Reproduction in broiler breeders has become a field of increased interest as continuing selection pressure for growth over the past decades has resulted in reproductive challenges specifically related to overfeeding. Yet the underlying endocrinological mechanisms of reproduction have not yet gained full attention. In broiler breeders, the process of sexual maturation before the onset of reproduction is affected by rearing photoperiod and BW or feed allowance before and after photostimulation. However, many of the underlying physiological and metabolic mechanisms as well as the dynamics of E2 remain unclear. Although rearing photoperiod is a major factor in the timing of sexual maturation in broiler breeders, no literature could be found investigating the effect of rearing photoperiod on E2 levels in broiler breeder hens during puberty. Therefore, the objective of this study was twofold. First, a model was developed as a tool to compare E2 levels in a holistic and integrative manner and to provide scientific insight into E2 profiles and dynamics. Second, the effect of rearing photoperiod and BW on plasma E2 levels in broiler breeders was interpreted using this novel methodology.
The experiment was a completely randomized design conducted as a 2 × 3 factorial arrangement of treatments with pullets reared either on a breeder-recommended target BW curve or an accelerated target BW curve reaching the 21 wk target BW at 18 wk (High), and maintained under 8L:16D, 10L:14D, or 12L:12D photoschedules during rearing. The High target BW was 22% higher than the Standard target BW at 21 wk of age. Ross 708 broiler breeder chicks (n = 180) were fed using a precision feeding (PF) system. The PF station recorded and controlled BW individually on a per visit basis after d 16. Cloacae of all hens were palpated daily just after lights turned on to detect the presence of a hard-shelled egg in the shell gland to measure AFE. From wk 20 to 28, weekly blood samples (2 mL) were taken from six randomly selected birds per BW × photoschedule treatment interaction for hormone extraction.
To our knowledge, this is the first time a mathematical methodology has been developed to describe and predict differences in E2 profiles and dynamics in broiler breeders. The model based on chronological age predicated that the duration between the E2-inflection point and AFE was longer in the Standard BW treatment compared to the High-BW treatment, whereas the model based on physiological age predicated that the duration between photostimulation and the E2-inflection point was longer in the Standard BW treatment compared to the High-BW treatment. In addition, the peak rate of E2 increase occurred consistently around 2.4 wk before AFE. The described methodology provides an example for other studies into endocrinological dynamics in poultry reproduction. The methodology is able to create value from less datapoints than previously possible and showed scientific insight into the dynamics of E2 concentration during sexual maturation in response to BW and rearing photoperiod. As the methodology is able to identify individual dynamics in E2 plasma concentration these individual parameters could potentially serve breeding purposes.
As broiler breeders face increased reproductive challenges specifically related to overfeeding, a clear understanding of the physiological effects of BW and rearing photoperiod on reproductive development is needed. The objective was to use mathematical models to compare plasma estradiol-17β (E2) concentration to characterize the effect of BW and rearing photoperiod on E2 levels. A 2 × 3 factorial arrangement of treatments was used. Hens (n = 180) were fed with a precision feeding system to allocate feed individually to achieve the breeder-recommended BW curve (Standard) or to a BW curve reaching the 21 wk target at 18 wk (High). Hens were on 8L:16D, 10L:14D, or 12L:12D photoschedules during rearing and were photostimulated at 21 wk. Age at first egg (AFE) was recorded. Plasma E2 levels were determined weekly between week 20 and 28. Two modified Gompertz models described E2 level as a function of (a) chronological or (b) physiological (relative to AFE) age. Timing of E2-inflection point was compared between models and treatments. Differences were reported as significant at P ≤ 0.05. The chronological age model inferred that High BW reduced the duration between the E2-inflection point and AFE, whereas the physiological age model inferred that High BW only reduced the duration between photostimulation and the E2-inflection point. Hens on the Standard BW treatment had a longer period between photostimulation and the E2-inflection point compared to hens on the High-BW treatment (11.03 vs. 1.50 wk, respectively, based on physiological age). Hens on the 12L:12D photoschedule had a longer period between photostimulation and the E2-inflection point compared to hens on the 8L:16D or 10L:14D photoschedule, both in the Standard and High BW (28.91 vs. 1.78 and 2.40 wk, 2.65 vs. 0.93 and 0.94 wk, respectively, based on physiological age). The described methodology and results provide quantitative insight into E2 dynamics and serves as a model for future endocrinological studies in poultry reproduction.
