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Broiler breeders are kept on a strict level of feed restriction to manage their reproductive performance. Especially during the rearing period when broiler breeders are most restricted, high competition for feed within broiler breeder flocks results in high BW variation. It is known that pullets that are underweight at photostimulation subsequently exhibit lower egg production. Flocks with a high variation in BW exhibit low production efficiency as a high proportion of hens weigh less than the target at photostimulation. As recent developments in feeding technology have allowed group housed hens to be reared toward individual target BW with less than 2% CV for BW, the aim of this research was to investigate the effects of BW and age at photostimulation on broiler breeder reproductive performance in group housed hens, when within-treatment variation in BW is minimized. It was hypothesized that hens following a higher BW profile would show faster dissipation of photorefractoriness at the same age at photostimulation and therefore show an increased egg production, due to a lengthened laying period because of an earlier onset of lay.
The experiment was conducted as a 2 × 2 factorial arrangement of treatments with pullets being reared following the breeder-recommended target BW curve or an accelerated target BW curve reaching the 21 wk BW at 18 wk (High), and photostimulated at either wk 18 (18WK) or wk 21 (21WK). As a result, the High target BW was 22% heavier than the Standard target BW at 21 wk of age. Ross 708 broiler breeder chicks (n = 120) were randomly allocated to one of 4 environmentally controlled rooms (30 chicks per room). Each room was equipped with a precision feeding (PF) station which controlled individual feed intake to achieve and adhere to the assigned target BW curves. At the start of the experiment, pairs of rooms were randomly assigned to either a 18WK or 21WK photostimulation treatment. Photostimulation was achieved in a single step to 16L:8D. The light source used was a 60% red, 20% green, and 20% blue LED light bulb set to provide 8 lux during the rearing phase and 25 lux during the production phase. The PF station recorded BW and feed intake individually on a per visit basis after individual feeding started. Because it would not be possible for floor eggs to be linked with individual hens, and hens on different BW treatments were housed in the same room, all hens were palpated daily via the cloaca to detect hard-shelled eggs in the shell gland. At wk 55, 16 hens per BW × photostimulation treatment were killed and dissected. The abdominal fat pad, full gastrointestinal tract (GIT), breast muscle (total weight of pectoralis major and pectoralis minor), heart, liver, oviduct (without content), and ovary weight were recorded. In addition, the number of yellow follicles larger than 10 mm (LYF) was recorded.
According to the analysis of variance, the CV in BW was dependent on age (P = 0.012) and on BW treatment (P < 0.001). All High BW hens had reached the 21 wk breeder recommended target BW at wk 18. At wk 20, the BW of High BW hens was higher than Standard BW hens (P < 0.001) and there was no effect of age at photostimulation on BW. At wk 20, the High BW hens were 2,423 and 2,417 g, and Standard BW hens were 1,978 and 1,975 g (±21 g) on the 18WK and 21WK treatments, respectively. As expected, there was a lower AFE for the High BW treatment compared to the Standard BW treatment (178.1 vs. 194.7 d, P = 0.036). AFE was not different for the High BW × 18WK birds compared to the Standard BW × 21WK birds (182.8 vs. 180.4 d), where it was expected that the High BW × 18WK treatment would have matured earlier, as it was anticipated that they would have reached the minimum BW target for sexual maturation. Looking at the rate at which birds started laying, from the rapid increase at wk 22 and the flattening of the curve after wk 24, it can be estimated that approximately 40% of the Standard BW birds and approximately 60% of the High BW birds were responsive to photostimulation at wk 18. These birds responded uniformly to photostimulation by sexually maturing, thus indicating that all 3 levels of the reproductive axis (hypothalamus, pituitary, and ovary) were in a ready state. All hens on the High BW × 21WK treatment laid their first egg before the end of the experiment, but 11.7% of the hens on the High BW × 18WK treatment never commenced egg production. For the Standard BW hens, 31.9 and 3.3% never commenced egg production on the 18WK and 21WK treatment, respectively. No interaction was found between BW and age at photostimulation for egg production. Cumulative egg production was higher in the High BW treatment and 21WK treatment compared to the Standard BW treatment and 18WK treatment, and the effect of photostimulation and BW were independent.
Even when within-treatment variation in BW was minimized, decreasing the age at photostimulation from wk 21 to 18 increased the variability in age at sexual maturity and decreased reproductive performance of broiler breeders. The current results indicate that the recommended breeder BW at wk 21 is below the optimal target for maturation after photostimulation. It is hypothesized that the hypothalamic responsiveness and dissipation of the photorefractory state might also be influenced by additional metabolic triggers resulting from a difference in feeding frequency and feed allocation.