EDUCATION AND PRODUCTION The Effects of Age at Photostimulation and Dietary Protein Intake on Reproductive Efficiency in Three s of Broiler Breeders Varying in Breast Yield N. S. Joseph,* A. A. J. Dulaney,* F. E. Robinson,*,1 R. A. Renema,* and M. J. Zuidhof *Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada, T6G 2P5; and Animal Industry Division, Alberta Agriculture, Food and Rural Development, Edmonton, Alberta, Canada, T6H 5T6 ABSTRACT The effects of strain, age at photostimulation, delayed sexual maturity but the rate of sexual maturation and dietary CP intake during the early lay period on sexual maturation and egg production parameters in broiler breeder hens were determined. Three Arbor Acres strains, Classic, Feather Sexable Yield (FSY), and an experimental line (EXP), were reared in floor pens. At 19 wk of age, pullets were moved into individual cages. At 20 wk of age one-half of the birds were photostimulated by increasing the day length from 8L:16D to 15L:9D. The remaining birds were photostimulated at 23 wk of age. The birds were fed 16 or 18% CP diets from 19 to 32 wk of age. The Classic hens achieved sexual maturity 4 d before the EXP hens. Delaying photostimulation to 23 wk of age was accelerated after photostimulation occurred. Egg weight at sexual maturity was higher for the later photostimulated birds. The FSY and EXP hens, selected for more white-meat yield than the Classic hens, were heavier in BW but had lower settable egg production than the latter. Feeding 18% CP resulted in a higher BW throughout the trial, as compared to the 16% CP diet. There was a response to dietary CP and photostimulation age on initial egg weight and the number of double-yolked eggs. Early photostimulation was not advantageous because egg production parameters were not affected. Both Classic and FSY strains performed well when fed 18% CP; however, EXP hens benefited from 16% CP intake. (Key words: broiler breeder hen, strain, age at photostimulation, crude protein intake, egg production) 2002 Poultry Science 81:597 607 INTRODUCTION The transition from a pullet to a hen involves a change in how much energy is partitioned between completing skeletal growth and muscle deposition to building fat reserves and developing reproductive organs for egg production. In response to increased genetic selection for growth traits over the past years, pullet growth potential has become increasingly larger. With feed restriction, pullets require more time than what was necessary in the past to attain the BW and level of body composition needed for sexual maturation (Leeson and Summers, 1983; Soller et al., 1984; Katanbaf et al., 1989). As a result, the age that birds were photostimulated 10 yr ago would be considered too early for today s modern strains. Most farms will now delay photostimulation until the flock is 21 to 23 wk of age. However, as new strains with even more meat yield are placed on the 2002 Poultry Science Association, Inc. Received for publication June 4, 2001. Accepted for publication December 28, 2001. 1 To whom correspondence should be addressed: frank.robinson@ ualberta.ca. market, the age at which these broiler breeders are photostimulated will have to be reassessed. Some of the effects of delaying photostimulation have been described in the literature (Yuan et al., 1994; Robinson et al., 1996). One effect of delaying photostimulation is that sexual maturity is also delayed (Yuan et al., 1994; Robinson et al., 1996). However this delay is an advantage as birds that are ready for egg production are forced to wait as those that are still growing are given more time to prepare for reproductive maturation. This delay then leads to an improvement in flock uniformity of BW and once photostimulation does occur; the length of time needed for the entire flock to achieve sexual maturity will be reduced (Hocking, 1996; Robinson et al., 1996). If BW is increased because photostimulation is delayed, egg weight may also increase, as BW and egg weight are positively correlated (McDaniel et al., 1981). Therefore delaying photostimulation would have the added benefit of increasing settable egg production. To date, delaying photostimulation has not been shown to Abbreviation Key: EXP = Arbor Acres experimental line; FSY = Arbor Acres Feather Sexable Yield; LYF = large yellow follicles. 597
598 JOSEPH ET AL. increase egg weight in broiler breeders (Yuan et al., 1994; Robinson et al., 1996). Alternatively, increasing dietary protein intake has been shown to increase egg weight in some studies but not in others (Cave, 1984; Brake et al., 1985; Spratt and Leeson, 1987; Summers and Leeson, 1994). Feeding 16% CP versus 14% CP in the prebreeder diet resulted in increased mean egg weight and increased settable egg production by four eggs per hen (Joseph et al., 2000). The effect of photostimulating at 20 and 23 wk of age was examined on three strains of meat-type pullets (Joseph, 2000). The later photostimulated hens had a higher BW at sexual maturity and produced larger eggs throughout the laying cycle but had the same settable egg production as early photostimulated hens (Joseph, 2000). The current study used the same strains and the same ages at photostimulation as Joseph (2000) but with the addition of two levels of dietary CP during the early lay period. The three strains used in both trials differed in growth rate, meat yield and, potentially, egg production as described previously (Joseph, 2000). Briefly, the Arbor Acres Classic hen is a smaller bird with a high rate of egg production. The Arbor Acres Feather Sexable Yield (FSY) is a larger hen than the Classic as it was designed