Correlated responses in reproductive fitness to selection in chickens

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Abstract
The correlated changes in reproductive fitness with selection for single traits for six generations in five Leghorn lines and five generations in three Fayoumi lines were analysed. The Leghorn lines, originating from crosses between four commercial strains, were: line A, selected for high egg production; lines B and C, selected for high and low nine-month body weight; and lines D and E, selected for high and low nine month egg weight. The Fayoumi lines, J, K and L, corresponding to the A, B and D Leghorn lines, respectively, were selected from a random mating population maintained over several years without selection. The breeding populations for the A and J lines each consisted of 16 sires mated to 9–16 dams each, while all the other lines were produced from eight sires, each mated to 9–16 dams. The A and J lines showed no consistent improvement in egg production or reproductive fitness and were used as control lines in this study. Body weight in the B and K lines increased by 55·8 and 45·2%, respectively, while egg weight in the D and L lines increased by 17·7 and 15·1%, respectively. The C line declined in body weight by 21·5%, while the E line declined in egg weight by 16·3%.A reproductive fitness index was measured as the product of rate of egg production, fertility, hatchability, and percent survival of offspring from housing to nine months of age. The mean values of the fitness index as percentages of the control lines were Leghorn lines B 66, C 81, D 74, E 85 and Fayoumi lines K 54 and L 74.Reproductive fitness and all fitness components except offspring survival declined more in the high lines than in the low lines. The body weight lines were lower in fitness than the egg weight lines, mainly because of lower rate of egg production The patterns of correlated changes in the fitness index were more regular in the Fayoumi than in the Leghorn lines. The linear regressions of fitness index on direc selection response in the K and L Fayoumi lines were statistically significant. Offspring survival contributed only 14% to the variation in the fitness index in the Leghorn lines and only 8% in the Fayoumi lines. The relative contributions to the variation in fitness index were 35% for rate of egg production, 24% for fertility and 23% for hatchability in the Leghorn lines. The corresponding figures were 35, 30 and 27% for the Fayoumi lines, respectively.The average linear regression coefficients of fitness index on body weight, calculated on an individual hen basis (selected breeders only) within lines and generations, were - 4·77 for the B line, 11·53 for the C line, and – 1·01 for the K line. The corresponding coefficients of fitness index on egg weight in the egg weight lines were – 0·543 for the D line, 0·947 for the E line, and – 0·712 for the L line. With the exception of the K line, the regressions on body weight were larger in the body weight lines than in the egg weight lines, and, similarly, the regressions on egg weight were larger in the egg weight lines than in the body weight lines. The differences between the regression coefficients of the high and low lines were statistically significant.