PHYSIOLOGICAL-RESPONSES OF HENS DIVERGENTLY SELECTED ON THE NUMBER OFCHICKS 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 increa sed (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<0.0000 1). A comparison of the overall laying performance and shell quality b etween 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 bre aking strength) than L+ eggs (P < 0.00001). These observations were as sociated with significantly higher percentages of early embryo death i n eggs from L- hens compared with L+ hens. Another series of experimen ts 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 sampl es from L+ males was compared with those in hens inseminated with samp les from L- males to determine whether the variations in oviductal spe rm storage between the two lines were male dependent. No significant d ifferences between the populations of spermatozoa present in the sperm storage tubules of either group of hens could be detected at any of t he intervals examined after insemination (days 1, 3 and 10). Finally, an experiment conducted on hens originating from the two selected line s indicated that the utero-vaginal junction of L+ hens contained signi ficantly more sperm storage tubules compared with L- hens (P < 0.01). It is concluded that selection based an overall reproductive performan ce modifies the number of eggs capable of developing viable embryos an d also influences the efficacy of initial sperm storage by increasing or altering the population of sperm storage tubules located in the ute ro-vaginal junction. Such changes have major consequences on the durat ion of the fertile period, which in avian species is directly dependen t on both the actual population of spermatozoa stored in the oviduct a nd on their rate of release from the storage sites.