Physiological responses of hens divergently selected on the number of chicks obtained from a single insemination.

A series of experiments was conducted in domestic fowl to investigate the consequences of five generations of divergent selection for increased (L+) or decreased (L-) numbers of hatched chicks. After artificial insemination with pooled ejaculates within the same line (L+ males x L+ hens or L- males x L- hens), significant differences were observed between L+ and L- hens for mean fertility rates (L+ 94.8%, L- 70.2%, P < 0.0001) and for effective and maximum duration of fertility (P < 0.00001). A comparison of the overall laying performance and shell quality between the two selected lines showed that L- hens laid fewer eggs than L+ hens (P < 0.00001) and L- eggs had poorer shell quality (shell breaking strength) than L+ eggs (P < 0.00001). These observations were associated with significantly higher percentages of early embryo death in eggs from L- hens compared with L+ hens. Another series of experiments revealed the presence of larger initial populations of spermatozoa in the sperm storage tubules as well as in the perivitelline layer of eggs from L+ hens. The populations of spermatozoa in the sperm storage tubules of commercial laying hens inseminated with pooled semen samples from L+ males was compared with those in hens inseminated with samples from L- males to determine whether the variations in oviductal sperm storage between the two lines were male dependent. No significant differences between the populations of spermatozoa present in the sperm storage tubules of either group of hens could be detected at any of the intervals examined after insemination (days 1, 3 and 10). Finally, an experiment conducted on hens originating from the two selected lines indicated that the utero-vaginal junction of L+ hens contained significantly more sperm storage tubules compared with L- hens (P < 0.01). It is concluded that selection based on overall reproductive performance modifies the number of eggs capable of developing viable embryos and also influences the efficacy of initial sperm storage by increasing or altering the population of sperm storage tubules located in the utero-vaginal junction. Such changes have major consequences on the duration of the fertile period, which in avian species is directly dependent on both the actual population of spermatozoa stored in the oviduct and on their rate of release from the storage sites.