Whole-body protein metabolism and energy expenditure in sheep selected for divergent wool production when fed above or below maintenance

Abstract

Realised wool growth relative to genetic potential depends on the availability of nutrients to the wool follicle (Hynd 1989; Williams 1978, 1995). Nutrient availability, in turn, is determined by the supply of nutrients absorbed from the gut (a function of diet) and those released from body stores of protein and fat (Adams et al. 2006). It has been proposed that Merino genotypes selected for or against wool growth have different rates of protein and energy metabolism (Adams et al. 2006; Adams and Cronjé 2003). Protein and energy metabolism play an important role in regulating body composition, which has consequences for the survival of the sheep when underfed, and later for the rate of repletion of body tissues when sufficient feed becomes available (Chilliard et al. 2000). Positively moderate genetic correlation between clean wool growth and liveweight (0.22 ± 0.09), and eye-muscle depth (0.35 ± 0.19) (Safari and Fogarty 2003) indicates that it is possible, genetically, to achieve improvements in both wool and meat production at the same time. However, without knowledge of the effects of such selection on feed intake, it is not clear whether the improvements in production result from an increased efficiency of ingested protein and energy utilisation for wool production and body-tissue growth, or whether these sheep simply consume more feed and use more nutrients. The present study was designed to determine whether sheep with different genetic capacity for wool production alter their energy and protein metabolism in response to different nutrient supply. This study contains estimates of whole-body protein and energy metabolism and their relationships with body composition in Merino sheep of known estimated breeding values (EBVs) for wool growth and liveweight in response to different feed intakes.