Genetic architecture of carcass composition in chickens

A genome-wide association study reveals dominance effects on number of teats in pigs

Dominance has been suggested as one of the genetic mechanisms explaining heterosis. However, using traditional quantitative genetic methods it is difficult to obtain accurate estimates of dominance effects. With the availability of dense SNP (Single Nucleotide Polymorphism) panels, we now have new opportunities for the detection and use of dominance at individual loci. Thus, the aim of this study was to detect additive and dominance effects on number of teats (NT), specifically to investigate the importance of dominance in a Landrace-based population of pigs. In total, 1,550 animals, genotyped for 32,911 SNPs, were used in single SNP analysis. SNPs with a significant genetic effect were tested for their mode of gene action being additive, dominant or a combination. In total, 21 SNPs were associated with NT, located in three regions with additive (SSC6, 7 and 12) and one region with dominant effects (SSC4). Estimates of additive effects ranged from 0.24 to 0.29 teats. The dominance effect of the QTL located on SSC4 was negative (−0.26 teats). The additive variance of the four QTLs together explained 7.37% of the total phenotypic variance. The dominance variance of the four QTLs together explained 1.82% of the total phenotypic variance, which corresponds to one-fourth of the variance explained by additive effects. The results suggest that dominance effects play a relevant role in the genetic architecture of NT. The QTL region on SSC7 contains the most promising candidate gene: VRTN. This gene has been suggested to be related to the number of vertebrae, a trait correlated with NT.

Further Investigations on the Role of Diet-Induced Thermogenesis in the Regulation of Feed Intake in Chickens: Comparison of Age-Matched Broiler versus Layer Cockerels

The aim of this study was to investigate the role of diet-induced thermogenesis (DIT) in the regulation of feed intake in age-matched broiler and layer cockerels. In addition, the effect of genotype on endocrine functioning and key metabolites of the intermediary metabolism as well as on the expression of muscular uncoupling protein (avUCP) was explored. One-day-old male broiler (Ross) and layer (ISA Brown) chicks were reared under standard conditions on commercial broiler starter and finisher diets. From 22 d of age, twice per week, 3 broiler and 6 layer cockerels were placed in open circuit respiratory chambers. After adaptation, the animals were feed-deprived for 24 h, and heat production was measured by indirect calorimetry. During the subsequent 7-h refeeding period, feed intake and DIT were measured. Blood samples were taken after feed deprivation and re-feeding. Muscle samples were taken after refeeding for determination of avUCP expression. A significantly higher heat production per metabolic BW (MBW) in the layer compared with the broiler cockerels, independent of nutritional state, suggests that the broilers used a greater proportion of the metabolizable energy intake for growth. The DIT per MBW and per gram of feed intake was higher for the layer than for the broiler cockerels. However, feed intake per MBW was also significantly higher in the layer cockerels. Thus, no feedback effect of DIT on feed intake per MBW was observed, and the model formulated for adult mammals relating feed intake to DIT could not be corroborated. The muscular expression of avUCP was not different between genotypes, which does not support the hypothesis of an involvement of avUCP in the higher DIT measured in layer cockerels. Circulating uric acid, glucose, triglyceride, and free fatty acid levels were significantly elevated in the layer compared with the broiler cockerels. As the diet was formulated according to broiler requirements, the higher metabolite levels of the layer cockerels might reflect a relative oversupply of dietary nutrients.

