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PRODUCTION, MODELING, AND EDUCATION |
* Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2P5; Alberta Agriculture, Food and Rural Development, Livestock Development Division, Edmonton, Alberta, Canada T6H 5T6; and Aviagen, Inc., Huntsville, AL 35805
1 Corresponding author: frank.robinson{at}ualberta.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Key Words: egg trait • chick quality • residual yolk • chick growth
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Chick quality is difficult to evaluate at hatch. Traditionally, hatch weight has been emphasized as a strong indicator of chick quality; however, recent research (Joseph et al., 2006) has shown that hatch weight differences among treatments are largely explained by variations in residual yolk mass. Noy et al. (1996) found that residual yolk in newly hatched chicks can account for as much as 20% (or 8 g) of hatch weight, whereas Vieira and Moran (1998) reported the yolk to constitute roughly 10% (4.5 to 5.0 g) of a chick’s BW. Residual yolk mass can be extremely variable among genetic strains at hatch, ranging from 0.8 to 10.6 g (Wolanski et al., 2006). Furthermore, length measurements of chicks at hatch have been found to correlate more strongly with 14-d BW than initial hatch weight and 14-d BW. In an effort to better quantify chick quality, an abdomen scoring system was previously implemented to estimate residual yolk mass of live chicks. This score correlated with actual residual yolk mass after dissection (r = 0.50; P < 0.0001; Wolanski et al., 2006).
The objectives of this study were to evaluate egg characteristics, internal chick components, and relative growth rate of 10 strains of broiler breeders. Residual yolk mass at hatch was believed to play a role in determining hatch weight. The genetic diversity evaluated in this trial would result in observable differences in egg characteristics that would be linked to hatch traits in the chicks. We anticipated that hatch traits would be related to the chick traits at 14 d of age.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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) were obtained from a primary breeding company (Aviagen North America, Huntsville, AL). Flock age ranged between 46 wk (strain 10) and 57 wk (strain 9) for the strains investigated in this trial. All eggs were collected in a 24-h period. The eggs were weighed on the day of lay at a central egg depot in the southern United States and shipped to the University of Alberta. Upon arrival (8 d after collection), eggs were reweighed and randomly allocated to 1 of 3 experimental determinations: 1) egg quality eggs (52 eggs/strain), 2) eggs for conductance (8 eggs/strain), or 3) incubation eggs (150 eggs/strain).
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Eggs for Conductance
Eggs destined for conductance were sealed in desiccators filled with Drierite dessicant (W. A. Hammond Drierite Company Ltd., Xenia, OH). Eggs from each strain were represented in each of the 8 desiccation chambers. Eggs were weighed at 24-h intervals for a total of 13 d to characterize moisture loss and calculate eggshell conductance.
Eggs for Incubation
The remaining eggs were placed in a 5,000-egg-capacity incubator (Jamesway Incubator Company Inc., Cambridge, Ontario, Canada). The flats were randomly placed in the incubator to minimize potential airflow or incubator effects. At 18 d of incubation, eggs were candled and infertile eggs and early dead embryos were removed. Macroscopic inspection of eggs removed at candling was performed to determine fertility status or the stage of embryonic death. Eggs were transferred into individual pedigree hatch baskets so that chick traits could be traced back to initial egg weight. The hatch was pulled at 21.5 d and chicks were processed. All unhatched eggs were broken open to macroscopically determine fertility or stage of development at death. At hatch, all chicks were neck tagged (Heartland Animal Health Inc., Fair Play, MO), weighed, and subjected to chick quality measures. Of particular interest was the consistency and fullness of the chick’s abdomen. Chick length and shank length were also characterized for each chick (Wolanski et al., 2006). Navel condition was also determined (data not presented).
