes more critical as the bird approaches maturity. (Key words: pullet, energy, protein)
DIETARY PROTEIN AND ENERGY DURING REARING
levels may improve growth rate (Stockland and Blaylock, 1974; Leeson and Summers, 1981). The situation is further influenced by the development of many strains of Leghorn pullets with substantially reduced feed intakes; these intakes are in turn related to reduced body weights (Leeson, 1986a). Little work has been reported on the effect of environmental temperature on the response of modern strain pullets to protein and energy in the diet. Four experiments were designed to investigate this effect.
Experiments 1 and 2. One-day-old Leghorn pullets of a commercial strain were wingbanded, weighed, and allocated to one of two rooms that provided environmental control. For the first 48 h, birds were exposed to constant light at 100 lx intensity; after this time the photoperiod was reduced to 8 h and the intensity to 10 lx. After a conventional brooding period of 21 days, one room was maintained at 22 C (Experiment 1); birds in an alternate room were subjected to 32 C during the light period (0800 to 1600 h) and 22 C during darkness (1600 to 0800 h; Experiment 2). Temperature adjustment was achieved within 1 h. All birds were housed at eight birds/60 x 50-cm cage and had free access to feed and water. Dietary treatments consisted of varying energy levels from 2,650 to 3,150 kcal ME/kg in increments of 100 kcal ME as shown in Table 1 for Diets 1 to 6. Each diet was offered to six replicate cage groups for each experiment. All birds were individually weighed at 7-day intervals to 84 days of age, and thereafter at 98, 112, and 140 days of age. Feed intake per cage group was also ascertained over these time periods. Shank length was measured when birds were 28, 36, and 84 days of age (Leeson and Summers, 1984b). Experiments 3 and 4. Leghorn pullets of the same commercial strain as used in Experiments 1 and 2 were wingbanded, weighed, and distributed to one of two environmental control rooms as described in Experiments 1 and 2. Experiments 3 and 4 commenced in the spring, which was the termination time of the previous experiments. Cage allocation, environmental conditions (conventional vs. hot), and management were as described in Experiments 1 and 2.
Dietary treatments consisted of varying protein levels from 15 to 20% in increments of 1% CP at a constant level of energy (2,850 kcal/kg) as shown in Table 1, for Diets 7 to 12, respectively. Each diet was tested with six replicate groups of eight caged birds located in conventional (Experiment 3) or cyclic-hot environments (Experiment 4). Birds were weighed weekly to 28 days of age, and again at 56, 84, 112, and 140 days of age. Feed intake was also ascertained over these time periods. Shank length, as described in Experiment 1, was measured at 28, 84, and 140 days of age. At the termination of the experiment when birds were 140 days of age, one bird per cage from each of the experiments was sacrificed by cervical dislocation, and the carcass was frozen. After partial thawing, carcasses were passed twice through a Hobart meat grinder (Hobart Manufacturing, Troy, OH) and then representative samples were freeze-dried to a constant weight After regrinding in a Waring blender (Fischer Scientific, Toronto), samples were assayed for crude protein, ether extractable fat, and moisture (Association of Official Analytical Chemists, 1975). Statistical Analysis. Data for each environmental room and energy level for Experiments 1 and 2 were handled as a completely randomized design. Data for the response variables body weight, feed intake, and shank length, at the appropriate time intervals, were subjected to a simple one-way analysis of variance (Cochran and Cox, 1957; Freund and Littel, 1981). Means of response variables with a significant F test were further analyzed using Duncan's multiple range test (Duncan, 1955). For Experiments 3 and 4, data were handled in the same manner as in Experiments 1 and 2 for each environmental room and protein level, with the exception of the addition of the response variables percentage of carcass protein and fat. RESULTS
Experiment 1. Higher dietary energy levels for birds reared in the conventional environment had little consistent effect on body weight until 98 days of age (Table 2). After this time, birds consuming diets providing 2,950 or more kcal ME/kg were heavier than birds fed diets providing 2,750 or fewer kcal ME/kg (P<.01). Dietary energy had no effect
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MATERIALS AND METHODS
547
2,750 18.0 .36 .91 1.01 .45
2,651 17.9 .35 .90 .98 .45
33.30 30.00 9.60 21.80 1.10 1.50 1.60 .30 .05 .75 2,849 18.0 .35 .92 .99 .44
3
3,044 18.1 .37 .95 .96 .43
2,947 18.1 .36 .93 .98 .44
69.50 25.20 1.10 1.50 1.60 .30 .05 .75
51.20 20.00
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23.50 1.10 1.50 1.60 .30 .05 .75
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66.40
3,148 18.1 .36 .96 .96 .43
6 (%) 33.00 38.50 9.60 13.70 1.00 1.50 1.60 .30 .05 .75 2,861 15.0 .31 .70 .98 .43
7
2
8
Calculated (Summers and Leeson, 1985).
'Provides per kilogram diet: vitamin A, 8,000 IU; vitamin D3, 1,600 IU; vitamin E, 11 IU; riboflavin, 7 mg; Ca-pantothenat chloride, 900 mg; vitamin K (menadione), 1.5 mg; folic acid, 1.5 mg; biotin, .25 mg; ethoxyquin, 125 mg; manganese, 55 mg contains .5% sand.
