PRODUCTION, MODELING, AND EDUCATION

PRODUCTION, MODELING, AND EDUCATION Hatching egg characteristics, chick quality, and broiler performance at 2 breeder flock ages and from 3 egg weights 1 A. M. Ulmer-Franco,* G. M. Fasenko, 2 and E. E. O Dea Christopher * * Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada, T6G2P5; and Department of Animal and Range Sciences, New Mexico State University, Las Cruces 88003-8003 ABSTRACT The objective of this study was to determine the effects of flock age and egg weight on hatching egg characteristics, fertility, hatchability, salable chick production, and broiler performance using a commercial Cobb 500 broiler breeder flock. Hatching eggs from the same breeder flock in 3 weight categories (light, average, and heavy) were obtained from a commercial hatchery when the birds were 29 and 59 wk of age. One group of eggs per age and weight category was selected to assess specific gravity and was broken open to weigh egg components. Another group of eggs was incubated for 21.5 d and incubation parameters were measured. At hatching, all salable chicks were individually weighed and placed in floor pens, where they were grown out for 41 d. Daily mortality, weekly feed consumption, and individual BW at 21 and 41 d were recorded. Irrespective of flock age and egg weight, all eggs had a specific gravity lower than 1.080, the commercial set value. Eggs from the young flock age had a smaller proportion of yolk and a greater proportion of albumen. Age affected fertility, with a lower value observed at 29 wk of age. Chicks from the flock at 59 wk hatched earlier than chicks from the flock at 29 wk, and light eggs hatched earlier than both average and heavy eggs. Broilers from the 29-wk-old breeders had a lower final BW than broilers from the 59-wk-old breeders. The smaller proportion of yolk in eggs from 29-wk-old broiler breeders may be associated with the low final BW observed in their offspring. This could mean that chicks hatching from young broiler breeders, which produce eggs with small yolks, may be at a disadvantage when reared under the same conditions as chicks produced by older broiler breeders whose eggs have larger yolks. Key words: broiler breeder, flock age, egg weight, chick quality, broiler performance 2010 Poultry Science 89 :2735 2742 doi: 10.3382/ps.2009-00403 INTRODUCTION The study of factors that influence the production of high-quality chicks is of great interest to hatching egg producers in Canada because they are paid based on the number of salable (not hatched) chicks produced. The effects of flock age and egg size (egg weight) on diverse production parameters have been studied (Wilson, 1991). However, in past research, these 2 factors have always been linked together (e.g., egg weight as a consequence of flock age; Wyatt et al., 1985). There is a lack of research evaluating the separate effects of flock age and egg weight on salable chick production. The quality of hatching eggs is imperative because eggs provide both physical protection and nutrition for 2010 Poultry Science Association Inc. Received August 15, 2009. Accepted September 6, 2010. 1 The use of trade names in this publication does not imply endorsement by the University of Alberta, New Mexico State University, or the authors of the products mentioned or criticism of similar products not mentioned. 2 Corresponding author: gfasenko@nmsu.edu the growing embryo. Shell quality (shell thickness and pore number) determines gas exchange and moisture loss during incubation (Wangensteen et al., 1970 1971). Poor shell quality has been associated with a higher percentage of egg moisture loss during incubation (Reis et al., 1997; Peebles et al., 2001) and low hatchability (Narushin and Romanov, 2002). It is known that as hens age, egg weight increases (Roque and Soares, 1994), shell thickness decreases (Peebles et al., 2000), and the proportion of yolk increases at the expense of albumen and eggshell (Suarez et al., 1997). The proportions of components of the hatching egg are also affected by egg size. Small eggs have a greater proportion of yolk than large eggs from the same flock age (Vieira and Moran, 1998). Because the yolk lipids supply more than 90% of the energy required by the developing embryo (Romanoff, 1960), a reduction in the proportion of yolk could be a disadvantage for embryos developing in eggs with small yolks. The quality of the newly hatched chick is a major factor in determining its livability, growth, and health. Since the 1950s, a highly significant correlation between 2735