for the further processing market. The last strain is an experimental line (EXP) that has more whitemeat yield than the FSY. The pullets were photostimulated at 20 or 23 wk of age and were fed 16 or 18% CP. The first objective of this trial was to determine differences between the strains in terms of sexual maturity, egg production, and carcass parameters. The second objective was to study the response of these strains to age at photostimulation and CP intake. In particular, discerning specific genotype by environment interactions would help characterize the strains and refine management techniques. For example, Bray et al. (1965) found that late maturing pullets had higher protein requirements because they had to maintain a higher rate of lay, egg weight, and BW than early maturing pullets. MATERIALS AND METHODS Stocks and Management The University of Alberta s Faculty of Agriculture, Forestry and Home Economics Animal Policy and Welfare Committee approved this protocol according to the Guide to the Care and Use of Experimental Animals (Canadian Council on Animal Care, 1984). One hundred fifty pullets of each strain were obtained from Aviagen Inc. 2 The strains were reared separately in floor pens (three pens/strain). From 0 to 3 d of age, the chicks received 24L:0D. From 4 d to 19 wk of age the photophase was shortened to 8L:16D. For the first 3 wk posthatch, the pullets were given a starter diet fed ad libitum (ME: 2,783 kcal/kg; CP: 18.1%). At 3 wk of age, the pullets 2 Aviagen Inc., 5015 Bradford Drive, Huntsville, AL 35805. were then fed a grower diet (ME: 2,711 kcal/kg; CP: 14.6%) and fed according to a skip-two-day feeding schedule. The amount of feed to be allocated was calculated by multiplying the daily allotment by 7 d to get the total amount of feed allocated over 1 wk. This amount of feed was then equally divided and fed for 5 d. For a more detailed description of the skip-two-day feeding program, refer to Joseph et al. (2000). Each week when BW was determined, feed allocation was recalculated accordingly. Feed ingredients and nutrient composition of the starter and grower diets have been described elsewhere (Joseph, 2000). Individual BW data were taken during rearing at 28- d intervals during the rearing period, beginning at 4 wk of age. On weeks when individual BW were not taken, a group weight was taken to determine a mean weekly BW for each pen. Feed allocation in each pen was based on the weekly BW as it compared to the FSY body weight curve (Anonymous, 1998) to reduce strain variability in BW at the time the trial commenced. Experimental Design Experimental Fate. At 18 wk of age, 96 pullets from each strain were selected for the laying trial. The birds were selected based on how close BW matched the target BW for 18 wk of age (1.820 kg) as recommended by the Arbor Acres Broiler Breeder Management Manual (Arbor Acres Farms, 1998). The pullets were randomly placed into individual laying cages in a light-tight facility. The cages were equipped with individual feeders. Dietary treatments also commenced at this time. The diets contained 16 or 18% CP (Table 1: prebreeder diets), with the 16% CP diet serving as the control. Whereas, the diets were formulated to be isocaloric, the level of supplemental amino acids was proportionate to the amount of CP in the diet. Feed allocation was identical for both treatments and was based on the mean weekly BW of the entire flock (Table 2). The prebreeder diets were fed daily from 18 to 23 wk of age. At 24 wk of age, the birds continued to receive 16 or 18% dietary CP; however, the level of calcium was increased in their diets (Table 1: breeder diets). From 32 wk of age to the termination of the trial, all birds were fed the 16% CP breeder diet. Before the pullets were placed in the laying house, the house was divided into two, using a curtain made of black polyethylene plastic, as described previously by Robinson et al. (1998). Light-tight ducting enabled air circulation between the two sides of the room. This technique ensured that despite the division of the room, the environment for all of the birds was very similar. At 20 wk of age, one-half of the birds were photostimulated by increasing the day length from 8L:16D to 15L:9D. The other half remained on 8L:16D until 23 wk of age when they received the same increase in photoperiod. At this time, the black curtain was removed. Sexual Maturity Data. Age at sexual maturity was determined for each bird and was defined as age at first
PHOTOSTIMULATION AGE AND CRUDE PROTEIN INTAKE IN BREEDER HENS 599 TABLE 1. Composition and nutrient intake of the prebreeder and breeder diets 1 Prebreeder diets (%) Breeder diets (%) Ingredient 16% CP 18% CP 16% CP 18% CP Ground corn 36.83 34.42 41.47 40.21 Wheat 25.50 24.50 25.29 22.38 Soybean meal (44% CP) 15.85 19.04 18.78 19.29 Oats 12.67 10.68...... Canola oil...... 1.63 1.67 Canola meal 2.32 2.50...... Corn gluten meal 0.41 2.16 1.50 5.11 Ground limestone 2.64 2.63 7.93 7.93 Dicalcium phosphate 1.79 1.76 1.80 1.79 Choline chloride premix 2 0.50 0.50 0.50 0.50 Layer premix 3 0.50 0.50 0.50 0.50 Salt 0.39 0.39 0.41 0.41 D,L-Methionine 0.10 0.08 0.09 0.07 L-Lysine......... 