16.00 33.58 25.00 20.20 1.00 1.50 1.60 .30 .07 .75
41.57 34.00 18.80 1.00 1.40 1.60 .30 .08 .75
Ground yellow corn Ground barley Ground oats Dehulled soybean meal (48% protein) Animal-vegetable fat Ground limestone Dicalcium phosphate (20% P, 20% Ca) Iodized salt (.015%) DL-Methionine Vitamin-mineral premix1 Analysis ME, kcal/kg2 CP, % Methionine, % Lysine, % Calcium, % Available phosphorus, %2
2
1
Ingredient
Experiments 1 and 2
TABLE 1. Diet composition
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of age, there was a consistent reduction in feed intake for birds fed diets of higher energy content (Table 4). Pullets fed diets providing 2,850, 2,950, or 3,050 kcal ME/kg consumed more energy than birds fed diets providing 2,750 or fewer kcal ME/kg (Table 5, P<.01). In the hot-cyclic environment, overall protein intake was maximized with diets of lower energy content (Table 5). Experiment 3. The higher protein concentration of diets of birds housed in the conventional environment generally resulted in increased body weights (P<.01) to 56 days of age; however, after this time, birds attained comparable body weights regardless of protein treatment (Table 6). This early increased growth response to dietary protein was reflected in increased shank length (P<.01) at 28 days, although after this time, shank length was not affected by diet (Table 7). Dietary protein level had no effect on feed intake (P> .05, Table 8). Although higher dietary protein levels resulted in higher overall protein intakes (P<.01, Table 9), energy intake was not influenced by diet (P>.05). Analysis of carcasses from pullets at 140 days of age revealed no differences in protein or fat content related to diet (Table 10). Experiment 4. Increasing the dietary protein concentration for birds in the hot-cyclic envi-
TABLE 3. Shank length of pullets at 28, 56, and 84 days of age as influenced by environmental temperature (Experiment 1-conventional; Experiment 2-hot-cyclic) and dietary energy Experiment no. 1
2
Dietary energy (kcal/kg) 2,650 2,750 2,850 2,950 3,050 3,150 Significance SD 2,650 2,750 2,850 2,950 3,050 3,150 Significance SD
28
84
56
, , 5.2 5.3 5.3 5.3 5.3 5.3 NS .12 5.1 5.0 5.1 5.2 5.1 5.1 NS .15
7.9 7.9 8.1 8.0 7.9 7.8 NS .16 7.8 7.6 7.8 7.7 7.7 7.7 NS .19
9.2 9.2 9.3 9.4 9.2 9.1 NS .17 9.2" 9.2b 9.4" 9 3
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"^Within experiments and columns, means with no common superscripts are significantly different (P<.05). *P<.05.
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on shank length (P>.05, Table 3). After 14 days of age, birds fed diets of higher energy content generally consumed less feed (Table 4). Over the 140-day experiment, birds fed diets providing 2,850 or less kcal ME/kg consumed more feed than birds fed diets providing 3,050 or more kcal ME/kg (P<.01). However, reduced intake of these higher energy diets resulted in increased energy intake and reduced protein intake (P<01, Table 5). Experiment 2. Increasing the energy content of the diet for birds housed in the hot-cyclic environment had little consistent effect on body weights of birds up to 77 days of age (Table 2). Up to this time, birds fed diets at 2,650 kcal ME/kg were most often as heavy as, or heavier (P<.05) than, birds fed the diet containing the highest energy concentration. However, after this time (84 to 140 days), the converse was seen: at 140 days, birds fed diets containing 2,750 or fewer kcal ME/kg were smaller than those fed diets containing 2,850 or more kcal ME/kg (Table 2, P<.01). At 84 days of age, birds fed diets providing 2,750 or fewer kcal ME/kg had shorter shank lengths than did birds fed a diet containing 2,850 or 3,050 kcal ME/kg. Pullets fed the diets with higher energy content initially consumed more feed (P<05, Table 4); although after 15 days
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ronment generally resulted in increased body weights to 84 days of age (P<.05), although after this time no significant (P>.05) effect was seen (Table 6). Diet had no effect on shank length (P>.05, Table 7), and with one notable exception at 29 to 56 days, no effect on feed intake (P>.05, Table 8). Increasing the protein content of the diet resulted in increased protein intake (P<.01, Table 9), although energy intake was not affected. Dietary protein level had no effect on carcass composition (P>.05, Table 10) of pullets at 140 days of age.
There is little doubt that hot-cyclic environmental conditions have a negative effect on growth and development of the pullet. It has been previously reported that pullet growth is quite acceptable when diets of low energy level are used (Cunningham and Morrison, 1976; Leeson and Summers, 1984a), although elevated environmental temperatures indicated the need for diets of higher nutrient density (Leeson, 1986a). Data from the present experiments suggest that pullet growth is increased with the use of high energy diets, regardless of environmental conditions. This apparent anomaly in results may relate to use in these experiments of earlier maturing, modern
TABLE 5. Cumulative energy and protein intake of pullets as influenced by environmental temperature (Experiment 1-conventional; Experiment 2-hot-cyclic) and dietary energy from 0 to 140 days of age Experiment no.
Dietary energy (kcal/kg) 2,650 2,750 2,850 2,950 3,050 3,150 Significance SD 2,650 2,750 2,850 2,950 3,050 3,150 Significance SD
Energy
Protein
(Meal ME/bird) 20.57c 20.92BC 21.76*8 22.08*" 21.40ABC 22.52*
(g/bird) 1,397* 1,370* 1,374* 1,347*8 l,263 c 1,2878°
**
**
.92 19.00° 18.84c 20.05*" 20.22** 20.50* 19.57BC
57 1,291* l^80 1.267*8 1,2348° U10c 1.118°
**
**
.59
36
*""DWithin experiments and columns, means with no common superscripts are highly significantly different (P<.01). **P<.01.
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DISCUSSION
strains of Leghorns, which react differently than the older strains used in earlier experiments, or it may be that the intended heatstress was not severe enough; possibly both considerations had an effect. Further, the birds' response to energy level of the diet is most pronounced in the 84 or 98 to 140-day age period as the bird approaches maturity. McNaughton et al. (1977) reported the difficulty commercial producers experienced in realizing required pullet weights of their flocks at 20 wk of age in summer months, and related this to inadequate levels of metabolizable energy in the diet. They reported higher energy intakes with higher levels of metabolizable energy in the diet, although the high energy intakes were not always accompanied by increases in body weight. With pullets identified as being small at 12 wk of age, these same authors show little differential response of birds to diets of 3,100 kcal ME/kg and 20% CP vs. diets of 2,700 kcal/kg and 14% CP. The above results are similar to those reported by Leeson and Summers (1984b), where genetically small pullets in a population failed to respond to dietary manipulation. Another factor that may confound experiments involved with the use of high energy diets is inadequacy of protein or amino acid intake. For example, Leeson and Summers (1982, 1985) indicate
4
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Significance
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SD 15 16 17 18 19 20
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56
Within experiments and columns, means with no common superscripts are highly significantly different (P<.01).