2736 egg weight and chick weight at hatching has been reported (Wiley, 1950). Even though egg weight at setting determines chick weight at hatching, controversy exists concerning the accuracy of day-old chick weight as a predictor of posthatch performance and market BW. Some authors have found chick weight to be an accurate predictor of final BW (Proudfoot and Hulan, 1981; Sklan et al., 2003), whereas for others, this has not been the case (Gardiner, 1973; Shanawany, 1987). Old breeder flocks produce a greater number of heavier chicks as a result of increased egg weight (Suarez et al., 1997; O Dea et al., 2004). However, the percentage of chicks with low quality scores was reported to be higher in older (45-wk) than in younger (35-wk) flocks (Tona et al., 2004). Poor chick quality, as reflected by a high number of culled chicks, has been associated with heavier than average egg weight for a particular flock age (Kumpula and Fasenko, 2004; Lawrence et al., 2004). The objective of this research was to study the effects of flock age (young vs. old) and egg weight (light, average, heavy) independently of each other and to determine the effect of these factors on hatching egg characteristics, fertility, hatchability, salable chick production, and broiler performance in a commercial Cobb 500 broiler breeder flock. It was hypothesized that 1) young hens would produce the highest number of salable chicks, 2) heavier than average eggs at each flock age would have the highest embryo mortality and the highest percentage of culled chicks, and 3) lighter than average eggs and eggs from the young flock age would produce the smallest chicks, but these chicks would reach the same BW at market age as those hatching from older flock ages and heavier eggs. MATERIALS AND METHODS All experimental procedures were approved by the Faculty of Agricultural, Life and Environmental Sciences Animal Policy and Welfare Committee at the University of Alberta, in accordance with the guidelines set forth by the Canadian Council on Animal Care (1993). Egg Collection Hatching eggs produced by a commercial Cobb 500 broiler breeder flock were obtained from a commercial hatchery, where they had been stored for 3 to 4 d at 18 C and 60 to 70% RH. Eggs from the same flock were obtained at young (29 wk) and old (59 wk) ages (n = 1,368 eggs/age). At each collection time, the average egg weight for that flock age was determined by randomly selecting and weighing 72 eggs. Eggs were weighed and classified in 3 weight ranges based on the average egg weight: light (L), average (A), and heavy (H). Eggs were categorized in the following way: the A eggs were within ±1.5 g from the average egg weight, the L eggs were 3.0 to 6.0 g lighter than the average, Ulmer-Franco et al. and the H eggs were 3.0 to 6.0 g heavier than the average egg. Egg Characteristics For each flock age, 30 eggs were randomly selected for each of the weight categories. Specific gravity (SG) was assessed by the flotation method (Bennett, 1992). Eggs were then weighed and broken open, and wet eggshell and wet yolk weights were recorded. Albumen weight was calculated by subtracting wet yolk and wet shell weights from the total egg weight. The eggshells and yolks were dried at 65 C for 3 d in a Despatch V Series Heat Processing Unit (Despatch Industries Inc., Minneapolis, MN) and the dry weights were determined. Incubation and Hatching At each flock age, settable eggs from each egg weight category were randomly divided into replicate trays of 18 eggs (n = 21 trays/egg weight per age). Doubleyolked eggs, misshapen eggs, eggs with poor shell quality, and dirty eggs are not considered settable in Canada (Fasenko et al., 2003). Each tray was placed, along with 1,134 additional eggs used for a parallel experiment, within a 5,000-egg-capacity Jamesway single-stage setter (Jamesway Incubator Company, Cambridge, Ontario, Canada). The trays were randomly placed to account for possible environmental differences caused by position in the incubator. All eggs were incubated for 18 d at a dry bulb