At hatch, 50 chicks/strain were randomly selected for characterization and euthanized by cervical dislocation. Chick weight, residual yolk mass, internal organ weights, and breast muscle fleshing were quantified after birds had been euthanized. The remaining birds were measured and placed in rearing pens for a period of 2 wk. Birds had ad libitum access to a broiler starter diet (Table 2) and water, with a photoperiod of 23 L:1 D. At 14 d, all birds were processed to measure breast muscle weight, residual yolk weight, and carcass morphometrics such as keel length, shank length, and total bird length.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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, along with the egg quality measures of egg specific gravity, shell thickness, and albumen height. The range in egg weight was 3.0 g for the 10 strains investigated in this trial. The largest hatching eggs (66.0 g) were produced by strain 2, a female line bird that has been selected for reproductive traits. In contrast, strains 4, 9, and 10 had mean egg weights of 63.0, 63.4, and 63.6 g, respectively. Interestingly, the flock ages of strains 4, 9, and 10 were 50, 57, and 46 wk of age, respectively. These data suggest that strain effects in this study had a more profound impact on egg weight than did breeder flock age.
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). Ultimately, the range in average yolk weight (1.5 g) accounted for approximately 50% of the variation observed in egg weight. The range in shell weight was approximately 0.5 g, with the remaining difference in egg weight being explained by variations in albumen weight. Albumen is the primary source of water in the egg, and Finkler et al. (1998) suggested that albumen weight in hatching eggs was the primary determinant of hatchling size. The female line (strain 2) had the greatest amount of albumen, both absolute (38.8 g) and proportionally (59.0%), and chicks from this strain hatched with the greatest BW. The youngest flock, strain 10 (46 wk), had the highest albumen height (5.22 mm) as compared with older flocks, strains 9, 5, 3 (57, 54, 52 wk), which had albumen heights of 4.36, 4.38, and 4.20 mm, respectively. The female line birds selected for yield (strain 2) had an albumen height of 5.23 mm. Both age and strain may influence the weight of internal contents (yolk and albumen) and composition of the hatching eggs, which may influence the hatching characteristics of each strain. Peebles et al. (2000) suggested that albumen height may be a factor in determining DM accumulation of chicken embryos, and these authors also reported that thick albumen may slow gas diffusion, limit nutrient availability, and decrease embryonic growth. Albumen height was 5.00 mm after 12 h of incubation, and by 60 h the albumen was 3.59 mm (Lapão et al. 1999).
The eggs of strain 1, which was a male line, had the highest shell weight and shell thickness (Table 3). This strain had a mean egg weight of 64.8 g. This heavily growth-selected strain had poorer egg production than all the other strains (F. E. Robinson, unpublished data), which may allow more calcium to be deposited onto an eggshell. The selection criteria used to develop the male line (strain 1) placed the majority of selection pressure on growth and BW, whereas selection on egg production was compromised. The frame size of male line females was larger (F. E. Robinson, unpublished data), potentially allowing for more storage of medullary bone used in the formation of eggshell constituents. Riczu et al. (2004) reported that BW was correlated with total bone density in laying hens. A report by Silversides et al. (2006) described strain effects on shell quality of layer chickens, with the largest strain producing the heaviest eggshell and the lightest strain producing a smaller egg with a lighter eggshell weight.
Eggshell Conductance
The initial egg weight, moisture loss, and percentage of moisture loss data are reported in Table 4. The oldest strain (strain 9) had the greatest moisture loss to 4 d (3.01 g) as compared with the female line (strain 3), which lost only 2.21 g in the same period of time. On a percentage basis, strain 1 eggs lost 4.60% of their weight compared with only 3.46% in the female line eggs. This result was likely associated with age, but some of the selection for reproductive traits in strain 3 may have had a positive impact on moisture retention in this line, because the 2 breeder flocks were only 5 wk apart in age. Tullett and Board (1977) reported that water loss from eggs was inversely proportional to shell thickness. The number and size of pores in each egg can influence the rate of moisture loss and heat conductance across the eggshell (Hulet et al., 2007). Strain 4 had the thickest shells (0.368 mm), which may explain why, for the egg desiccation data, this strain lost only 3.55 and 2.71% of moisture from d 0 to 4 and d 4 to 11, respectively (Table 4).