**P<01.
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Experiment
no.
TABLE 6. Body weight of pullets from 7 to 140 days of age as influenced by environ (Experiment 3-conventional; Experiment 4-hot-cyclic) and dietary prote
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554
LEESON AND SUMMERS
temperatures. March and Biely (1972) suggest that high environmental temperatures accentuate amino acid deficiencies. These workers indicated that at suboptimal levels of dietary lysine, growth rates were depressed when energy supply was increased either by increasing the energy concentration of the diet or by increasing the environmental temperature. It has previously been suggested that energy is the major nutrient influencing growth rate of the pullet (Leeson and Summers, 1981). Results from Experiments 1 and 2 tend to confirm this observation, as final body weight is more closely correlated to energy than to protein intake. In Experiments 1 and 2, increased body weight of pullets at 140 days of age correlates well with higher energy intake (Table 5), but does not bear any relationship to protein intake (Table 5). In Experiments 3 and 4, comparable body weights of pullets at 140 days of age (Table 6) corresponded to overall energy intakes that are similar (Table 9), whereas birds' corresponding protein intakes are quite dissimilar (Table 9). Results from Lodhi et al. (1975) can also be explained on the basis of energy intake. If one compares pullets reared under hot vs. conventional environmental conditions that consumed similar quantities of lysine or methionine, then birds from the hot environment are still smaller
TABLE 7. Shank length of pullets at 28, 84, and 140 days of age as influenced by environmental temperature (Experiment 3-conventional; Experiment 4-hot-cyclic) and dietary protein Experiment no.
Dietary protein
28
84
140
4.63 c 4.76BC 4.82"* 4.92A 4.93A
9.0 9.1 9.1 9.1 9.1 9.2 NS .13 8.9 9.1 9.1 9.2 9.0 9.2 NS .19
9.6 9.5 9.4 9.5 9.6 9.2 NS .21
(%) 3
15 16 17 18 19 20 Significance SD 15 16 17 18 19 20 Significance SD
4
A
489AB
** .12 4.6 4.8 4.8 4.6 4.8 4.8 NS .24
9.5 9.5 9.3 9.5 9.4 9.6 NS .19
°Within experiments and columns, means with no common superscripts are highly significantly different (P<,01). **P<.01.
Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on March 31, 2015
poor growth rate of pullets fed high energy-low protein diets, relating this effect to a deficiency of protein. As birds' appetite control is governed mainly by energy intake, high energy diets usually reduce feed intake. Leeson and Summers (1982) report a cumulative protein intake of less than 1 kg for the small-weight pullet, which is much lower than that required to maximize body weight gain in the present study. It is likely that pullets' requirement for protein (amino acids) is increased (as a percentage of the diet) in hot weather conditions. Lodhi et al. (1975) suggest the protein requirement of pullets at 32 and 20 C mean environmental temperatures to be 18.5 and 15% CP, respectively. Leeson and Summers (1981) indicate that early growth rates of pullets reared in warm environments can be improved by providing higher concentrations of protein in the diet. In an environment with conventional temperatures, Leeson and Summers (1981) show a 4% increase in 8-wk body weight as a result of a 61% increase in protein intake. However, in a warm environment, a 30% increase in protein intake resulted in a 10% increase in body weight. Stockland and Blaylock (1974) likewise concluded that the pullet's protein requirement in terms of dietary specifications was greater at higher ambient
DIETARY PROTEIN AND ENERGY DURING REARING
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LEESON AND SUMMERS TABLE 9. Protein and energy intake of pullets as influenced by environmental temperature (Experiment 3-conventional; Experiment 4-hot-cyclic) and dietary protein from 0 to 140 days of age
Experiment no.
Dietary protein
Protein
Energy
(%)
(g/bird) 1,280° 1,284° 1,369°° l,388 c 1,526B 1,619*
(Meal ME/bird) 24.32 22.86 22.94 21.98 22.89 23.06 NS
SD
** 76 1,108° l,168 c U44B 1,292B 1,381A 1,434A
** 50
1.28 21.05 20.81 20.85 20.46 20.72 20.43 NS .84
A
°Within experiments and columns, means with no common superscripts are highly significantly different (P<.01). **P<.01.
in weight (Lodhi et al., 1975). Again, this effect likely relates to a simple deficiency of energy. In Experiments 3 and 4, the fact that birds were of similar 140-day body weight regardless of dietary protein specification likely
relates to an adequate protein intake (greater than 1 kg) provided by all diets. Little difference is seen in the birds' responses to dietary specifications regardless of environmental conditions. It is possible that the hotcyclic environment used in the present experi-
TABLE 10. Effect of environmental temperature (Experiment 3-conventional; Experiment 4-hot-cyclic) and dietary protein on 140-day carcass composition of pullets Experiment no.
Dietary protein
Crude Protein
Fat
(%) 15 16 17 18 19 20 Significance SD 15 16 17 18 19 20 Significance SD
51.2 52.9 54.1 54.4 54.6 54.3 NS 5.4 57.3 54.2 53.6 53.7 55.2 50.9 NS 5.0
40.6 38.2 37.2 36.1 36.6 38.3 NS 6.9 32.3 37.3 39.0 37.6 34.5 41.4 NS 5.8
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15 16 17 18 19 20 Significance SD 15 16 17 18 19 20 Significance
DIETARY PROTEIN AND ENERGY DURING REARING
ACKNOWLEDGMENT
This work was supported by the Ontario Ministry of Agriculture and Food, the Natural Sciences and Engineering Research Council of Canada, and the Ontario Egg Producers' Marketing Board. REFERENCES Association of Official Analytical Chemists, 1975. Official Methods of Analysis. 12th ed. Assoc. Offic. Anal. Chem., Washington, DC. Cochran, W. G., and G. M. Cox, 1957. Experimental Design. 2nd ed. Wiley and Sons, New York, NY. Cunningham, D. C , and W. D. Morrison, 1976. Dietary energy and fat content as factors in the nutrition of developing egg strain pullets and young hens. 1. Effect on several parameters and body composition at sexual maturity. Poultry Sci. 55:85-97. Duncan, D. D., 1955. Multiple range and multiple F tests. Biometrics 11:1-42. Freund, R. J., and R. C. Littel, 1981. SAS for linear models. SAS Institute Inc., Cary, NC. Leeson, S., 1986a. Nutrient requirements of egg layer replacements and layers. Pages 110-117 in: Proc.