temperature of 37.5 and a wet bulb temperature of 29.4 C. At 7 d of incubation, all eggs were removed from the incubator and candled. Any egg not containing a viable embryo was removed and broken open to assess fertility. If fertile, the day of embryonic death was estimated. At 18 d of incubation, all eggs were removed from the incubator, individually weighed, and transferred to a 5,000-egg-capacity Jamesway hatcher (Jamesway Incubator Company, Cambridge, Ontario, Canada). The eggs were further incubated for an additional 3.5 d at a dry bulb temperature of 35.2 C and a wet bulb temperature of 29.4 C. At each flock age, 1 tray of 18 eggs/egg size category was randomly selected and the eggs were placed in pedigree hatch baskets (dimensions = 8 8 cm) so that hatching time could be assessed in individual eggs and the chick could be traced back to the egg from which it hatched. These eggs were checked for external pipping (beak of the embryo through the shell) and hatching (chick free of shell and the down dry) after 476, 484, 492, 500, 508, and 516 h of incubation. Hatchability and Salable Chick Production After 21.5 d (516 h) of incubation, all hatched chicks were counted and chick quality was visually assessed according to commercial hatchery standards. Chicks that had physical abnormalities, were weak, had unhealed navels, or had red hocks were considered unsalable and

FLOCK AGE, EGG WEIGHT, AND BROILER CHICK QUALITY 2737 were culled. Hatchability was calculated based on salable chicks only. All salable chicks were individually weighed and neck tagged (Mark III Swiftack Tagging Gun, Avery Dennison, Pasadena, CA). All unhatched eggs were broken open to determine the approximate day of embryonic death. Embryonic mortality was grouped into 3 categories: early (1 to 7 d of incubation), mid (8 to 14 d of incubation), and late (15 to 21 d of incubation). Broiler Performance For each flock age, 2 groups of salable chicks per egg weight category were randomly placed in floor pens (n = 119 chicks per pen). Chicks from the same egg weight category were grouped and reared together to prevent competition for feed and resources between chicks of different egg weights. Temperature, humidity, and ventilation were strictly controlled in all pens to be the same. Stocking density was 0.07 m 2 /bird. The broilers were reared on wood shavings for 42 d with a photoperiod of 23 h of light and 1 h of darkness. Birds were fed a crumbled starter diet (23.0% CP and 3,067 kcal of ME/kg) from 1 to 14 d; a crumbled grower diet (20.2% CP and 3,152 kcal of ME/kg) from 15 to 28; and a crumbled finisher diet (19.0% CP and 3,196 kcal of ME/kg) from 29 to 41 d. Feed and water were provided ad libitum throughout the experiment. Daily mortality, weekly feed consumption, and individual BW at 21 and 41 d were recorded. Early, late, and overall feed conversion ratio [FCR, g/g (g of BW gain/g of feed consumed)] were determined. Statistical Analysis The experimental design was a 2 3 factorial arrangement with age (29 and 59 wk of age) and egg weight (L, A, and H) as main effects. The statistical model for the ANOVA was as follows: Y ijk = µ + A i + W j + A i W j + ε ijk, where Y ijk is the characteristic that was measured, µ is the overall mean, A i is the main effect of flock age, W j is the main effect of egg weight, A i W j is the effect of the interaction between flock age and egg weight, and ε ijk is the random error term. The experimental unit differed according to the parameter that was measured. For hatching egg characteristics, the experimental unit was each egg; for incubation parameters, it was each tray of 18 eggs; and for broiler performance, it was each pen of broilers. Body weight was individually measured. All percentage data were subjected to angular transformation to stabilize variances (arcsine square root percentage transformation) before statistical analysis. All data were analyzed using the MIXED model of SAS software (SAS Institute, 2002 2003), with egg tray and broiler pen as random factors. The probability level was set at P 0.05. Where the model indicated significance, the least squares means were separated using the pdiff procedure of SAS. Where the interaction effect was significant, the effects of the main factors were not discussed. RESULTS AND DISCUSSION Hatching