Effect of age at photostimulation on sexual maturation in broiler breeder pullets

Genetic selection programs in broiler stocks have contributed to changes in the responsiveness of the broiler breeder female to nutritional factors. A study was conducted to investigate the effect of photostimulation (PS) age on sexual maturity traits in current broiler breeder pullets. Seven hundred twenty Ross 308 pullets with full access to feed to 1 or 3 wk of age were divided among 8 environmental rooms to 16 wk of age. At this time, 128 pullets (64 per initial feeding treatment) were randomly assigned to individual cages in 8 rooms with 16 pullets per room. Pullets were PS at 17 (PS17), 19 (PS19), 21 (PS21), or 23 (PS23) wk of age by increasing the day length from 8L:16D to 16L:8D (multiple steps). Each hen was dissected on the morning after first oviposition (sexual maturity). Data were analyzed by 2-way ANOVA with additional analysis of covariance analysis done using sexual maturity on BW as a covariate. Sexual maturation occurred later with later PS ages. Rate of sexual maturation was accelerated at later PS ages, although age at PS did not affect variation in age at sexual maturity. Despite a 42 d difference in PS age between the PS17 and PS23 groups, only a 25 d difference existed in mean age at sexual maturity among these groups. Initial egg weight, breast muscle weight, frame size, and abdominal fat pad weight at sexual maturity increased in birds with later PS ages. Covariate analysis indicated that these effects were influenced primarily by increased age and BW at sexual maturity in later PS birds. Pullets PS at 17 wk of age had 11.0 large white ovarian follicles, compared with 14.9 in PS23 birds. Comparison to previous studies demonstrates that the years of genetic selection for growth traits have not changed the interval between PS and sexual maturation. Previous research published on this theme likely had a confounding effect of mean BW on estimates of variation at various PS ages. Photostimulating later presented advantages with regard to development of the reproductive tract; these effects were BW dependent.