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). Although strain 10 was a female line, the breeder flock age at which these eggs were obtained was 6 and 7 wk younger than that of strain 2 or 3 breeders, respectively. This suggests that the nearly 3.3 and 1.8 g difference in egg weight in relation to the other female lines (strains 2 and 3) may be partially explained by differences in flock age. However, it should be noted that strain 9, which was the oldest flock (57 wk), had hatching eggs that were among the lightest (63.8 g) of all strains examined, demonstrating that age, along with other factors, may contribute equally to differences in egg weight. Joseph et al. (2002) compared 3 breeder strains and reported that the strain with the best rate of lay produced significantly lighter eggs.
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), which demonstrated a different ranking among strains.
Hatch Weight Relative to Egg Weight
Egg weight is a dominant factor in determining chick hatch weight (Wyatt et al., 1985). In this study the large male line hens (strain 1) produced one of the largest hatching eggs (65.8 g) and had the greatest hatch weight (46.3 g; Table 6). Egg weights for female lines (strains 2 and 3) were significantly different (66.9 and 65.4 g, respectively), whereas both hatch weights (46.6 and 45.4 g, respectively) and carcass weights of dissected chicks (36.2 and 35.4 g, respectively) did not differ. In addition, strain 10 (a female line with emphasis on reproduction) produced eggs that were nearly 2 g smaller than those of strain 3; however, the chick weight and carcass weight were comparable to strain 3. Pal et al. (2002) showed that the wet weight as well as dry weight of prehatched chicks was significantly altered by genotype as well as by stage of incubation in broiler and layer chicks. Furthermore, Hardin (1972) stated that the weight of prehatched chicks of different breeds was not merely associated with differences in egg weight, but also reflected true genetic differences. Christensen et al. (2002) reported that line, age, and storage factors, along with the interaction of these factors, affect embryonic body and organ weights differently. Although numerous factors influence chick weight at hatch, Ricklefs and Starck (1998) stated that egg weight and the length of the incubation period have the greatest impact on hatch weight.
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Yolk utilization, on a percentage of chick weight basis, showed that the male line (strain 1) had 13.5% of total hatch weight in the form of unused residual yolk as compared with the commercial strains 5, 6, 7, which had only 9.4, 9.2, and 9.7% of initial hatch weight in the form of residual yolk (Table 6). The early mobilization of yolk lipids in these strains may explain why they had the greatest proportion of liver at the time of hatch (2.84, 2.80, 2.83%, respectively). Strains that had a significantly greater amount of residual yolk (strains 1 and 2) had significantly lower liver weights on a percentage basis (2.56 and 2.37%, respectively). In a report by Sklan et al. (2003), hepatic size was relatively larger in chicks that exhibited greater growth, which the author suggested may reflect the level of metabolic activity. It is well recognized that the liver is necessary for the remodeling of residual yolk lipids into lipoprotein particles that are exported into circulation (Sklan et al., 2003).
14-d Growth Data
Hatch weight is reported in Table 6, whereas growth data, external morphometrics, and dissection results are reported in Table 7. Male line chicks (strain 1) had the longest shank, keel, and total length measurements at 14 d of age (Table 7). Strain 1 was among the heaviest chicks at hatch, and by 7 d of age this strain was 15% heavier than the second heaviest strain (slow-feathering commercial cross, strain 8). The 15% relative difference between strains 1 and 8 generally remained constant throughout the grow-out period. The male line strain (strain 1) had the highest relative BW gain from hatch to 14 d (765%). Commercial strains (strains 4, 5, and 6) had relative BW gains of 615, 546, and 518%, respectively. Strain 6 had the greatest degree of selection on white meat yield and the slowest relative growth rate of all strains investigated. Wilson (1991) reported that every 1-g increase in hatch weight resulted in an 8- to 13-g advantage in broiler market weight in the 1980s. This emphasizes the importance of suiting incubation conditions to allow for optimal broiler growth during incubation. Scheuermann et al. (2003) compared chick growth and muscle development in 8 strain crosses and suggested that different growth curves exist among commercially available strain crosses.