Egg Characteristics Flock Age. Eggs laid at 29 wk of age had smaller wet and dry yolk percentages and greater albumen proportion than those laid at 59 wk of age (Table 1). These results agree with the findings of Suarez et al. (1997). It is well known that the egg yolk provides the nutrients for the developing embryo (Romanoff, 1960). Thus, the result that eggs from 29-wk-old hens had a smaller proportion of dry yolk matter (and perhaps less nutrient content as a percentage of the egg) may have had a negative effect on the embryonic development and posthatch performance of these chicks. Egg Weight. Heavy eggs had a greater proportion of albumen than A and L eggs (Table 1). Light eggs had a greater proportion of wet yolk than H eggs, but A eggs did not differ from L or H eggs in the proportions of wet yolk. No differences were found in dry yolk weights and in dry shell weights between egg sizes. These results agree with those of Vieira and Moran (1998), who compared heavy and light eggs from 4 different strains. It is known that the major component of the albumen is water (approximately 88% of the total weight; Ahn et al., 1997); thus, the difference in the proportion of albumen observed between H vs. L and A eggs should be considered when setting eggs for incubation to guarantee optimal moisture loss. The lack of difference in dry yolk weights between egg sizes suggests that, in terms of nutrient content of the yolk, egg size by itself was not a determining factor. Flock Age Egg Weight Interaction. Egg weight was significantly affected by the interaction of flock age and egg size (Table 1). The differences in egg weight were expected to be significant because eggs were purposely selected according to weight. Because a positive correlation between flock age and egg weight has been reported since the 1950s (Wiley, 1950; Tona et al., 2004), it was not surprising to find that egg weight increased with flock age. Shell quality, as estimated using SG, was significantly affected by the interaction of flock age and egg size (Table 1). All treatment groups had a SG lower than 1.080, which is the value that has been reported as the minimum SG indicating good shell quality (Bennett, 1992; Roque and Soares, 1994). The fact that all treatment groups were below this minimum industry standard may indicate that shells from eggs of modern strains are different from shells from eggs of strains from 15+ yr ago, when this threshold value was proposed. These data suggest that values for acceptable SG of eggs from modern strains may need to be reevaluated. Regardless of size, eggs laid by the old hens had greater SG than eggs laid by the young hens. Light eggs from the young hens had the lowest

2738 Ulmer-Franco et al. Table 1. Effects of 2 breeder flock ages, 3 egg weights, and their interaction on hatching egg characteristics in a commercial Cobb 500 broiler breeder flock Item n 1 weight (g) Egg Specific gravity Albumen weight Wet yolk weight Dry yolk weight Dry shell weight 29 wk 90 53.8 b 1.069 b 63.3 a 27.8 b 13.9 b 8.8 59 wk 90 71.3 a 1.075 a 58.5 b 31.3 a 16.7 a 8.6 SEM 0.1 0.001 0.2 0.2 0.2 0.1 Egg weight 2 Light (L) 60 58.3 c 1.071 b 60.5 b 30.0 a 15.6 8.6 Average (A) 60 62.6 b 1.073 a 60.7 b 29.7 ab 15.3 8.8 Heavy (H) 60 66.8 a 1.072 a 61.6 a 29.1 b 15.1 8.7 SEM 0.1 0.001 0.2 0.2 0.2 0.1 Age egg weight 29 wk L 30 49.8 f 1.066 c 63.0 28.5 14.4 8.5 b 29 wk A 30 53.6 e 1.071 b 63.0 27.8 13.8 9.0 a 29 wk H 30 58.0 d 1.070 b 64.0 27.1 13.6 8.8 b 59 wk L 30 66.8 c 1.075 a 58.0 31.5 16.7 8.7 b 59 wk A 30 71.5 b 1.076 a 58.4 31.5 16.9 8.6 b 59 wk H 30 75.6 a 1.075 a 59.2 31.0 16.5 8.5 b SEM 0.1 0.001 0.3 0.3 0.3 0.1 a f Means within a column lacking a common superscript differ significantly (P 0.05). 