Nitrogen and sulfur mobilised from body tissue can be used for wool growth

Abstract

The hypothesis was tested that when nitrogen (N) and sulfur (S) are mobilised from body tissue the rate of wool growth is determined both by the amount of N and S apparently digested and by the amount of N and S mobilised from body tissue. The experiment consisted of seven 5-day periods (Periods 1–7) and N and S retention were measured during each period in 12 mature Merino wethers. During Periods 2 and 3, casein and methionine were infused into the abomasum of all sheep to increase the amount of N and S absorbed in the small intestine. Three nutritional regimes were used in order to manipulate the amount of N and S mobilised from body tissues after the cessation of the abomasal infusion: one group of sheep (n = 4) was fed ad libitum throughout the experiment (Group A), another group (n = 4) was fed at a level calculated to maintain liveweight throughout the experiment (Group M), and the third group (n = 4) was fed ad libitum pre-infusion and during the infusion but was then fed at a level calculated to maintain liveweight after the infusion ceased (Group AM). The infusion of the supplement increased wool growth, and when the infusion ceased, it took 10 days for wool growth to return to a steady rate of growth. Wool growth per unit of apparently digested N or S increased markedly when N or S was mobilised from body tissue, indicating that mobilised protein was used for wool growth. Based on the relationship between the apparent retention of N and S in body tissues excluding wool, and changes in plasma amino acid concentrations, it appeared that the supply of S-containing amino acids from body tissue limited the use of mobilised protein for wool growth.