Poultry Nutr. and Disease Control Technical Symposium. MSD-Ag Vet., Princeton, NJ. Leeson, S., 1986b. Nutritional considerations of poultry during heat stress. World's Poult. Sci. J. 42:619-81. Leeson, S., and J. D. Summers, 1981. Effect of rearing diet on performance of early maturing pullets. Can. J. Anim. Sci. 61:743-749. Leeson, S., and J. D. Summers, 1982. Use of single-stage low protein diets for growing leghorn pullets. Poultry Sci. 61:1684-1691. Leeson, S., and J. D. Summers, 1984a. Effects of cage density and diet energy concentration on the performance of growing leghorn pullets subjected to early induced maturity. Poultry Sci. 63:875-882. Leeson, S., and J. D. Summers, 1984b. Influence of nutrient density on growth and carcass composition of weightsegregated leghorn pullets. Poultry Sci. 63:17641772. Leeson, S„ and J. D. Summers, 1985. Early reproductive characteristics of leghorn pullets reared on least-cost diets formulated to protein and/or amino acid specifications. Can. J. Anim. Sci. 65:205-210. Lodhi, G. N., J. S. Chawla, A. K. Ahuja, and J. S. Ichhponani, 1975. Influence of climate conditions on protein and energy requirements of poultry. Indian J. Anim. Sci. 45:664-668. March, B. E., and J. Biely, 1972. The effect of energy supplied from the diet and from environment heat on the response of chicks to different levels of dietary lysine. Poultry Sci. 51:665-668. McNaughton, J. L., L. F. Kubena, J. W. Deaton, and F. N. Reece, 1977. Influence of dietary protein and energy on the performance of commercial egg-type pullets reared under summer conditions. Poultry Sci. 56:1391-1398. Payne, C. G„ 1967. Environmental Control of Poultry Production. T. C. Carter, ed. Langmans, London, UK. Stockland, W. L., and L. G. Blaylock, 1974. The influence of temperature on the protein requirement of cage-reared replacement pullets. Poultry Sci. 53:1174-1187. Summers, J. D., and S. Leeson, 1983. Factors influencing early egg size. Poultry Sci. 62:1155-1159. Summers, J. D., and S. Leeson, 1985. Poultry Nutrition Handbook. Univ. Guelph, Guelph, Ontario, Canada. Wells, R. G., 1980. Pullet feeding systems during rearing in relation to subsequent laying performance. Pages 87-94 in: Recent Advances in Animal Nutrition 1980. W. Haresign, ed. Butterworths, London, England UK.
Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on March 31, 2015
merits was not severe enough to stress the pullets, and that the pullets adapted to the daily regimen of 22 to 32 C cyclic conditions (Leeson, 1986b). Under hot-cyclic daily ambient temperature involving a maximum value of 32 C, it is suggested that growth of pullets to 140 days of age will be optimum with cumulative intakes of about 21 Meal ME and 1,200 g CP. It would appear that these intakes can be achieved with mean diet specifications of around 2,900 kcal ME/kg and 18% CP. However, with multidiet programs commonly used in the industry, it appears that pullet growth is initially most sensitive to dietary protein, whereas energy intake becomes more critical as the bird approaches maturity.
557
1989 Poultry Science 68:546-557 INTRODUCTION
Pullets that mature earlier dictate the need for growing programs that maximize both growth rate and optimum development of associated body components (Summers and Leeson, 1983). Nutrient intake plays a major role in dictating growth rate; the major nutrients, energy and protein (amino acids), are of economic importance and thus can be manipulated to control growth according to commercial requirements. Qualitative and quantitative nutrient restriction invariably result in reduced body weight (Wells, 1980) and most often uneconomical adult performance (Leeson, 1986a). It is generally assumed that birds will regulate their energy intake when offered a diet providing a range of energy concentrations. At moderate environmental temperatures, the mechanism is quite precise (Payne, 1967). However, there are indications that the energy intake of pullets can be maximized through use
of low energy diets. Leeson and Summers (1984a) show comparable or improved growth rates of pullets reared on diets with 2,085 vs. 3,050 kcal ME/kg. Similarly, Cunningham and Morrison (1976) give evidence for increased energy intake of birds fed diets with lower energy content. Under hot weather conditions, however, optimum pullet growth cannot be achieved with low energy diets (Leeson, 1986a), and invariably high energy diets containing fat are essential. These observations suggest that pullets react differently to energy concentration of the diet depending upon environmental temperature. Protein intake is also a factor influencing growth and development of pullets, and again environmental temperature may have a confounding effect on requirements. Under moderate environmental conditions, birds show little response to elevated protein intake (Leeson and Summers, 1981); although under hot weather conditions, increased dietary protein
546
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ABSTRACT Four experiments were conducted to determine the growing pullet's response to graded levels of dietary ME (2.650 to 3,150 kcal ME/kg) or CP (15 to 20% CP) when reared in conventional (constant 22 C) or hot-cyclic temperatures (22 to 32 C). Each diet was tested with six replicate groups of eight caged birds from 1 day to 140 days of age. Body weight and feed intake were monitored periodically. Increasing the energy concentration of the diet resulted in increased body weight, with this effect being most pronounced in the hot environment. At 140 days of age, birds fed diets providing 2,650 or 2,750 kcal ME/kg were smaller than birds fed diets providing 2,950 or more kcal ME/kg. Increasing the dietary energy concentration resulted in reduced feed intake and a general tendency to increase energy intake. In other experiments, increasing the dietary protein level had an initial effect of increasing body weights, although by 140 days of age, differential dietary protein levels of 15 to 20% had no effect on body weight. Increasing the protein level of the diet resulted in increased protein intake but had no effect on energy intake. It is concluded that given adequate protein intake, pullet growth is most responsive to energy intake. Pullet growth to 140 days of age will be maximized with cumulative nutrient intakes of 21 Meal ME and 1,200 g CP. It appears that such intakes can be achieved with mean dietary specifications during rearing of 2,900 kcal ME/kg and 18% CP. However, with multidiet programs, as commonly used in the industry, it appears that pullet growth is initially most sensitive to dietary protein level, whereas energy intake becomes more critical as the bird approaches maturity. (Key words: pullet, energy, protein)
DIETARY PROTEIN AND ENERGY DURING REARING
levels may improve growth rate (Stockland and Blaylock, 1974; Leeson and Summers, 1981). The situation is further influenced by the development of many strains of Leghorn pullets with substantially reduced feed intakes; these intakes are in turn related to reduced body weights (Leeson, 1986a). Little work has been reported on the effect of environmental temperature on the response of modern strain pullets to protein and energy in the diet. Four experiments were designed to investigate this effect.
