Effect of age and body weight at photostimulation on the sexual maturation of broiler breeder pullets transferred from 8L:16D to 16L:8D

Abstract

1. In two trials, broiler breeders were grown to a mean body weight of 2.0 to 2.2 kg at 20 weeks on 8L:16D and transferred to 16L:8D at various ages between 69 and 175 d of age at mean body weights varying between 1.0 and 2.7 kg. Other groups were grown to have a mean 20-week body weight 0.87, 1.15, 1.25 or 1.37 of the normal birds and photostimulated simultaneously with the normal weight birds. Controls were held on 8L:16D in each weight group. In a third trial, broiler breeders were transferred from 8L:16D to 16L:8D at 45 (ad libitum fed), 75 or 90 d (mild feed control), when the mean body weight of each group had reached 2.1 kg. Controls weighing 2.0 kg at 20 weeks were held on 8L:16D or 16L:8D. 2. In trial 1, photostimulating normal weight birds at 69, 76, 83 or 97 d delayed maturity and widened the spread of individual maturities, whilst transferring to 16L:8D at 111 or 125 d advanced maturity. Increasing the mean 20-week body weight to 2.8 kg advanced maturity in birds transferred to 16L:8D at 97, 111 or 125 d. Birds photostimulated at 69, 76 and 83 d matured at a similar age to controls held on 8L:16D. 3. In trial 2, transferring normal weight birds to 16L:8D at 91, 112, 133, 147, 161 or 175 d advanced maturity. Reducing the 20-week mean body weight to 1.91 kg delayed maturity in birds photostimulated at 91 or 112 d, but advanced it in birds photostimulated at 112, 133, 147, 161 or 175 d. Sexual maturity was similar for birds with a 20-week mean body weight of 2.54 or 2.74 kg, with advances for all groups photostimulated between 91 and 175 d. 4. In trial 3, birds maintained on 16L:8D matured later than birds held on 8L:16D, but maturity was advanced when birds were transferred to 16L:8D at 45, 75 or 90 d at a common body weight of 2.1 kg.

The effect of age at photostimulation of male broiler breeders on testes growth and the attainment of sexual maturity

“…Therefore, it would appear that the age at photostimulation had no influence on the mean age when semen production was first observed. The age at which testis growth was most rapid (t*) was earlier, in all cases, than the predicted age at 50% production in females, which was calculated as a more accurate indicator of sexual maturity in females than age at first egg, as not all pullets lay on consecutive days after the first egg (Lewis et al, 2007) (Figure 1). It appears as if males and females exhibit a similar response to photostimulation when using t* as an indicator of sexual maturity, rather than the age at which semen is first observed or produced, although t* is a measure of testis development, rather than sexual maturity.…”

“…Although it may be expected that males would naturally reach sexual maturity before their female counterparts, the apparent earlier development may not necessarily be accompanied by the production of functional spermatozoa during the rapid growth phase -this aspect possibly being delayed until the testes have reached their mature weight. Figure 1 Estimated age at 50% production of female broiler breeders ( ) (Lewis et al, 2007) and the age at which testis growth was most rapid in male broiler breeders ( ) photostimulated at different ages, with curves fitted by eye.…”

“…Females photostimulated from 8 to 16 h light/d at 69, 76, 83 or 97 d of age did not respond to photostimulation, and dissipation of photorefractoriness was delayed so that birds matured at the same rateas if they had been maintained on long days from hatch, as the delay in ASM was similar to the delay seen in females reared on 16 h light (Lewis et al, 2004). An advance in ASM was observed when females were photostimulated at 111 and 125 d, but a bimodal distribution was observed when the females were photostimulated at 111 d, as the ASM in some had been advanced while others matured as if reared on 16 h light/d (Lewis et al, 2007), suggesting that there is some genetic variation in the acquisition of photosensitivity. By 124 d the entire population had become photosensitive.…”

“…A controlled feeding programme recommended by the primary breeding company (Aviagen, 2006) was used throughout the growing period, where the average weight of birds in each room was used to allocate feed to keep body weights on the target growth curve. This was in an attempt to minimise the effect of body weight, as there is an expected interaction between body weight and acquisition of photosensitivitywhere birds become photosensitive earlier when on an accelerated growth curve Lewis et al, 2007). From 123 to 224 d of age, one or two birds from each room were sacrificed weekly, and the left and right testis were removed and weighed.…”

“…This suggests that photorefractoriness is dissipated at different rates, and that dissipation could occur at a faster rate when broiler breeders are exposed to short daylengths for a period of time. The length of exposure to short daylengths and the age at which birds acquire photosensitivity was previously investigated in female broiler breeders by Lewis et al (2007). Females photostimulated from 8 to 16 h light/d at 69, 76, 83 or 97 d of age did not respond to photostimulation, and dissipation of photorefractoriness was delayed so that birds matured at the same rateas if they had been maintained on long days from hatch, as the delay in ASM was similar to the delay seen in females reared on 16 h light (Lewis et al, 2004).…”