1 Number of experimental units; each experimental unit = 1 individual egg. 2 L = light (3.0 to 6.0 g lighter than the average egg weight); A = average (±1.5 g from the average egg weight); H = heavy (3.0 to 6.0 g heavier than the average egg weight). SG. It was expected that SG values would be related to shell percentage (i.e., that eggs with a high shell percentage would have a high SG); however, this was not the case in this experiment. Dry shell percentage was greater in A eggs laid by young hens than in all other egg groups. Overall, these results question the accuracy and reliability of SG as a method for determining shell quality. Fertility, Hatchability, and Salable Chick Production Flock Age. Fertility was significantly lower at 29 wk than at 59 wk of age (Table 2). Previous research has reported a reduction in fertility as breeder flocks age (Elibol et al., 2002; Zakaria et al., 2005). Because feed and breeder management can influence fertility and these parameters were not included in the experimental design, comparison of results from the present study with previous research may not be applicable. Eggs laid by the young hens also had a greater percentage of weight loss at transfer than eggs from the old hens. did not affect hatchability of fertile eggs, embryonic death, or percentage of culled chicks at hatching. The present results are different from those reported by Tona et al. (2001). In a continuous study of a Cobb broiler breeder flock (from 27 to 60 wk of age), Tona et al. (2001) reported the highest total hatchability and the lowest total embryo mortality at 40 wk of age. The lowest hatchability and highest rates of embryo mortality were observed toward the end of the study at older flock ages (Tona et al., 2001). The authors hypothesized that these results might have been due to a combined effect of high embryonic heat production and lower than optimal ventilation in the setter because of increased egg size. In the present study, the likelihood of reduced ventilation in the setter was low because the number of eggs set at each flock age was below the capacity of the setter and hatcher. Egg Weight. The percentage of egg weight loss at transfer decreased as egg size increased (Table 2). Because of the increased surface-to-volume ratio in L eggs (inferred from egg size), it was not surprising to observe that the L eggs lost the highest percentage of moisture. These results could also be related back to the abovementioned finding that H eggs had a greater proportion of albumen (and thus greater moisture content) than L eggs. Late embryonic mortality increased as egg weight increased. The highest late embryonic mortality in H vs. L eggs is in agreement with that reported by Lawrence et al. (2004). These authors analyzed the effect of egg size on the hatchability of a 43-wk-old Cobb 500 flock and reported low hatchability in eggs that were larger than the average egg. This was a consequence of both high late embryo mortality and culled chicks (Lawrence et al., 2004). Fertility, hatchability of fertile eggs, early and mid embryonic mortality, and percentage of culled chicks at hatching were not affected by egg weight. Flock Age and Egg Weight Interaction. The only parameter affected by the interaction of egg flock age and egg weight was percentage of culled chicks at hatching (Table 2). The lowest number of culled chicks was observed in the L and A eggs from the young flock age (the smallest in egg weight). The number of culled chicks from the H eggs at 59 wk (the largest in egg weight) did not differ from any of the other egg groups. Tona et al. (2004) reported a higher percentage of highquality chicks in young vs. old flocks. Even though Tona et al. (2004) did not report percentage of culled chicks, these 2 findings together support the concept that chick quality should not be affected in chicks hatching from young breeders (not even in the smallest chicks). These

FLOCK AGE, EGG WEIGHT, AND BROILER CHICK QUALITY 2739 birds would likely have the same strength and potential of chicks hatching from older breeders and larger eggs. External Pipping and Hatching Time Flock Age. Chicks from the young flock pipped and hatched later than chicks from the old flock (Table 3). This agrees with the results of Hudson et al. (2004) and Hamidu et al. (2007), who also reported delayed hatching in chicks from 29-wk-old hens. This prolonged incubation length could be related to the lower embryonic metabolism in young vs. older flocks reported by Hamidu et al. (2007). Hudson et al. (2004) showed that, after 480 h of incubation, only 10% of chicks from 29-wk-old breeders had hatched compared with 24.6% of chicks from 41-wk-old breeders. In commercial operations, chicks are usually removed from the hatcher after 21 d of incubation (504 h) to allow for most of the chicks to hatch and thus optimize the number of salable