Estimation of Nitrogen Maintenance Requirements and Potential for Nitrogen Deposition in Fast-Growing Chickens Depending on Age and Sex

Experiments were conducted to estimate daily N maintenance requirements (NMR) and the genetic potential for daily N deposition (ND(max)T) in fast-growing chickens depending on age and sex. In N-balance studies, 144 male and 144 female chickens (Cobb 500) were utilized in 4 consecutive age periods (I: 10 to 25 d; II: 30 to 45 d; III: 50 to 65 d; and IV: 70 to 85 d). The experimental diets contained high-protein soybean meal and crystalline amino acids as protein sources and 6 graded levels of protein supply (N1 = 6.6%; N2 = 13.0%; N3 = 19.6%; N4 = 25.1%; N5 = 31.8%; and N6 = 37.6% CP in DM). The connection between N intake and total N excretion was fitted for NMR determination by an exponential function. The average NMR value (252 mg of N/BW(kg)0.67 per d) was applied for further calculation of ND(max)T as the threshold value of the function between N intake and daily N balance. For estimating the threshold value, the principle of the Levenberg-Marquardt algorithm within the SPSS program (Version 11.5) was applied. As a theoretical maximum for ND(max)T, 3,592, 2,723, 1,702, and 1,386 mg of N/BW(kg)0.67 per d for male and 3,452, 2,604, 1,501, and 1,286 mg of N/BW(kg)0.67 per d for female fast-growing chickens (corresponding to age periods I to IV) were obtained. The determined model parameters were the precondition for modeling of the amino acid requirement based on an exponential N-utilization model and depended on performance and dietary amino acid efficiency. This procedure will be further developed and applied in the subsequent paper.

Correlated responses in body composition to divergent selection for exponential growth rate to 14 or 42 days of age in chickens

Chicks divergently selected for 14-d (14H and 14L) or 42-d (42H and 42L) exponential growth rate (EGR) over five generations were used to determine correlated responses between growth at different ages and body composition. Regression coefficient estimates across five generations of selection were not significant for any line at either age for percentage total body water or protein. Genetic correlations between EGR from hatching to 14 d of age (EGR14) and 42-d percentage carcass fat were -0.18, -0.57, 0.63, and -0.79 among the 14H, 14L, 42H, and 42L lines, respectively. Genetic correlations between EGR from hatching to 42 d of age (EGR42) and 42-d percentage carcass fat were 0.09, -0.67,0.50, and -0.75 among the 14H, 14L, 42H, and 42L lines, respectively. During the short-term selection experiment, selection for fast EGR14 or EGR42 increased fat at the age of selection. However, selection for fast EGR42 increased body weight and percentage fat at 42 d of age (DOA), whereas selection for fast EGR14 increased body weight but not fat at 42 DOA. Therefore, it is possible to simultaneously select for high body weight at, or near, the inflection point of the growth curve without increasing fat deposition or obesity by taking advantage of the lack of a genetic correlation between EGR14 and body fat percentage at later ages.

Genetic parameters of growth curve parameters in male and female chickens

1. Individual growth curves of 7143 chickens selected for the form of the growth curve were fitted using the Laird form of the Gompertz function, BW4=BW0xe(L/K)(1-e-Kt) where BWt is the body weight at age t, BW0 the estimated hatching weight, L the initial specific growth rate and K the maturation rate. 2. Line and sex effects were significant for each parameter of the growth curve. In males, L, BW0, age and body weight at inflection (T(I)and BWI) were higher whereas K was lower than in females. Lines selected for high adult body weight had higher BW0 and BW(I) whereas lines selected for high juvenile body weight had larger estimates of L and lower estimates of T(I). 3. Data from 38,474 animals were included in order to estimate the genetic parameters of growth curve parameters in males and females, considering them as sex-limited traits. Genetic parameters were estimated with REML (REstricted Maximum Likelihood) and an animal model. Maternal genetic effects were also included. 4. Heritabilities of the growth curve parameters were moderate to high and ranged between 0.31 and 0.54, L, BW0 in both sexes and BW(I) in males exhibited significant maternal heritability. Heritabilities differed between males and females for BWI and T(I). Genetic correlations between sexes differed significantly from one for all parameters. L, K and T(I) were highly correlated but correlations involving BW0 and BW(I) were low to moderate. 5. Sexual dimorphism of body weight at 8 and 36 weeks and of L, K and T(I) was moderately heritable. Selection on growth curve parameters could modify the difference between sexes in precocity and thus in body weight at a given age.