Experiments 1 and 2. One-day-old Leghorn pullets of a commercial strain were wingbanded, weighed, and allocated to one of two rooms that provided environmental control. For the first 48 h, birds were exposed to constant light at 100 lx intensity; after this time the photoperiod was reduced to 8 h and the intensity to 10 lx. After a conventional brooding period of 21 days, one room was maintained at 22 C (Experiment 1); birds in an alternate room were subjected to 32 C during the light period (0800 to 1600 h) and 22 C during darkness (1600 to 0800 h; Experiment 2). Temperature adjustment was achieved within 1 h. All birds were housed at eight birds/60 x 50-cm cage and had free access to feed and water. Dietary treatments consisted of varying energy levels from 2,650 to 3,150 kcal ME/kg in increments of 100 kcal ME as shown in Table 1 for Diets 1 to 6. Each diet was offered to six replicate cage groups for each experiment. All birds were individually weighed at 7-day intervals to 84 days of age, and thereafter at 98, 112, and 140 days of age. Feed intake per cage group was also ascertained over these time periods. Shank length was measured when birds were 28, 36, and 84 days of age (Leeson and Summers, 1984b). Experiments 3 and 4. Leghorn pullets of the same commercial strain as used in Experiments 1 and 2 were wingbanded, weighed, and distributed to one of two environmental control rooms as described in Experiments 1 and 2. Experiments 3 and 4 commenced in the spring, which was the termination time of the previous experiments. Cage allocation, environmental conditions (conventional vs. hot), and management were as described in Experiments 1 and 2.
Dietary treatments consisted of varying protein levels from 15 to 20% in increments of 1% CP at a constant level of energy (2,850 kcal/kg) as shown in Table 1, for Diets 7 to 12, respectively. Each diet was tested with six replicate groups of eight caged birds located in conventional (Experiment 3) or cyclic-hot environments (Experiment 4). Birds were weighed weekly to 28 days of age, and again at 56, 84, 112, and 140 days of age. Feed intake was also ascertained over these time periods. Shank length, as described in Experiment 1, was measured at 28, 84, and 140 days of age. At the termination of the experiment when birds were 140 days of age, one bird per cage from each of the experiments was sacrificed by cervical dislocation, and the carcass was frozen. After partial thawing, carcasses were passed twice through a Hobart meat grinder (Hobart Manufacturing, Troy, OH) and then representative samples were freeze-dried to a constant weight After regrinding in a Waring blender (Fischer Scientific, Toronto), samples were assayed for crude protein, ether extractable fat, and moisture (Association of Official Analytical Chemists, 1975). Statistical Analysis. Data for each environmental room and energy level for Experiments 1 and 2 were handled as a completely randomized design. Data for the response variables body weight, feed intake, and shank length, at the appropriate time intervals, were subjected to a simple one-way analysis of variance (Cochran and Cox, 1957; Freund and Littel, 1981). Means of response variables with a significant F test were further analyzed using Duncan's multiple range test (Duncan, 1955). For Experiments 3 and 4, data were handled in the same manner as in Experiments 1 and 2 for each environmental room and protein level, with the exception of the addition of the response variables percentage of carcass protein and fat. RESULTS
Experiment 1. Higher dietary energy levels for birds reared in the conventional environment had little consistent effect on body weight until 98 days of age (Table 2). After this time, birds consuming diets providing 2,950 or more kcal ME/kg were heavier than birds fed diets providing 2,750 or fewer kcal ME/kg (P<.01). Dietary energy had no effect
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MATERIALS AND METHODS
547
2,750 18.0 .36 .91 1.01 .45
2,651 17.9 .35 .90 .98 .45
33.30 30.00 9.60 21.80 1.10 1.50 1.60 .30 .05 .75 2,849 18.0 .35 .92 .99 .44
3
3,044 18.1 .37 .95 .96 .43
2,947 18.1 .36 .93 .98 .44
69.50 25.20 1.10 1.50 1.60 .30 .05 .75
51.20 20.00
5
23.50 1.10 1.50 1.60 .30 .05 .75
4
25.80 3.60 1.50 1.60 .30 .05 .75
66.40
3,148 18.1 .36 .96 .96 .43
6 (%) 33.00 38.50 9.60 13.70 1.00 1.50 1.60 .30 .05 .75 2,861 15.0 .31 .70 .98 .43
7
2
8
Calculated (Summers and Leeson, 1985).