chicks. However, because all chicks do not hatch at the same time, the period of time between the hatching of the first and the last chick (the hatch window) can last up to 48 h (Sklan et al., 2000). Consequently, variation exists in the number of hours between hatching and placement of chicks on the farm (Vieira et al., 2005). Under commercial conditions, where transportation to the farm could take several hours, a proportion of the chicks will have their first access to feed 50 or more hours after hatching. It has been reported that prolonged posthatch holding time decreases chick BW (Pinchasov and Noy, 1993; Sklan et al., 2000), and that this early reduction in BW is still significant at 21 d of age (Sklan et al., 2000). This should be taken into account when determining the appropriate time to pull the hatch and thus avoid dehydration of chicks from old hens or removal of wet chicks from young hens. Egg Weight. Chicks from L eggs pipped and hatched earlier than chicks from A of H eggs. These results agree with those reported by Wilson (1991) and Kumpula and Fasenko (2004) and confirm earlier studies (Rahn and Ar, 1974). In summary, regardless of flock age, L eggs hatched earlier, but regardless of egg size, eggs produced at the young flock age took longer to hatch. This means that when setting eggs for incubation, both parameters should be considered to minimize the hatch window and thus produce a more uniform hatch. The effects of the interaction on external pipping and hatching time were not significant (P = 0.214 and 0.447 respectively; data not shown). Broiler Performance Flock Age. Body weight at 21 d (early) and 41 d (final) as well as early and overall BW gains were significantly lower in chicks hatching at the young breeder flock age than in chicks hatching at the older flock age (Table 4). This could be related to the lowest feed con- Table 2. Effects of 2 breeder flock ages, 3 egg weights, and their interaction on fertility, hatchability, egg weight loss, embryonic mortality, and culled chicks in a commercial Cobb 500 broiler breeder flock Item n 1 Fertility 2 Weight loss 3 Hatch of fertile 4 Early dead 5 Mid dead 6 Late dead 7 Culls 8 29 wk 63 76.7 b 12.8 a 88.0 4.8 0.6 5.2 1.2 59 wk 63 94.4 a 11.9 b 87.0 4.0 0.8 4.6 2.0 SEM 1.0 0.1 1.1 0.6 0.3 0.7 0.4 Egg weight 9 Light (L) 42 84.2 12.7 a 88.3 5.6 0.6 2.8 c 1.3 Average (A) 42 85.1 12.3 b 89.9 3.0 0.8 4.8 b 1.2 Heavy (H) 42 87.5 11.9 c 84.3 4.6 0.8 7.0 a 2.4 SEM 1.2 0.1 1.4 0.8 0.4 0.9 0.4 Age egg weight 29 wk L 21 74.4 13.1 90.2 6.2 0.5 2.4 0.3 b 29 wk A 21 75.7 12.7 90.7 3.5 1.1 4.6 0.3 b 29 wk H 21 80.2 12.3 83.1 4.8 0.3 8.7 3.2 a 59 wk L 21 94.0 12.3 86.3 5.1 0.7 3.2 2.3 a 59 wk A 21 94.4 11.9 89.1 2.6 0.5 5.1 2.1 a 59 wk H 21 94.9 11.5 85.6 4.4 1.4 5.3 1.6 ab SEM 1.6 0.1 2.0 1.0 0.5 1.2 0.7 a c Means within a column lacking a common superscript differ significantly (P 0.05). 1 Number of experimental units; each experimental unit = 1 group of 18 eggs. 2 Fertility = (number of fertile eggs/number of eggs set) 100. 3 Weight loss at 18 d of incubation = [(egg weight at setting egg weight at transfer)/egg weight at setting] 100. 4 Hatch of fertile = (number of salable chicks hatched/number of fertile eggs set) 100. 5 Early dead = (number of embryos that died between 1 and 7 d of incubation/total number of eggs set) 100. 6 Mid dead = (number of embryos that died between 8 and 14 d of incubation/total number of eggs set) 100. 7 Late dead = (number of embryos that died between 15 and 21 d of incubation/total number of eggs set) 100. 8 Culls = (number of nonsalable chicks culled at hatching/total number of eggs set) 100. 9 L = light (3.0 to 6.0 g lighter than the average egg weight); A = average (±1.5 g from the average egg weight); H = heavy (3.0 to 6.0 g heavier than the average egg weight).