Further studies on the influence of genotype and dietary protein on the performance of broilers

An experiment was conducted to quantify genetic differences in response to dietary protein level of male vs female broilers. Broilers (1 d old) from a "high-yield" strain cross (Ross x Ross 208) and a "fast-growing" strain cross (Peterson x Arbor Acres) were placed on fresh pine shavings in floor pens. From Day 0 to 18, all birds were fed a 23% CP starter diet. During Days 18 to 53 male birds were fed either a 16, 18, 20, 22, 24, or 26% CP diet (3,200 kcal ME/kg) and female birds were fed the 16, 20, or 24% CP diet. Significant differences (P < 0.05) were noted in the performance of the different strains. Ross x Ross 208 male birds had a higher body weight (3.37 vs 3.16 kg), higher feed intake (7.08 vs 6.78 kg), higher breast yield (31.76 vs 29.25%), higher carcass yield (73.90 vs 71.85%), and a lower adjusted feed conversion ratio (FCR; 2.10 vs 2.16 g:g) than Peterson x Arbor Acres males at 53 d of age. As compared to Peterson x Arbor Acres females, Ross x Ross 208 female broilers also had a higher body weight (2.68 vs 2.55 kg), higher breast meat yield (33.61 vs 30.80%), higher carcass yield (75.31 vs 73.91%), and lower adjusted FCR (1.97 vs 2.04 g:g). Qualitative differences in the response of these strains were confirmed and better qualitative data is presented that can be used to predict the important output parameters from the import inputs in broiler production.

Influence of broiler strain cross and dietary protein on the performance of broilers

The purpose of this experiment was to quantify the responses of two broiler strain crosses to different dietary protein levels on performance and carcass yields. Day-old broiler chicks from a high-yield strain cross (Ross x Ross 208) and a fast-growing strain cross (Peterson x Arbor Acres) were placed in floor pens on fresh pine shavings. All birds were fed a 23% CP starter for the first 18 d. During Days 18 to 53, birds were fed either 16, 20, or 24% CP diets (3,200 kcal ME/kg). At 53 d of age, significant differences (P < 0.05) were noted in the performance of the strains. Overall, Ross x Ross birds had higher body weights (3.29 vs 3.10 kg), higher feed intakes (6.40 vs 6.11 kg), and higher carcass yields (72.51 vs 71.17%), although the differences were dependent on dietary protein levels. Strain cross had no significant effect on feed conversion (1.95 g feed: g gain vs 1.97 g:g). Results indicated that both strain cross and protein level had effects on body weight and feed intake, and there were significant strain cross by protein level interactions for body weight and carcass weight. Increasing dietary protein level increased body weights more for the Ross x Ross 208 than for the Peterson x Arbor Acres broilers. Feed conversion was indirectly proportional to dietary protein level, but was not affected by strain cross. Percentage carcass yield was greatly affected by strain (P < 0.0001), but protein level had no significant effect (P = 0.68). The significant interactions indicate that different strain crosses should have different feeding programs to maximize profitability.

Effects of dietary sorghum of different tannin concentrations and tallow supplementation on the performance of broiler chicks

1. The chemical compositions and tannin contents of 17 varieties of grain sorghum were determined. 2. Commercial broiler chicks were fed from 7 to 49 d of age on 15 experimental diets which varied in their concentrations of tannins and animal fat (tallow), by the inclusion of different amounts of high and a low protein varieties of sorghum. 3. Substitution of a low tannin sorghum completely for maize did not have any significantly adverse effect on performance. Broiler chicks could tolerate up to 2.6 g tannin/kg above which their performance was reduced. Addition of tallow did not significantly affect broiler performance except at the highest tannin content and added fat intake, where weight gain and efficiency appeared to improve significantly.