ABSTRACT

Understanding factors affecting metabolizable energy (ME) availability for (re)productive processes is an important step in optimal feed formulation and breeding in poultry. The first experiment in this thesis compared a mathematical modelling methodology with the comparative slaughter technique (CST) to estimate energy partitioning to heat production (HP) and energy retention (RE) and investigated differences in heat dissipation. Two diets were used with differing energy content. HP and RE were estimated by CST and with a non-linear mixed model explaining daily ME intake as a function of metabolic body weight (BW) and daily gain. The model underestimated HP and overestimated RE compared to the CST. The second experiment studied the interaction between BW and rearing photoperiod on reproductive performance in precision fed broiler breeders. The effect of rearing photoperiod depended on BW; age at sexual maturity did not differ between hens on 8L:16D or 10L:14D photoschedules at a higher BW target, but the 12L:12D rearing photoperiod delayed sexual maturity at the higher BW. Birds at the breeder recommended targets had delayed sexual maturity with both the 10L:14D and the 12L:12D rearing photoschedule treatments. All hens on the higher BW treatment laid at least one egg before the end of the experiment. Almost 40% of the hens on the breeder recommended target and the 12L:12D rearing photoschedule treatment did not commence egg production. Mathematical models were developed to compare plasma estradiol-17β (E2) concentration and characterize the effect of BW and rearing photoperiod on E2 levels. Plasma E2 levels were determined weekly between week 20 and 28. Two modified Gompertz models described E2 level as a function either chronological or physiological age (i.e. relative to age at first egg). Hens on the breeder recommended BW target had a longer period between photostimulation and the E2 inflection point compared to hens on a higher BW target. 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 in both BW target treatments. Finally, an energy partitioning model was fitted to the data of experiment 2. The model included a random effect for individual maintenance requirement and age-related maintenance requirements in broiler breeders. This model provided a biologically sound estimation of life-time energy partitioning. Although hens on the recommended BW target with a 12L:12D rearing photoschedule were most energetically conservative, their reproductive performance was the poorest. It is concluded that current recommended breeder BW may be too low for optimal sexual maturation after photostimulation in precision fed broiler breeders. It is also concluded that increased BW partially counteracts the effect of longer photoschedules on sexual maturity in broiler breeders and that dissipation of the photorefractory state depends on BW. The modeling methodologies and results in this thesis provided quantitative insight into E2 dynamics and energetic partitioning during the broiler breeder hens’ life-time.