'Provides per kilogram diet: vitamin A, 8,000 IU; vitamin D3, 1,600 IU; vitamin E, 11 IU; riboflavin, 7 mg; Ca-pantothenat chloride, 900 mg; vitamin K (menadione), 1.5 mg; folic acid, 1.5 mg; biotin, .25 mg; ethoxyquin, 125 mg; manganese, 55 mg contains .5% sand.
16.00 33.58 25.00 20.20 1.00 1.50 1.60 .30 .07 .75
41.57 34.00 18.80 1.00 1.40 1.60 .30 .08 .75
Ground yellow corn Ground barley Ground oats Dehulled soybean meal (48% protein) Animal-vegetable fat Ground limestone Dicalcium phosphate (20% P, 20% Ca) Iodized salt (.015%) DL-Methionine Vitamin-mineral premix1 Analysis ME, kcal/kg2 CP, % Methionine, % Lysine, % Calcium, % Available phosphorus, %2
2
1
Ingredient
Experiments 1 and 2
TABLE 1. Diet composition
from http://ps.oxfordjournals.org/ at Michigan State University on March 31, 2015
549
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of age, there was a consistent reduction in feed intake for birds fed diets of higher energy content (Table 4). Pullets fed diets providing 2,850, 2,950, or 3,050 kcal ME/kg consumed more energy than birds fed diets providing 2,750 or fewer kcal ME/kg (Table 5, P<.01). In the hot-cyclic environment, overall protein intake was maximized with diets of lower energy content (Table 5). Experiment 3. The higher protein concentration of diets of birds housed in the conventional environment generally resulted in increased body weights (P<.01) to 56 days of age; however, after this time, birds attained comparable body weights regardless of protein treatment (Table 6). This early increased growth response to dietary protein was reflected in increased shank length (P<.01) at 28 days, although after this time, shank length was not affected by diet (Table 7). Dietary protein level had no effect on feed intake (P> .05, Table 8). Although higher dietary protein levels resulted in higher overall protein intakes (P<.01, Table 9), energy intake was not influenced by diet (P>.05). Analysis of carcasses from pullets at 140 days of age revealed no differences in protein or fat content related to diet (Table 10). Experiment 4. Increasing the dietary protein concentration for birds in the hot-cyclic envi-
TABLE 3. Shank length of pullets at 28, 56, and 84 days of age as influenced by environmental temperature (Experiment 1-conventional; Experiment 2-hot-cyclic) and dietary energy Experiment no. 1
2
Dietary energy (kcal/kg) 2,650 2,750 2,850 2,950 3,050 3,150 Significance SD 2,650 2,750 2,850 2,950 3,050 3,150 Significance SD
28
84
56
, , 5.2 5.3 5.3 5.3 5.3 5.3 NS .12 5.1 5.0 5.1 5.2 5.1 5.1 NS .15
7.9 7.9 8.1 8.0 7.9 7.8 NS .16 7.8 7.6 7.8 7.7 7.7 7.7 NS .19
9.2 9.2 9.3 9.4 9.2 9.1 NS .17 9.2" 9.2b 9.4" 9 3
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Downloaded from http://ps.oxfordjournals.org/ at Michigan State University on March 31, 2015
on shank length (P>.05, Table 3). After 14 days of age, birds fed diets of higher energy content generally consumed less feed (Table 4). Over the 140-day experiment, birds fed diets providing 2,850 or less kcal ME/kg consumed more feed than birds fed diets providing 3,050 or more kcal ME/kg (P<.01). However, reduced intake of these higher energy diets resulted in increased energy intake and reduced protein intake (P<01, Table 5). Experiment 2. Increasing the energy content of the diet for birds housed in the hot-cyclic environment had little consistent effect on body weights of birds up to 77 days of age (Table 2). Up to this time, birds fed diets at 2,650 kcal ME/kg were most often as heavy as, or heavier (P<.05) than, birds fed the diet containing the highest energy concentration. However, after this time (84 to 140 days), the converse was seen: at 140 days, birds fed diets containing 2,750 or fewer kcal ME/kg were smaller than those fed diets containing 2,850 or more kcal ME/kg (Table 2, P<.01). At 84 days of age, birds fed diets providing 2,750 or fewer kcal ME/kg had shorter shank lengths than did birds fed a diet containing 2,850 or 3,050 kcal ME/kg. Pullets fed the diets with higher energy content initially consumed more feed (P<05, Table 4); although after 15 days
DIETARY PROTEIN AND ENERGY DURING REARING
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552
LEESON AND SUMMERS
ronment generally resulted in increased body weights to 84 days of age (P<.05), although after this time no significant (P>.05) effect was seen (Table 6). Diet had no effect on shank length (P>.05, Table 7), and with one notable exception at 29 to 56 days, no effect on feed intake (P>.05, Table 8). Increasing the protein content of the diet resulted in increased protein intake (P<.01, Table 9), although energy intake was not affected. Dietary protein level had no effect on carcass composition (P>.05, Table 10) of pullets at 140 days of age.
There is little doubt that hot-cyclic environmental conditions have a negative effect on growth and development of the pullet. It has been previously reported that pullet growth is quite acceptable when diets of low energy level are used (Cunningham and Morrison, 1976; Leeson and Summers, 1984a), although elevated environmental temperatures indicated the need for diets of higher nutrient density (Leeson, 1986a). Data from the present experiments suggest that pullet growth is increased with the use of high energy diets, regardless of environmental conditions. This apparent anomaly in results may relate to use in these experiments of earlier maturing, modern
TABLE 5. Cumulative energy and protein intake of pullets as influenced by environmental temperature (Experiment 1-conventional; Experiment 2-hot-cyclic) and dietary energy from 0 to 140 days of age Experiment no.
Dietary energy (kcal/kg) 2,650 2,750 2,850 2,950 3,050 3,150 Significance SD 2,650 2,750 2,850 2,950 3,050 3,150 Significance SD
Energy
Protein
(Meal ME/bird) 20.57c 20.92BC 21.76*8 22.08*" 21.40ABC 22.52*
(g/bird) 1,397* 1,370* 1,374* 1,347*8 l,263 c 1,2878°
**
**
.92 19.00° 18.84c 20.05*" 20.22** 20.50* 19.57BC
57 1,291* l^80 1.267*8 1,2348° U10c 1.118°
**
**
.59
36
*""DWithin experiments and columns, means with no common superscripts are highly significantly different (P<.01). **P<.01.