0.06 Generic enzyme 0.10 0.10 0.10 0.10 Nutrient composition CP (analyzed), % 16.2 17.4 16.4 17.9 ME (calculated), kcal per kg 2,830 2,845 2,830 2,845 Calcium (analyzed), % 1.69 1.47 4.31 4.30 Available phosphorus (calculated), % 0.45 0.45 0.45 0.45 Methionine (calculated), % 0.36 0.38 0.36 0.38 Lysine (calculated), % 0.75 0.84 0.75 0.83 1 The prebreeder diets were fed from 19 to 23 wk of age; the breeder diets were fed from 24 to 32 wk of age. 2 Choline chloride premix supplied at 100 mg/kg of diet. 3 Layer premix supplied the following per kilogram of diet: vitamin A, 12,000 IU; vitamin D 3, 3,000 IU; vitamin E, 40 IU; vitamin K, 2.0 mg; pantothenic acid, 14.0 mg; riboflavin 6.5 mg; folacin, 1.0 mg; niacin, 40 mg; thiamine, 3.3 mg; pyridoxine, 6.0 mg; vitamin B 12, 0.02 mg; biotin, 0.2 mg; iodine, 0.5 mg; Mn, 75.0 mg; Cu, 15.0 mg; Zn, 80.0 mg; Se, 0.1 mg; Fe, 100.0 mg. oviposition. Body weight, shank length, keel length, and chest width at sexual maturity was also recorded. Shank length was determined, using a vernier caliper, by measuring the length of the tibiotarsus (from the top of the hock joint to the footpad), and keel length was determined by measuring the distance from the hypocleidoclavical joint to the caudal end of the sternum. A chest width measurement was taken using vernier calipers placed under the birds wings, 2.5-cm below the clavicle bones at approximately the widest point on the chest. Egg Weight and Production. All eggs were collected daily and weighed. A code was assigned to each egg based on its weight, shell quality, and the number of yolks. A defective egg was categorized as an egg with any one of the following: more than one yolk, a soft shell, a membranous shell, or an abnormal shell. Eggs that were broken by the handler or pecked by the bird were considered normal. A settable egg was defined as an egg with an intact shell, a single yolk, and that weighed over 52 g. The assignment of a code allowed for the calculation of weekly hen-day egg production, total egg number, and settable egg number. Prime sequence length, sequence number, mean sequence length, mean weekly sequence length, and intersequence pause length were determined using a software program, the Egg Production and Sequence Analyzer. 3 Mean se- 3 Version 3.00. Copyright 1999. Alberta Agriculture, Food and Rural Development, Edmonton, AB, T6H 5T6, Canada. quence length was the mean length of all sequences for the laying period. Mean weekly sequence length was calculated by assigning each day a sequence length value based on the length of sequence the bird was in at that time and then summarizing these daily values for each week. The weekly values were then averaged for the laying period to get the mean weekly sequence length. Egg weight and egg production were calculated for the 23-to-31-wk period, when the CP diets differed, for the 32-to-48-wk period, when all birds were on the 16% CP diet, and for the overall 23-to-48-wk production period. The egg characteristics of the first three normal eggs laid by each hen were determined. Thereafter, at 32, 36, 40, and 44 wk of age, two normal eggs from each hen were assessed. Egg yolk, albumen, and shell weights were determined on an absolute and percentage basis. Egg specific gravity was determined by the flotation method on the day following oviposition to limit the effects of variable storage times. Prior to measurement, eggshells were rinsed with tap water, air-dried at room temperature for 4 d, and then weighed. A decrease in egg weight between the time the egg was laid and the time it was opened was assumed to be due to decreased albumen weight. Therefore, albumen weight was determined by subtracting yolk and shell weights from the weight of the egg when it was laid. Carcass Parameters. Prior to processing at 48 wk of age, shank length, keel length, and chest width were measured on each bird as described above as an assess-
600 JOSEPH ET AL. TABLE 2. Total feed allocation for female broiler breeders and protein intake for breeders fed 16 or 18% dietary protein Age (wk) Variable Treatment 18 19 20 21 22 23 24 25 26 27 28 29 30 31 Feed allocation Total All birds 87 92 98 102 103 110 119 131 143 150 159 163 163 161 16% CP 14.1 14.9 15.9 16.5 16.7 17.8 19.5 21.5 23.5 24.6 26.1 26.7 26.7 26.4 18% CP 15.1 16.0 17.1 17.7 17.9 19.1 21.3 23.4 25.6 26.9 28.5 29.2 29.2 28.8 1 Dietary protein intake was calculated based on the CP analysis (Table 1). (g/d) ment of frame size. Birds were euthanatized by cervical dislocation and weighed individually. The pectoralis major and pectoralis minor, liver, abdominal fat pad, ovary, and oviduct were removed from the carcass and individually weighed. Total breast muscle weight was calculated by adding the pectoralis major weight to the pectoralis minor weight. Contents of the oviduct were removed before oviduct weight was determined. A follicle in the ovary that was greater than 10 mm in diameter was classified as a large yellow follicle (LYF). Follicles in this category establish the ovarian hierarchy, as they are all in the process of rapid follicular growth and maturation (Gilbert and Wells, 1984). Each LYF was individually weighed. The incidence of atretic LYF, characterized by a discolored or shrunken appearance, was also noted (Gilbert et al., 1983). A stroma weight was determined by weighing the ovary after the LYF and atretic LYF had been removed. Statistical Analyses The experiment was set up as a 3 2 2 factorial design with three strains, two ages at photostimulation, and two dietary CP treatments as the main effects. Pullets within a strain were assigned to a cage and a treatment in a completely randomized design. The data were analyzed by three-way ANOVA with the general linear models procedure of SAS software (SAS Institute, 1999)., photostimulation treatment, dietary treatment, and their interactions were tested against the experimental unit cage (strain age at photostimulation CP level). The significance of differences between means was determined with the PDIFF option of the LSMEANS statement of SAS software (SAS Institute, 1999). Pearson