2740 Table 3. Pipping and hatching times in a commercial Cobb 500 broiler breeder flock at 2 flock ages and from 3 egg sizes Item n 1 time 2 (h) Pipping Hatching time 3 (h) 29 wk 54 494.4 a 503.8 a 59 wk 54 490.2 b 500.4 b SEM 1.2 1.0 Egg weight 4 Light 36 488.9 b 498.0 b Average 36 493.7 a 503.3 a Heavy 36 494.3 a 505.0 a SEM 1.3 1.0 a,b Means within a column lacking a common superscript differ significantly (P 0.05). 1 Number of experimental units; each experimental unit = 1 individual egg. 2 Time at which beak of embryo punctured through shell. 3 Time at which chick was completely out of shell and the down was dry. 4 Light (3.0 to 6.0 g lighter than the average egg weight); average (±1.5 g from the average egg weight); heavy (3.0 to 6.0 g heavier than the average egg weight). Ulmer-Franco et al. sumption during the first 21 d of the grow-out period observed in chicks from the young flock age (Table 5). The results for BW observed in chicks hatching from the young flock agree with those of Proudfoot and Hulan (1981) and Sklan et al. (2003), who considered chick weight at hatching an accurate predictor of final broiler BW. Whether a connection exists between the smaller dry yolk content (and perhaps less available energy) they observed in eggs produced by the young breeders and the lower final BW they observed in their broilers remains an unexplored hypothesis. No significant differences were found in early, late, or overall FCR between broilers hatched at the different breeder flock ages (Table 5). This means that, regardless of breeder flock age, Cobb broilers had the same FCR potential, and that the lighter final BW was a consequence of low feed consumption by small chicks. These results disagree with those of Hulet et al. (2007). Using a commercial Cobb flock, Hulet et al. (2007) compared the cumulative FCR (up to 44 d of age) of chicks hatching from old (57 wk) vs. young (29 wk) breeder flocks. Even though the largest chicks (those hatching from the old flock) were heavier throughout production, they were less efficient (higher FCR) than the chicks from the young flock. However, in their study, the effects of breeder flock, breeder age, and egg weight were confounded statistically and thus were not analyzed as separate effects (Hulet et al., 2007). Egg Weight. Significant differences were observed in early and final BW as well as in late and overall BW gain between egg size treatment groups (Table 4). Broilers hatched from H eggs were the heaviest at 21 d. Despite differences in chick weight at hatching, broilers hatched from L and A eggs did not differ in BW at 21 d. Even though at market age broilers from H eggs were still heavier than those from L eggs, broilers from A eggs had caught up in BW with those from H eggs. When looking back at BW at hatching, chicks from A eggs were approximately 3.1 g heavier than chicks from L eggs, and approximately 3.5 g lighter than chicks from H eggs. This means that H chicks were approximately 6.6 g heavier than L chicks at hatching, perhaps a large Table 4. Effects of 2 breeder flock ages, 3 egg sizes, and their interaction on average broiler BW and BW gains in a commercial Cobb 500 flock Item Chick BW 1 (g) 21-d BW (g) 41-d BW (g) Early BW gain 2 (g) Late BW gain 3 (g) Overall BW gain 4 (g) 29 wk 37.3 b (714) 5 716.9 b (686) 2,411.1 b (665) 680.0 b 1,699.2 2,373.8 b 59 wk 48.9 a (714) 825.2 a (692) 2,505.9 a (678) 776.4 a 1,678.0 2,457.3 a SEM 0.1 4.2 15.5 4.2 13.4 15.6 Egg weight 6 Light (L) 39.9 c (476) 760.8 b (461) 2,412.6 b (448) 722.4 1,654.7 b 2,374.6 b Average (A) 43.0 b (476) 767.6 b (461) 2,490.6 ab (447) 724.5 1,720.8 a 2,447.8 a Heavy (H) 46.5 a (476) 784.7 a (456) 2,472.3 a (448) 737.7 1,690.3 ab 2,423.1 a SEM 0.1 5.2 19.0 5.2 16.4 19.2 Age weight 29 wk L 34.1 f (238) 697.1 (229) 2,346.4 (221) 665.0 1,654.4 2,316.1 29 wk A 37.0 e (238) 715.8 (230) 2,458.7 (221) 678.8 1,744.2 2,421.3 29 wk H 40.1 d (238) 737.7 (227) 2,428.1 (223) 696.6 1,698.8 2,381.9 59 wk L 45.6 c (238) 824.5 (232) 2,478.8 (227) 778.9 1,654.9 2,433.2 59 wk A 48.9 b (238) 819.5 (231) 2,522.5 (226) 770.7 1,697.3 2,474.6 59 wk H 52.1 a (238) 831.6 (229) 2,516.4 (225) 779.5 1,681.8 2,464.3 SEM 0.1 7.4 27.0 7.4 23.3 27.0 a f Means within a column lacking a common superscript differ significantly (P 0.05). 1 BW at hatching. 2 Early BW gain = 21 d BW chick BW. Same number of experimental units as early BW. 3 Late BW gain = 41 d BW 21 d BW. Same number of experimental units as final BW. 4 Overall BW gain = 41 d BW chick BW. Same number of experimental units as final BW. 5 (n) = number of experimental units; each experimental unit = 1 individual broiler. 6 L = light (3.0 to 6.0 g lighter than the average egg weight); A = average (±1.5 g from the average egg weight); H = heavy (3.0 to 6.0 g heavier than the average egg weight).