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DISCUSSION
strains of Leghorns, which react differently than the older strains used in earlier experiments, or it may be that the intended heatstress was not severe enough; possibly both considerations had an effect. Further, the birds' response to energy level of the diet is most pronounced in the 84 or 98 to 140-day age period as the bird approaches maturity. McNaughton et al. (1977) reported the difficulty commercial producers experienced in realizing required pullet weights of their flocks at 20 wk of age in summer months, and related this to inadequate levels of metabolizable energy in the diet. They reported higher energy intakes with higher levels of metabolizable energy in the diet, although the high energy intakes were not always accompanied by increases in body weight. With pullets identified as being small at 12 wk of age, these same authors show little differential response of birds to diets of 3,100 kcal ME/kg and 20% CP vs. diets of 2,700 kcal/kg and 14% CP. The above results are similar to those reported by Leeson and Summers (1984b), where genetically small pullets in a population failed to respond to dietary manipulation. Another factor that may confound experiments involved with the use of high energy diets is inadequacy of protein or amino acid intake. For example, Leeson and Summers (1982, 1985) indicate
4
3
SD
Significance
3 51 54 53 51 54 52 NS 5
SD 15 16 17 18 19 20
6
*
85* 94* 90*"
91"" 37.bc
6 82c
**
**
Significance
83 B C 88*"
93* 91* 90*
76 c
50*"
14
48"
7
52* 53* 53* 52*
15 16 17 18 19 20
(%)
Dietary protein
11
**
* 12
12 208 c 229" 238*" 231" 247* 242*"
**
** 11 133b 147"b 146"" 144* 157" 151"
197c 216" 223*" 238* 232* 234*
28
127c 138"° 149*" 158* 152* 150*"
21
23
**
22 531 c 574" 595*" 604* 613* 620*
**
580 610 c 626"° 639*" 627"° 660*
(&) D
56
Within experiments and columns, means with no common superscripts are highly significantly different (P<.01).
**P<01.
*P<.05.
AD
'"'Within experiments and columns, means with no common superscripts are significantly different (P<.05).
Experiment
no.
TABLE 6. Body weight of pullets from 7 to 140 days of age as influenced by environ (Experiment 3-conventional; Experiment 4-hot-cyclic) and dietary prote
from http://ps.oxfordjournals.org/ at Michigan State University on March 31, 2015
554
LEESON AND SUMMERS
temperatures. March and Biely (1972) suggest that high environmental temperatures accentuate amino acid deficiencies. These workers indicated that at suboptimal levels of dietary lysine, growth rates were depressed when energy supply was increased either by increasing the energy concentration of the diet or by increasing the environmental temperature. It has previously been suggested that energy is the major nutrient influencing growth rate of the pullet (Leeson and Summers, 1981). Results from Experiments 1 and 2 tend to confirm this observation, as final body weight is more closely correlated to energy than to protein intake. In Experiments 1 and 2, increased body weight of pullets at 140 days of age correlates well with higher energy intake (Table 5), but does not bear any relationship to protein intake (Table 5). In Experiments 3 and 4, comparable body weights of pullets at 140 days of age (Table 6) corresponded to overall energy intakes that are similar (Table 9), whereas birds' corresponding protein intakes are quite dissimilar (Table 9). Results from Lodhi et al. (1975) can also be explained on the basis of energy intake. If one compares pullets reared under hot vs. conventional environmental conditions that consumed similar quantities of lysine or methionine, then birds from the hot environment are still smaller
TABLE 7. Shank length of pullets at 28, 84, and 140 days of age as influenced by environmental temperature (Experiment 3-conventional; Experiment 4-hot-cyclic) and dietary protein Experiment no.
Dietary protein
28
84
140
4.63 c 4.76BC 4.82"* 4.92A 4.93A
9.0 9.1 9.1 9.1 9.1 9.2 NS .13 8.9 9.1 9.1 9.2 9.0 9.2 NS .19
9.6 9.5 9.4 9.5 9.6 9.2 NS .21
(%) 3
15 16 17 18 19 20 Significance SD 15 16 17 18 19 20 Significance SD
4
A
489AB
** .12 4.6 4.8 4.8 4.6 4.8 4.8 NS .24
9.5 9.5 9.3 9.5 9.4 9.6 NS .19
°Within experiments and columns, means with no common superscripts are highly significantly different (P<,01). **P<.01.
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poor growth rate of pullets fed high energy-low protein diets, relating this effect to a deficiency of protein. As birds' appetite control is governed mainly by energy intake, high energy diets usually reduce feed intake. Leeson and Summers (1982) report a cumulative protein intake of less than 1 kg for the small-weight pullet, which is much lower than that required to maximize body weight gain in the present study. It is likely that pullets' requirement for protein (amino acids) is increased (as a percentage of the diet) in hot weather conditions. Lodhi et al. (1975) suggest the protein requirement of pullets at 32 and 20 C mean environmental temperatures to be 18.5 and 15% CP, respectively. Leeson and Summers (1981) indicate that early growth rates of pullets reared in warm environments can be improved by providing higher concentrations of protein in the diet. In an environment with conventional temperatures, Leeson and Summers (1981) show a 4% increase in 8-wk body weight as a result of a 61% increase in protein intake. However, in a warm environment, a 30% increase in protein intake resulted in a 10% increase in body weight. Stockland and Blaylock (1974) likewise concluded that the pullet's protein requirement in terms of dietary specifications was greater at higher ambient
DIETARY PROTEIN AND ENERGY DURING REARING
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LEESON AND SUMMERS TABLE 9. Protein and energy intake of pullets as influenced by environmental temperature (Experiment 3-conventional; Experiment 4-hot-cyclic) and dietary protein from 0 to 140 days of age
Experiment no.