correlation coefficients (Steel et al., 1997) were determined for sequence and egg production parameters using pooled data. The error variation for strain, age at photostimulation, and dietary CP level was the variation between birds. Five birds that had ceased laying for 5 wk or more were removed from the trial. Twelve birds were also removed from the flock because of leg problems or other health issues. For reporting in tables, the SEM shown was the one for the group with the least number of birds. Unless otherwise stated, all statements of significance are based on testing at P 0.05. RESULTS AND DISCUSSION Sexual Maturity The Classic strain matured earlier than the strain selected for the most meat yield, EXP. After photostimulation, the EXP birds achieved sexual maturity 4 d after the Classic birds, on average (Table 3). Sexual maturation for the FSY strain was intermediate between the other two strains. A previous study using these strains found no strain difference in age at sexual maturity (Joseph, 2000). However, the sample size in the previous study was smaller than that of the current study. The delay in age at sexual maturity for the EXP birds was accompanied by a 94-g increase in BW, a 3.9 mm longer keel, a 7.1 mm greater chest width, and a 2.7 g larger initial egg weight as compared to the birds of the Classic strain (Table 3). In the previous study, it was observed that EXP pullets also had a greater chest width than their counterparts prior to photostimulation (19 wk of age) (Joseph, 2000). The age at which the birds were photostimulated also affected age at first oviposition. Sexual maturity for the 20-wk and 23-wk treatment occurred at 180.5 and 186.1 d of age, respectively. This finding is consistent with the literature (Yuan et al., 1994; Robinson et al., 1996; Joseph, 2000). One advantage for delaying photostimulation is that the birds become more uniform in terms of reproductive maturation (Robinson et al., 1996). That is, birds that are overweight and ready for egg production will likely wait while those that are underweight complete growth or catch-up in development. Plasma estradiol-17β concentration increased more rapidly when photostimulation was delayed from 19 wk of age to 21 wk of age (R. A. Renema, unpublished data). Therefore, once photostimulation does occur, the birds respond quickly as they are all at a similar stage of development. The results of the current study support this argument because the pullets that were photostimulated later required only 25.1 d (postlighting) on average, to achieve sexual maturity, whereas the early-photostimulated group took 40.5 d (Table 3). The birds in the 23- wk treatment had a larger frame size at first egg, as evidenced by a 139 g increase in BW, a 2.2 mm longer keel length, and a 1.7 mm wider chest measurement. The increase in BW and frame size in response to delaying photostimulation agrees with other studies (Yuan et al.,
PHOTOSTIMULATION AGE AND CRUDE PROTEIN INTAKE IN BREEDER HENS 601 TABLE 3. Sexual maturity parameters for three strains of female broiler breeders photostimulated at 20 or 23 wk of age and fed 16 or 18% CP diets Age at Shank Keel Chest Egg SM 1 Days from BW length length width weight 3 Source (d) PS 2 to SM 1 (kg) (mm) (mm) (mm) (g) Classic 181.2 b 30.7 b 2.994 b 105.4 154.2 b 66.0 c 45.1 b FSY 4 183.6 ab 33.1 ab 3.026 b 105.9 158.7 a 69.6 b 46.9 ab EXP 5 185.2 a 34.7 a 3.088 a 106.3 158.1 a 73.1 a 47.8 a SEM 1.0 1.0 0.029 0.4 0.6 0.6 0.5 20 wk 180.5 b 40.5 a 2.966 b 106.0 155.9 b 68.7 b 45.4 b 23 wk 186.1 a 25.1 b 3.105 a 105.7 158.1 a 70.4 a 47.8 a SEM 0.8 0.8 0.023 0.3 0.5 0.5 0.4 16% CP 183.2 32.7 3.001 b 105.5 156.7 69.7 46.6 18% CP 183.5 33.0 3.070 a 106.2 157.3 69.4 46.6 SEM 0.8 0.8 0.023 0.3 0.5 0.5 0.4 a c Means within a column and source with no common superscript differ significantly (P 0.05). 1 SM = sexual maturity; defined as first oviposition. 2 PS = photostimulation. 3 Egg weight = of the first three eggs laid, only those with a single yolk and a hard, intact shell were used to calculate egg weight. 4 FSY = Arbor Acres Feather Sexable Yield. 5 EXP = Arbor Acres experimental line selected for high breast muscle yield. 1994; Robinson et al., 1996; Joseph, 2000). Despite the significant differences in age at sexual maturity for strain and age at photostimulation, no interactions were observed. intake during the prelay period did not advance or delay the age at sexual maturity. However, birds fed 18% CP were heavier at sexual maturity than birds fed 16% CP (Table 3), demonstrating that increased dietary protein (in an isocaloric diet) can increase BW. The increase in BW had no lasting effect on frame size or initial egg weight. The interaction of age at photostimulation and CP intake showed that when birds were fed 16% CP, those that were photostimulated at 20 wk of age produced smaller eggs (44.7 g) at sexual maturity than those photostimulated at 23 wk of age (48.4 g). Whereas, in terms of feeding 18% CP, there was no difference in egg weight between hens photostimulated at 20 or 23 wk of age (46.1 and 47.1 g, respectively). Feeding more dietary protein has been shown to increase the albumen weight of the egg (Fisher, 1969; Keshavarz and Nakajima, 1995; Joseph et al., 2000) and to increase the total nitrogen content in the albumen (Butts and Cunningham, 1972). Feeding 18% dietary CP increased the amount of albumen in the eggs so that the earlyphotostimulated hens were able to produce the samesized eggs as the later photostimulated hens. Body Weight and Egg Weight Two strains differed in initial egg weight (Table 3), which was most likely a function of BW as the