Feed intake, body composition, and plasma metabolic hormones in Merino sheep that differ genetically in fleece weight or fibre diameter

Abstract

Negative genetic correlations result in sheep selected for fleece weight having reduced fatness and reproductive turnoff. Both of these characteristics depend mainly on energy metabolism, but the links between wool growth and energy metabolism are poorly defined. The present study examined aspects of energy metabolism (body composition, feed intake, and metabolic hormones) in Merino sheep with high or low estimated breeding values for clean fleece weight (CFW) or fibre diameter (FD). The groups were selected to have similar mean liveweights. High CFW sheep had a lower proportion of fat (P < 0.01) and more lean tissue (P < 0.05) in their body, and ate more of a medium-quality diet (P < 0.05). Intake of a straw diet with low digestibility that imposed a physical constraint on feed intake was not associated with CFW. When fed at the same level (1.2 × calculated maintenance), high CFW sheep had lower plasma concentrations of glucose (P < 0.01), insulin (P < 0.05), and insulin-like growth factor-1 (P < 0.05), and higher concentrations of growth hormone (P < 0.001). Responses of plasma insulin or glucose to treatment with insulin, adrenaline, or propranolol were independent of CFW. No differences were found between the high and low FD groups, except that the plasma cortisol response to adrenocorticotrophic hormone (ACTH) was higher in sheep with low FD. Calculations indicate that the lower fat reserves and plasma insulin and glucose concentrations in the high CFW sheep are sufficient to account for previous reports of reduced number of lambs weaned in high CFW sheep. Furthermore, the results indicate that high CFW sheep have smaller metabolic energy reserves to withstand unfavourable feed conditions, and so may be less able to thrive and reproduce successfully in harsh feed environments.