Dietary protein
Protein
Energy
(%)
(g/bird) 1,280° 1,284° 1,369°° l,388 c 1,526B 1,619*
(Meal ME/bird) 24.32 22.86 22.94 21.98 22.89 23.06 NS
SD
** 76 1,108° l,168 c U44B 1,292B 1,381A 1,434A
** 50
1.28 21.05 20.81 20.85 20.46 20.72 20.43 NS .84
A
°Within experiments and columns, means with no common superscripts are highly significantly different (P<.01). **P<.01.
in weight (Lodhi et al., 1975). Again, this effect likely relates to a simple deficiency of energy. In Experiments 3 and 4, the fact that birds were of similar 140-day body weight regardless of dietary protein specification likely
relates to an adequate protein intake (greater than 1 kg) provided by all diets. Little difference is seen in the birds' responses to dietary specifications regardless of environmental conditions. It is possible that the hotcyclic environment used in the present experi-
TABLE 10. Effect of environmental temperature (Experiment 3-conventional; Experiment 4-hot-cyclic) and dietary protein on 140-day carcass composition of pullets Experiment no.
Dietary protein
Crude Protein
Fat
(%) 15 16 17 18 19 20 Significance SD 15 16 17 18 19 20 Significance SD
51.2 52.9 54.1 54.4 54.6 54.3 NS 5.4 57.3 54.2 53.6 53.7 55.2 50.9 NS 5.0
40.6 38.2 37.2 36.1 36.6 38.3 NS 6.9 32.3 37.3 39.0 37.6 34.5 41.4 NS 5.8
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15 16 17 18 19 20 Significance SD 15 16 17 18 19 20 Significance
DIETARY PROTEIN AND ENERGY DURING REARING
ACKNOWLEDGMENT
This work was supported by the Ontario Ministry of Agriculture and Food, the Natural Sciences and Engineering Research Council of Canada, and the Ontario Egg Producers' Marketing Board. REFERENCES Association of Official Analytical Chemists, 1975. Official Methods of Analysis. 12th ed. Assoc. Offic. Anal. Chem., Washington, DC. Cochran, W. G., and G. M. Cox, 1957. Experimental Design. 2nd ed. Wiley and Sons, New York, NY. Cunningham, D. C , and W. D. Morrison, 1976. Dietary energy and fat content as factors in the nutrition of developing egg strain pullets and young hens. 1. Effect on several parameters and body composition at sexual maturity. Poultry Sci. 55:85-97. Duncan, D. D., 1955. Multiple range and multiple F tests. Biometrics 11:1-42. Freund, R. J., and R. C. Littel, 1981. SAS for linear models. SAS Institute Inc., Cary, NC. Leeson, S., 1986a. Nutrient requirements of egg layer replacements and layers. Pages 110-117 in: Proc.
Poultry Nutr. and Disease Control Technical Symposium. MSD-Ag Vet., Princeton, NJ. Leeson, S., 1986b. Nutritional considerations of poultry during heat stress. World's Poult. Sci. J. 42:619-81. Leeson, S., and J. D. Summers, 1981. Effect of rearing diet on performance of early maturing pullets. Can. J. Anim. Sci. 61:743-749. Leeson, S., and J. D. Summers, 1982. Use of single-stage low protein diets for growing leghorn pullets. Poultry Sci. 61:1684-1691. Leeson, S., and J. D. Summers, 1984a. Effects of cage density and diet energy concentration on the performance of growing leghorn pullets subjected to early induced maturity. Poultry Sci. 63:875-882. Leeson, S., and J. D. Summers, 1984b. Influence of nutrient density on growth and carcass composition of weightsegregated leghorn pullets. Poultry Sci. 63:17641772. Leeson, S„ and J. D. Summers, 1985. Early reproductive characteristics of leghorn pullets reared on least-cost diets formulated to protein and/or amino acid specifications. Can. J. Anim. Sci. 65:205-210. Lodhi, G. N., J. S. Chawla, A. K. Ahuja, and J. S. Ichhponani, 1975. Influence of climate conditions on protein and energy requirements of poultry. Indian J. Anim. Sci. 45:664-668. March, B. E., and J. Biely, 1972. The effect of energy supplied from the diet and from environment heat on the response of chicks to different levels of dietary lysine. Poultry Sci. 51:665-668. McNaughton, J. L., L. F. Kubena, J. W. Deaton, and F. N. Reece, 1977. Influence of dietary protein and energy on the performance of commercial egg-type pullets reared under summer conditions. Poultry Sci. 56:1391-1398. Payne, C. G„ 1967. Environmental Control of Poultry Production. T. C. Carter, ed. Langmans, London, UK. Stockland, W. L., and L. G. Blaylock, 1974. The influence of temperature on the protein requirement of cage-reared replacement pullets. Poultry Sci. 53:1174-1187. Summers, J. D., and S. Leeson, 1983. Factors influencing early egg size. Poultry Sci. 62:1155-1159. Summers, J. D., and S. Leeson, 1985. Poultry Nutrition Handbook. Univ. Guelph, Guelph, Ontario, Canada. Wells, R. G., 1980. Pullet feeding systems during rearing in relation to subsequent laying performance. Pages 87-94 in: Recent Advances in Animal Nutrition 1980. W. Haresign, ed. Butterworths, London, England UK.
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merits was not severe enough to stress the pullets, and that the pullets adapted to the daily regimen of 22 to 32 C cyclic conditions (Leeson, 1986b). Under hot-cyclic daily ambient temperature involving a maximum value of 32 C, it is suggested that growth of pullets to 140 days of age will be optimum with cumulative intakes of about 21 Meal ME and 1,200 g CP. It would appear that these intakes can be achieved with mean diet specifications of around 2,900 kcal ME/kg and 18% CP. However, with multidiet programs commonly used in the industry, it appears that pullet growth is initially most sensitive to dietary protein, whereas energy intake becomes more critical as the bird approaches maturity.
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