The effect of age at photostimulation of male broiler breeders on testes growth and the attainment of sexual maturity
Male broiler breeders were photostimulated at different ages (56, 77, 98, 119, 147 and 161 d) during the period of growth prior to achieving sexual maturity to observe their response to photoperiod stimulation. Birds were sampled at regular intervals to measure the average testis weight - these values responding curvilinearly to the age at photostimulation, similar to the attainment of sexual maturity observed previously in females. However, males reached sexual maturity at an earlier age than females, as measured by the start of semen production. Based on the age at first semen production, no differences in the mean age at sexual maturity (ranging from 164 to 172 d) resulted from the different ages at photostimulation. Testis weights from males on different photostimulation treatments showed that some males respond to early stimulation, but from 189 d of age some of these early photostimulated males still had an average testis weight of less than 10 g, while males photostimulated 98 d and later all had an average testis weight over 10 g. Serum testosterone concentration measured at 351 d also showed that early photostimulation resulted in two groups of birds that either responded or not. At 351 d the serum testosterone concentration was more evenly distributed in males photostimulated after 98 d, while males photostimulated before 98 d had either high or low serum testosterone concentrations. Directional asymmetry was observed in testis weights, with the left testis being bigger than the right. Other bilateral traits measured also showed directional asymmetry, an indication of developmental instability. However, the degree of relative asymmetry was not related to treatment. All males should respond to photostimulation by 98 d but some respond earlier, and these could be used by broiler breeding companies to eliminate seasonality in these birds.