EXP hens were heavier than the Classic hens and hens that are heavier tend to lay larger eggs (McDaniel et al., 1981). During the early lay period of 23 to 31 wk of age, the EXP hens weighed more than the Classic hens (3.024 and 2.974 kg, respectively). In addition, the Classic hens continued to produce smaller eggs than the FSY and EXP hens (Table 4). This trend continued in the late-lay period of 32 to 48 wk of age (Table 4). During this period, Classic hens remained lighter than the FSY and EXP hens (3.607, 3.670, and 3.703 kg, respectively). As egg weight was significantly greater for FSY and EXP hens (Table 4), strain differences in egg characteristics were observed (Table 5). The EXP hens had proportionately more albumen (65.6%) than Classic and FSY hens (60.8 and 61.7%, respectively). However, Classic hens had proportionately more yolk and shell than either FSY or EXP hens. The higher shell weight corresponded to a higher specific gravity value for the Classic eggs. Like strain, age at photostimulation affected egg weight at sexual maturity and for the duration of the production cycle (Table 4). Joseph (2000) found that photostimulation at 23 versus 20 wk of age resulted in a higher mean egg weight throughout the entire laying cycle, an effect that was attributed mainly to differences in BW. In the current study, the hens photostimulated at 23 wk of age were heavier at sexual maturity but BW was only different in the early lay period and not the late lay period. BW during the early-lay period was 2.984 and 3.021 kg, for the 20 wk and 23 wk treatments, respectively. Hens photostimulated at 23 wk had larger eggs than those photostimulated at 20 wk of age. Other studies have observed a difference in BW in response to delayed photostimulation with no subsequent differences in egg size (Yuan et al., 1994; Robinson et al., 1996). The eggs that were opened to determine egg characteristics were of a similar weight, therefore, effects of age at photostimulation on egg characteristics were not observed (Table 5).
602 JOSEPH ET AL. TABLE 4. Mean egg weight of Classic, Feather Sexable Yield (FSY) and Experimental line (EXP) birds photostimulated at 20 or 23 wk of age and fed 16 or 18% dietary CP Egg weight 1 (g) Early lay period Late lay period Total lay period Source (23 to 31 wk of age) (32 to 48 wk of age) (23 to 48 wk of age) Classic 54.0 b 61.6 b 59.4 b FSY 55.3 a 62.7 a 60.6 a EXP 2 55.3 a 62.7 a 60.7 a SEM 0.4 0.4 0.4 20 wk 54.2 b 62.0 b 59.7 b 23 wk 55.5 a 62.7 a 60.8 a SEM 0.3 0.3 0.3 16% 54.9 62.4 60.3 18% 54.8 62.2 60.1 SEM 0.3 0.3 0.3 a,b Means within a column and within a source with no common superscript differ significantly (P 0.05). 1 Egg weight = only eggs with a single yolk and a hard, intact shell were used to determine mean egg weight. 2 Arbor Acres experimental line selected for high breast muscle yield. At the onset of the trial there was no difference in BW among the birds assigned to the 16 and 18% CP treatments (1.702 and 1.701 kg, respectively). Within 1 wk of initiating the feeding regimens, the hens fed 18% CP weighed more (2.049 kg) than those fed 16% CP (2.014 kg). Although all of the birds were changed to the same CP diet at 32 wk of age, BW continued to differ for the remainder of the trial (mean BW overall: 3.613 and 3.221 kg, respectively). Despite the effect of 18% CP intake on BW, dietary protein had no effect on mean egg weight during any part of the production cycle (Table 4). BW is not normally influenced by CP intake (Keshavarz and Nakajima, 1995; Cave, 1984; Lilburn and Myers- Miller, 1990; Joseph et al., 2000). However, Harms and Russell (1995) observed that the BW gain of older hens increased in response to increased CP in the diet. It is possible then, that CP intake can affect the BW of broiler breeder hens. The dietary CP treatments had no effect on albumen, yolk, and shell weight (Table 5). Egg Production and Sequence Analysis There were significant differences between strains in the mean rate of weekly egg production during the early lay period (Table 6). Classic hens had the highest rate (64.8%), followed by FSY (60.0%) and then EXP hens (55.4%). Egg production for the Classic hens during the late laying period continued to remain higher than the other strains, with no difference in egg production between FSY and EXP hens. By 48 wk of age, the Classic hens produced 9.1 more settable eggs per hen than the EXP strain and 3.3 more settable eggs per hen than the FSY strain. Joseph (2000) found no strain differences in hen-day egg production, with the exception of the late TABLE 5. Egg weight, albumen weight, yolk weight, shell weight, and specific gravity of eggs 1 from broiler breeders Egg Albumen Yolk Shell weight Weight Percentage Weight Percentage Weight Percentagae Specific Source (g) (g) (%) (g) (%) (g) (%) gravity Classic 61.4 b 37.3 b 60.8 b 18.7 a 30.6 a 5.39 8.80 a 1.0772 a FSY 2 62.3 ab 38.3 b 61.7 b 18.7 a 30.1 b 5.27 8.49 b 1.0751 b EXP 3 64.1 a 40.7 a 65.6 a 18.3 b 29.4 c 5.25 8.42 b 1.0750 SEM 0.7 0.7 1.2 0.1 0.2 0.05 0.07 0.0005 20 wk 62.8 39.0 63.6 18.5 30.1 5.29 8.61 1.0757 23 wk 62.5 38.5 61.8 18.6 29.9 5.32 8.53 1.0759 0.6 0.6 0.9 0.1 0.1 0.04 0.05 0.0004 16% 62.6 38.7 62.6 18.6 30.0 5.28 8.54 1.0757 18% 62.7 38.8 62.8 18.6 30.0 5.32 8.60 1.0759 SEM 0.6 0.6 1.0 0.1 0.1 0.04 0.05 0.0004 a c Means within a column and source with no common superscript differ significantly (P 0.05). 1 Eggs with a single yolk and a hard, intact shell were opened to determine the weight of the egg components. 2 FSY = Arbor Acres Feather Sexable Yield. 3 EXP = Arbor Acres experimental line selected for high breast muscle yield.