FLOCK AGE, EGG WEIGHT, AND BROILER CHICK QUALITY 2741 Table 5. Effects or 2 breeder flock ages and 3 egg sizes on early, late, and total feed consumption and feed conversion ratio (FCR) in commercial Cobb 500 broilers Item n 1 Early feed consumption 2 (g/bird) Early FCR 3 (g/g) Late feed consumption 4 (g/bird) Late FCR 5 (g/g) Total feed consumption 6 (g/bird) Total FCR 7 (g/g) 29 wk 6 958.9 b 1.41 3,107.2 1.84 4,071.4 1.71 59 wk 6 1,099.7 a 1.41 3,115.6 1.86 4,212.0 1.71 SEM 15.4 0.01 51.4 0.02 60.3 0.01 Egg weight 8 Light 4 1,024.2 1.41 3,060.0 1.86 4,079.2 1.72 Average 4 1,030.7 1.42 3,154.3 1.84 4,185.1 1.71 Heavy 4 1,031.6 1.40 3,119.8 1.86 4,160.8 1.72 SEM 19.0 0.01 62.2 0.03 72.4 0.02 a,b Means within a column lacking a common superscript differ significantly (P 0.05). 1 Number of experimental units; each experimental unit = pen of 119 broilers. 2 Early feed consumption = cumulative feed intake per bird from 1 d to 21 d. 3 Early FCR = BW gain/feed consumption from 1 d to 21 d. 4 Late feed consumption = cumulative feed intake per bird from 22 d to 41 d. 5 Late FCR = BW gain/feed consumption from 22 d to 41 d. 6 Total feed consumption = cumulative feed intake per bird from 1 d to 41 d. 7 Total FCR = BW gain/feed consumption from 1 to 41 d. 8 L = light (3.0 to 6.0 g lighter than the average egg weight); A = average (±1.5 g from the average egg weight); H = heavy (3.0 to 6.0 g heavier than the average egg weight). enough difference to affect final BW. Chick weight was not an accurate predictor of final BW, as had been previously reported (Gardiner, 1973; Shanawany, 1987). Egg weight did not affect feed consumption or FCR parameters (Table 5). Flock Age and Egg Weight Interaction. The only broiler performance parameter affected by the interaction was chick weight at hatching (Table 4). Chick weight followed the same pattern as egg weight at setting. Thus, L eggs produced by the young flock hatched the lightest chicks and H eggs produced by the old flock hatched the heaviest chicks. This result was expected because a strong positive correlation between egg weight and chick weight has long been reported (McNaughton et al., 1978). Broiler mortality was not affected by the main effects of flock age and egg weight or by their interaction (data not shown). Conclusions It is known that during incubation, when the embryo is enclosed in the egg, the yolk content (more specifically, the lipids contained in it) is the main source of energy for embryonic development (Romanoff, 1960). Over the first few days posthatch, the remaining content of the yolk sac provides the newly hatched chicks with energy for growth and development of the small intestine (Noy and Sklan, 1999). Furthermore, a direct positive correlation between the nutrient content of the yolk sac and the subsequent performance of broilers has been reported (Murakami et al., 1992; Vieira and Moran, 1999). In the present study, eggs laid by a 29-wk-old broiler breeder flock had a smaller proportion of yolk than those laid at 59 wk of age. The offspring produced by this young breeder flock were of equal chick quality at hatching, but they had lower final BW than the offspring of the breeder flock at an older age. All chicks were reared in separate groups according to egg weight and flock age; thus, competition between small and large chicks was avoided. In addition, the chicks were reared under the same conditions and fed the same feed. It could be concluded that, when reared under the same conditions, chicks produced at a young breeder age (when eggs with smaller yolks are produced) could be at a disadvantage when compared with chicks produced at older breeder ages (when eggs with larger yolks are produced). Further research determining the ideal rearing conditions for chicks from young breeders is advised. Cobb 500 is a strain commonly used for broiler production in Canada; because of the normal egg production cycle, at any given moment chicken producers would likely manage broiler chicks from young breeder flocks. This research provided basic information that could be useful when making decisions for broiler chick management. ACKNOWLEDGMENTS The authors acknowledge the financial support of this research by the Natural Sciences and Engineering Research Council of Canada (Ottawa, Ontario, Canada), Canadian Hatching Egg Producers (Ottawa, Ontario, Canada), Alberta Chicken Producers (Edmonton, Alberta, Canada), Alberta Livestock Industry Development Fund (Edmonton, Alberta, Canada), and Alberta Innovation and Science (Edmonton, Alberta, Canada). The donation of hatching eggs from Maple Leaf Hatchery (Wetaskiwin, Alberta, Canada) is greatly appreciated. The authors also thank M. MacKenzie (Department of Agricultural, Food and Nutritional Science,

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