Controlling the partitioning of energy intake in meat-type chickens

Abstract

Meat-type chickens have been bred for prioritizing energy partitioning to rapid gain and lean tissue growth, which required feed restriction to be commonly applied in the industry to optimize egg production. This thesis studied the effects of controlling energy intake on energy partitioning to maintenance, growth, and reproduction in meat-type chickens (broilers and broiler breeders). Precision feeding was used as a tool to control feed intake or body weight (BW) and collect the required feed intake and BW data of each individual bird. A modelling methodology was compared with the comparative slaughter technique (CST; industry standard) in broilers to estimate energy partitioning to maintenance and growth. Maintenance and growth were estimated by CST and with a non-linear mixed model explaining daily metabolizable energy intake as a function of metabolic BW and daily gain. The estimated values from the model approached the values estimated by the CST. BW and lighting both have a large effect on sexual maturation and egg production in broiler breeders, but their interaction was hitherto unknown. It was hypothesized that an increase in BW above the breeder-recommended target and an advanced age at which photoperiod would be increased (photostimulation) would advance the onset of lay (sexual maturity) and increase egg production. Sexual maturity was advanced and maturation interval between photostimulation and sexual maturity was shorter for hens with higher BW compared to hens on the breeder recommended BW target. Hens photostimulated at week 21 matured earlier and had a higher egg production compared to hens photostimulated at week 18. It was also hypothesized that the effect of the light schedule during the rearing phase would depend on BW. Therefore, two growth curves and three rearing photoperiod treatments were compared. The age at sexual maturity did not differ between hens on an 8 h or 10 h rearing light schedule at a higher BW target, but the 12 h rearing photoperiod delayed sexual maturity at the higher BW. Hens at the breeder recommended target had delayed sexual maturity with the 8 h, 10 h, and the 12 h 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 12 h treatment did not commence egg production during the experiment. The plasma concentration of estradiol-17β (E2), an important hormone involved in sexual maturation, was measured to study the underlying cause of the differences between treatments. Two models were developed based on modified Gompertz curves, to describe 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 age at which E2 increased at the highest rate compared to hens on a higher BW target. Hens on the 12 h rearing photoperiod treatment had a longer period between photostimulation and the age at which E2 increased at the highest rate compared to hens on the 8 h and 10 h in both BW target treatments. It was hypothesized that hens on the shorter rearing photoperiod and with decreased BW, would be more energetically conservative. The model for energy partitioning in broiler breeders included a random effect for individual maintenance requirement and age-related maintenance requirements and provided a biologically sound estimation of life-time energy partitioning. Although it was estimated that hens on the recommended BW target with a 12 h rearing photoperiod were most energetically conservative, their egg production was the poorest. This thesis concludes that current recommended breeder BW could be too low for optimal sexual maturation after photostimulation in precision fed broiler breeders. Even when BW variation is minimized through precision feeding, early photostimulation is not recommended. Increased BW partially counteracted the effect of longer photoschedules on sexual maturity in broiler breeders and that this effect depended on BW. The described modeling methodologies and results provide quantitative insight into E2 dynamics and energetic partitioning during the broiler breeder hens’ life-time.