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. Breast meat weight for the 10 strains ranged from 51 g in the heavily growth-selected male line (strain 1) to a low of 30 g in the strain 3 birds, which are predominately selected based on female reproductive performance. Remignon et al. (1994, 1995) reported that fast-growing lines of chickens have 15 to 20% more myofibers than slower growing lines. Therefore, some of the difference in breast muscle yield in this trial may relate to more myofibers in the heavily growth-selected strain 1 chicks relative to other strains.
In the industry, the balance between BW and reproduction has been well documented, and the selection criteria used to develop the divergent genotypes in this study appeared to have an impact on traits of extremely young birds. On a percentage basis, the breast muscle weight of strain 1 accounted for 13% of BW as compared with 10% in strain 3 at 14 d. With the commercial products (strains 4, 5, and 6), there is an increasing gradient in the degree of selection for white meat yield, with strain 4 having the least emphasis and strain 6 having the greatest. This was demonstrated by the white meat yield performance of strain 4, with 10.9% breast muscle, compared with strains 5 and 6, with 11.6 and 11.8%, respectively. The liver, which is a supply organ, showed a great deal of variation based on body size, with the largest male line strain (strain 1) having a mean liver weight of 15.5 g as compared with the commercial birds (strains 4, 5, and 6), which had mean liver weights of 11.3, 10.4, and 10.1 g, respectively. However, when liver was expressed on a percentage basis, female line strains 10 and 2 had an advantage when compared with the commercial strains (5, 6, and 7; Table 7). Heart weight ranged from 2.2 g in strains 6 and 10 to a high of 3.3 g in strain 1. Cardiovascular fitness in commercial broilers may help to prevent mortality associated with ascites or other metabolic diseases. When examining the relative heart weight in the 10 strains surveyed, the commercial lines (strains 5, 6, and 7) had the greatest percentage of heart (0.84, 0.83, and 0.81%, respectively). The internal organ weights were correlated with BW and consistently showed that the male line (strain 1) had the largest absolute internal organ weights by 14 d.
Correlations and CV
Correlations among egg characteristics and fresh egg weight are reported in Table 8. Both albumen weight and yolk weight were significantly correlated with fresh egg weight. However, albumen weight had a correlation coefficient of r = 0.84, whereas yolk weight had a coefficient of r = 0.48. In this trial, shell weight was strongly correlated with shell thickness (r = 0.78). This finding was in partial agreement with a report by Zhang et al. (2005), who described a moderate but significant correlation between eggshell weight and eggshell thickness.
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). The correlation between fresh egg weight and hatch weight was r = 0.87, corroborating previous reports demonstrating 86 to 97% of the variation in chick weight to be explained by set egg weight (Tullett and Burton, 1982; Burke, 1992). Hatch weight was found to correlate with both wet residual yolk weight (r = 0.67) and wet carcass weight (r = 0.83) in chicks dissected at hatch. In agreement with a previous study by Wolanski et al. (2006), the live abdomen score was significantly correlated with dissected residual yolk mass (r = 0.50).
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). The range in CV of chick weight at hatch was comparable to that of the CV for egg weight, being 7.1% in strain 8 and 9.5% in strain 7 (Table 10). It is interesting to note that the variation increased in strain 7, whereas chick weight was less variable than for the eggs set for strain 8. The greatest CV for all traits examined in this trial was found for wet residual yolk content of hatched chicks. This variable had a CV ranging from 25.9% in strain 2 (female line-selected bird) to a high of 42.5% in strain 7. This high degree of variation suggests that some chicks were efficiently utilizing yolk reserves prior to hatch, whereas others had not utilized residual yolk reserves to the same extent. From the CV data, yolk utilization was 5 times more variable than the variation observed in initial egg weight data and chick weight data.
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ACKNOWLEDGMENTS |
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Received for publication November 10, 2006. Accepted for publication March 17, 2007.
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