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In broiler breeders, rearing photoperiod and BW both affect sexual maturation and productivity, and to date those effects have been reported as independent. Photoperiod needs to be controlled during rearing to dissipate juvenile photorefractoriness. Independent of photoperiod, higher than recommended BW at the end of rearing accelerated sexual maturity (age at first egg), whereas a lower than recommended BW delayed sexual maturity. Therefore, both target BW and the timing and level of feed restriction may affect sexual maturity and reproductive efficiency. The aim of the current research was to investigate the interaction between BW and rearing photoperiod on group housed broiler breeder reproductive performance with minimal BW variation. It was hypothesized that onset of lay would be delayed in lower BW hens under extended photoperiods, and egg production would be reduced. Conversely, within photoschedule treatments, higher BW hens would dissipate photorefractoriness and mature more quickly, thereby increasing total egg production.
The experiment was conducted as a 2 × 3 factorial arrangement of treatments with pullets reared either on a breeder-recommended target BW curve (Standard) or an accelerated target BW curve reaching the 21 wk BW at 18 wk (High), and maintained under 8L:16D, 10L:14D, or 12L:12D photoschedules during rearing. The resulting High target BW was 22% higher than the Standard target BW at 21 wk of age. Ross 708 broiler breeder chicks (n = 180) were randomly allocated in 6 environmentally controlled rooms. Each room was equipped with a precision feeding (PF) system, which controlled individual feed intake to achieve and adhere to the assigned target BW curves. Hens from all treatments were photostimulated at week 21 with a single abrupt step to 16L:8D. At week 18, a nest box was installed in each room, which identified eggs of individual hens. At week 21, 3 roosters were introduced to each room. For the first 2 wk, pullets were weighed manually on a daily basis. After individual feeding started, the PF system recorded individual BW and feed intake on a per visit basis. Because it would not be possible for floor eggs to be linked with individual hens because hens on different BW treatments were housed in the same room, cloaca of all hens were palpated daily to detect hard-shelled eggs in the shell gland to measure age at first egg and individual egg production from 20 to 36 wk. Eggs assigned to individual hens were weighed daily. At week 55, all remaining hens were killed and dissected. Abdominal fat pad, filled gastrointestinal tract (GIT), breast muscle (total weight of pectoralis major and pectoralis minor), heart, liver, oviduct (without content), and ovary weight were recorded.
Actual Standard and High BW profiles closely matched their target profiles up to 24 wk of age. BW CV of the High BW treatment increased after photostimulation compared to the Standard BW treatment. We hypothesize that this was mainly because hens on the High BW treatment started laying earlier compared to hens on the Standard BW treatment. Hens on the High BW treatment started laying 34 d earlier than hens on the Standard BW treatment. The advance of sexual maturity was 1.6, 9.1, and 12.7 d per 100 g increase in BW at 20 wk of age for the 8L:16D, 10L:14D, and 12L:12D rearing photoschedule, respectively. Sexual maturity was delayed and egg production reduced in hens reared on increased photoperiods. Hens reared on 10L:14D tended to start laying later than hens on the 8L:16D photoschedule (15 d, P = 0.082) and hens reared on the 12L:12D photoschedule started laying 61 d later (P < 0.001) than hens on the 8L:16D photoschedule. This confirms that modern broiler breeders are photorefractory at hatch, and that the photorefractory state was dissipated by a short photoperiod in a photoperiod dependent manner. We found that the effect of rearing photoschedule on sexual maturity and egg production was dependent on BW. High BW × 8L:16D or 10L:14D hens did not differ in age at sexual maturity (173 vs. 172 d, respectively), but Standard BW hens showed a significant delay in sexual maturity when photoperiod increased from 8L:16D to 12L:12D (180 vs. 266 d, respectively), while the 10L:14D treatment was intermediate (212 d). There was a significant interaction between the effect of BW and rearing photoschedule on egg weight (P < 0.001). Egg weight was not different between the 8L:16D and 12L:12D photoschedules, but for the 10L:14D photoschedule, egg weight of Standard BW hens was 1.5 g lower compared with High BW hens. Standard BW hens had a higher proportional breast weight and a lower proportional fatpad weight compared with High BW hens (27.5 vs. 25.8%; P = 0.006, and 1.5 vs. 2.4%; P < 0.001), respectively, which coincided with lower egg production. In addition, Standard BW hens had a lower proportional ovary weight and lower number of LYF compared to High BW hens.
To our knowledge, this is the first time an interaction has been shown between the effects of BW and rearing photoperiod on reproductive performance in broiler breeders. These results suggest that greater BW or feed intake might override negative signals such as increased photoperiods against sexual maturation. In addition, increasing the target BW for breeder hens could increase egg production, particularly when managing BW with a PF system, and might counteract the negative effects of increased photoperiod during rearing in open housed facilities. We suggest further studies to investigate body fat thresholds in broiler breeders. In addition, we suggest further investigation into physiological and neuroendocrinological cues behind the effects of BW and rearing photoperiod.