PHOTOSTIMULATION AGE AND CRUDE PROTEIN INTAKE IN BREEDER HENS 603 TABLE 6. Weekly hen-day egg production during the early and late laying periods, cumulative total egg number, and settable egg number of hens photostimulated at 20 or 23 wk of age and fed either 16 or 18% dietary CP Hen-day egg production 1 (%) Egg parameters (23 to 48 wk of age) Early lay period Late lay period Overall Total egg Settable egg Parameter (23 to 31 wk of age) (32 to 48 wk of age) (23 to 48 wk of age) number 2 number 3 Classic 64.8 a 81.3 a 75.5 a 137.5 a 119.7 a FSY 4 60.0 b 77.6 b 71.5 b 130.1 b 116.4 b EXP 5 55.4 b 76.4 b 69.1 b 125.6 b 110.6 b SEM 1.5 1.0 0.9 1.6 1.7 20 wk 63.8 77.6 72.8 132.6 115.0 23 wk 56.3 79.2 71.2 129.7 116.1 SEM 1.2 0.8 0.7 1.3 1.4 16% 60.6 79.0 72.6 132.2 116.9 18% 59.5 77.9 71.5 130.1 114.2 SEM 1.2 0.8 0.7 1.3 1.4 a,b Means within a column and within a source with no common superscript differ significantly (P 0.05). 1 Hen-day egg production: all eggs, including defective eggs, were used in the calculation of hen day egg production. 2 Total egg number = calculated by counting the number of eggs laid per hen (included both defective and normal eggs). 3 Settable egg number = calculated by counting the number of eggs with a single yolk and a hard intact shell, weighing over 52.0 g. Eggs with a hard shell but broken were not included in the analysis. 4 FSY = Arbor Acres Feather Sexable Yield. 5 EXP = Arbor Acres experimental line selected for high breast muscle yield. laying period in which the Classic and FSY hens had a higher rate of egg production than the EXP hens, but strain had no effect on the number of settable eggs produced. The Classic strain was expected to have a higher rate of egg production than the FSY strain, but to lay smaller eggs, as was found in the present study. Shortly after peak egg production, the EXP hens began to gain more weight than the Classic hens, which may have contributed to their lower rate of egg production during the postpeak period. Robinson et al. (1990) found that hens with the largest BW gains had the poorest rate of egg production. Prime sequence length was significantly affected by strain, whereas age at photostimulation and dietary CP had no effect (Table 7). Classic hens had the longest prime sequence length of 30.7 d compared to 23.8 d for the EXP hens. The mean prime sequence length of FSY hens was intermediate between the other two strains (26.0 d). Average and weekly sequence lengths were longer for Classic hens than FSY and EXP hens. The number of days between sequences, known as intersequence pause length, was greatest for EXP hens, followed by FSY, and lowest for Classic hens. For this trial, the correlation coefficient of prime sequence length and total egg production was + 0.58, higher than the value that has been previously reported (+ 0.40) (Robinson et al., 1990). This relationship indicates that hens laying long prime sequences have a higher persistency of lay and are likely to produce more eggs. Intersequence pause length was negatively correlated with total egg production ( 0.51). This result, intuitively, is logical because hens with longer pauses between sequences have fewer days within the production cycle to lay their eggs. The strain differences in prime sequence length and intersequence pause length can be explained by their differences in growth potential. The three strains were reared according to the FSY BW curve (Arbor Acres Farms, 1998). Therefore, the Classic pullets were reared on a heavier curve than may be appropriate in commercial practice. The EXP line was a newly created strain and did not have a recommended BW curve. Presumably, the EXP pullets were reared on a lighter curve than what was optimal for this strain because it was genetically selected for more white meat yield than the FSY. It is possible, that at the time of photostimulation, the EXP pullets were struggling between attaining an appropriate body composition and sexual development. If this was true, then they were physiologically unprepared for egg production, which would explain why the EXP hens had shorter prime sequence lengths and longer pause lengths. The same observations were been made previously, in which Classic hens had a longer prime sequence and a shorter intersequence pause length than the EXP hens, lending support to this theory (Joseph, 2000). The rate of hen-day egg production was not affected by age at photostimulation during the laying cycle (Table 6). Not surprisingly, there were no differences in settable egg numbers. Therefore, early photostimulation advanced the age at sexual maturity with no subsequent effect on the number of eggs produced, as described previously (Yuan et al., 1994; Robinson et al., 1996; R. A. Renema, unpublished data). Sequence parameters were unaffected by age at photostimulation (Table 7). The beneficial effect of delaying photostimulation was that the hens were able to produce as many eggs as the earlyphotostimulated hens.
604 JOSEPH ET AL. TABLE 7. Sequence parameters measured on three strains of hens, photostimulated at either 20 or 23 wk of age, and fed either 16 or 18% dietary CP Prime Mean Mean weekly Average sequence Number sequence sequence pause length 1 of length length length Source (d) sequences (d) (d) (d) Classic 30.7 a 25.6 6.43 a 12.30 1.12 c FSY 2 26.0 ab 27.7 5.28 b 8.93 1.22 b EXP 3 23.8 b 28.0 5.21 b 8.31 1.32 a SEM 2.0 0.9 0.36 1.10 0.03 20 wk 26.0 27.9 5.46 9.51 1.22 23 wk 27.7 26.3 5.81 10.18 1.22 SEM 1.6 0.7 0.29 0.87 0.02 16% 28.0 26.8 5.79 10.47 1.19 18% 25.7 27.4 5.49 9.23 1.25 SEM 1.6 0.8 0.29 0.88 0.02 a c Means within a column and within a source with no common superscript differ significantly (P 0.05). 1 Prime sequence length: longest sequence of eggs laid, typically laid early in lay; therefore only strain data are presented. 2 FSY = Arbor Acres Feather Sexable Yield. 3 EXP = Arbor Acres experimental line selected for high breast muscle yield. did not affect egg production during the early lay period (Table 6). Likewise, total and settable egg numbers were similar among groups. In a study comparing 14, 16, and 18% CP diets there were no differences in egg production in the first 5-wk of lay (Joseph et al., 2000). There was a response at 29 wk of age when hens fed 14% CP had a lower rate of lay compared to hens fed 16 or 18% CP (Joseph, 2000). In the current study, hens that were photostimulated at 20 wk of age had the same number of eggs with two yolks, regardless of CP intake (0.56 and 0.89 double-yolked eggs/hen, for 16 and 18% CP, respectively). However, for hens photostimulated at 23 wk of age, feeding 18% CP resulted in significantly fewer double-yolked eggs compared to feeding 16% CP (0.81 and 0.38 double-yolked eggs/hen, respectively). Intersequence pause length was influenced by strain and the level of dietary CP. EXP hens fed 18% CP had the longest intersequence pause length (1.41 d), whereas Classic hens fed 18% CP had the shortest pause length (1.11 d). The higher level of CP had an additive effect on the pause length of EXP hens. Carcass Characteristics at 48 Wk of Age FSY and EXP hens weighed more and had a longer keel at processing (48 wk of age) than the Classic hens (Table 8). Chest width measurement differed among strains. Chest width was highest in the EXP hens and lowest in the Classic hens. Relative breast weight was greater in the EXP hens than the FSY or Classic hens, reflecting differences in pectoralis major weight. Body weight at processing was not affected by age at photostimulation but keel length was. At sexual maturity the delay in photostimulation resulted in an increase in keel length and chest width. By the end of the production cycle, however, this trend was reversed. The keel length and chest width were higher in the 20-wk treatment than in the 23-wk treatment. Note that hens photostimulated later produced the same number of eggs as those photostimulated at 20 wk of age. Also, broiler breeders with a high rate of lay tend to gain less weight and can even lose weight at high production points in the laying cycle (Robinson et al., 1995), which would explain the decrease in keel length and chest width in the later photostimulated birds. BW was affected by CP intake. Birds fed 18% CP weighed 3.835 kg compared to a BW of 3.766 kg in the 16% CP treatment. Despite this BW difference, there were no other dietary CP effects on any carcass parameters measured. It is likely that there was increased tissue or muscle mass but it was in areas of the carcass that were not measured. Joseph et al. (2000) found no difference in the amount of carcass protein in hens fed 16 versus 18% CP in the early lay period. However, in that trial, there was also no difference in BW (Joseph et al., 2000). Differences in reproductive organ weights were minimal (Table 9). The FSY and EXP hens had a higher oviduct weight than Classic hens, mainly because they were heavier in BW (as there were no proportionate differences). Ovary weight and the number of LYF were not influenced by strain, age at photostimulation, or CP intake. Differences in age at photostimulation are likely to have a greater effect during the early lay period rather than at the end of lay. However, it has been shown that birds photostimulated at 23 wk of age had greater ovary weights and more LYF in the ovary late in production (54 wk of age) than birds photostimulated at 20 wk of age (Joseph, 2000), which explained why the later photostimulated hens were able to produce as many eggs as the early-photostimulated hens. As the same
PHOTOSTIMULATION AGE AND CRUDE PROTEIN INTAKE IN BREEDER HENS 605 TABLE 8. BW, shank length, keel length, chest width, and breast muscle parameters measured at 48 wk of age on broiler breeder hens Pectoralis Pectoralis BW at Shank Keel Chest Breast major minor processing length length width 1 weight 2 weight weight Source n (kg) (mm) (mm) (%) (%) (%) (%) Classic 92 3.734 b 105.3 155.1 b 60.3 c 15.30 c 11.62 c 3.68 FSY 3 93 3.810 a 105.7 161.9 a 64.2 b 16.11 b 12.29 b 3.82 EXP 4 86 3.857 a 106.1 161.5 a 66.0 a 17.03 a 13.09 a 3.93 SEM 0.026 0.4 0.7 0.6 0.16 0.12 0.09 20 wk 134 3.801 106.0 160.4 a 65.4 a 16.23 12.34 3.89 23 wk 137 3.800 105.4 158.7 b 61.6 b 16.06 12.33 3.73 SEM 0.021 0.3 0.6 0.5 0.13 0.10 0.71 16% CP 138 3.766 b 105.5 159.1 63.3 16.14 12.34 3.79 18% CP 133 3.835 a 105.8 159.9 63.7 16.16 12.33 3.83 SEM 0.021 0.3 0.6 0.5 0.13 0.10 0.07 a c Means within a column and within a source with no common superscript differ significantly (P 0.05). 1 Chest width = measurement taken across the chest of the bird, 2.5-cm below the clavicle bones. 2 Breast weight = pectoralis major and pectoralis minor weight. 3 FSY = Arbor Acres Feather Sexable Yield. 4 EXP = Arbor Acres experimental line selected for increased breast muscle yield. observations were not made in the present study, it is difficult to conclude that this relationship between egg production and ovarian morphology exists for hens photostimulated at older ages. It is possible that there were differences in ovarian parameters during other parts of the laying cycle but without processing birds at various ages, it is difficult to know for certain. When characteristics such as appetite, growth rate, and meat yield are favored as selection criteria for a breeding program, it is usually at the cost of reproductive efficiency (Siegel and Dunnington, 1985). The strain differences observed in this study are demonstrative of this relationship between growth and reproduction. EXP hens, selected for the purposes of providing white-meat yield at the lowest possible cost, took 4 d more to achieve sexual maturity than the Classic hens. The EXP hens had a larger frame size at sexual maturity, which affected egg weight because they produced larger eggs than the Classics. The FSY hens were intermediate between the other two strains in terms of age and egg size at sexual maturity, but overall they too were heavier and produced a higher egg weight than the Classic hens. Classic hens had a 6.9 d longer prime sequence length than the EXP hens. The number of days between sequences, also TABLE 9. Fat pad weight, liver weight, ovary weight, oviduct weight, stroma weight, large yellow follicle (LYF) number, and LYF atresia measured at 48 wk of age on broiler breeder hens Abdominal fat pad Liver Ovary Oviduct Stroma Number of weight weight weight weight weight Number of atretic LYF 2 Source n (%) (%) (g) (g) (g) LYF 1 (%) Classic 92 4.75 1.44 b 56.2 65.6 b 9.6 5.23 1.71 FSY 3 93 4.70 1.51 a 53.5 70.1 a 9.7 4.99 1.53 EXP 4 86 4.46 1.53 a 55.3 69.1 a 9.3 5.29 1.17 SEM 0.13 0.02 1.5 1.3 0.3 0.1 0.4 20 wk 134 4.68 1.52 56.0 67.8 9.4 5.11 1.60 23 wk 137 4.60 1.47 54.1 68.8 9.6 5.23 1.34 SEM 0.11 0.02 1.2 1.0 0.2 0.08 0.36 16% CP 138 4.67 1.52 55.1 67.3 9.5 5.22 1.39 18% CP 133 4.60 1.47 54.9 69.3 9.6 5.13 1.54 SEM 0.11 0.02 1.2 1.0 0.2 0.08 0.35 a,b Means within a column and within a source with no common superscript differ significantly (P 0.05). 1 LYF = large yellow follicles greater than 10 mm in diameter. 2 Atretic LYF = atretic large yellow follicles; follicles greater than 10 mm in diameter, characterized by a shrunken or discolored appearance. 3 FSY = Arbor Acres Feather Sexable Yield. 4 EXP = Arbor Acres experimental line selected for increased breast muscle yield.
606 JOSEPH ET AL. known as intersequence pause length, was greatest for EXP hens, followed by FSY hens, and then Classic hens. These differences may have influenced egg production because the Classic strain had the highest rate of egg production, producing an additional 9.1 settable eggs per hen than the EXP strain and 3.3 more settable eggs per hen than the FSY strain. Certainly selection for greater meat yield led to a strain (EXP) that was heavier in BW and pectoralis major weight than strains with less selection pressure for meat-yield (Classic or FSY). However, reproductive performance suffered as the EXP hens had the lowest settable egg production. In addition, delaying photostimulation or increasing the amount of CP in the diet was unable to increase egg production in this strain. Delaying photostimulation by 3-wk delayed sexual maturity but hens in both age at photostimulation treatments commenced lay within 5.6 d of each other, on average. The hens on the 20-wk treatment were lighter in BW and were not adequately prepared for egg production as the length of time needed to achieve sexual maturity was significantly increased by 15.4 d. Overall, the delay in photostimulation increased egg weight, even though there was no difference in settable egg number. In addition, both treatments had an equivalent rate of egg production, thus demonstrating that there was no real advantage to photostimulating at 20 wk of age. Based on observations from this study, it is clear that the best combination of age at photostimulation and CP intake may be strain dependent. Overall, most strains will perform well if photostimulation occurs at 23 wk of age compared to 20 wk of age. For strains that characteristically have less fleshing and better reproductive performance, a prebreeder diet with 18% CP is recommended. At this level, both Classic and FSY strains had increased initial egg weight and reduced incidence of double-yolked eggs. 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