Effects of protein and energy intakes on the wool growth of Merino wethers

Abstract

The results of an experiment are presented which show that the supply of both protein and energy to the small intestine of adult sheep affects the rate of wool growth, and that the direction and magnitude of the response to energy absorption is dependent upon the particular level of protein given. Thirty fine-wool Merino wethers received their entire diets, which consisted of reconstituted spray-dried cow's milk, sodium caseinate, glucose, butterfat, and micronutrients, by direct abomasal infusion in an experiment consisting of two parts. In part I the effect of protein intake (20,40, 60, 80, or 100 g/day) at constant energy intake (5.02 MJ/day) was investigated, while in part II the effect of energy intake (3.01, 4.02, 5.02,7.53, or 10.04 MJ/day) at three levels of protein intake (20, 60, or 100 g/day) was studied. A nitrogen balance trial was conducted with selected animals during 7 days near the end of each part of the experiment. In part I, as the daily intake of protein was increased from 20 to 80 g, the wool growth rate increased almost linearly from 2.3 to 9.0 g/sheep/day. With a further rise in protein intake to 100 g/day the mean value fell to 8 .O g/sheep/day. The overall trend in wool growth with increasing protein intake was significantly quadratic. In part II, for sheep receiving 100 g protein/day, an increase in energy intake from 3.01 to 10.04 MJ/day caused a highly significant, linear rise in wool growth rate of 52 %. For animals receiving 60 g protein/day, wool growth rate tended to increase at first and then fell again as energy intake was raised from 3.01 to 10.04 MJ/day, but the differences were not significant. When sheep were given only 20 g protein/day, an increase in energy intake from 3.01 to 10.04 MJ/day resulted in a decline in wool growth rate of 45 %. The results suggest that there is an optimum ratio of protein to energy absorption required for maximum wool growth in sheep. If protein is limiting, an increase in protein absorption stimulates wool growth, but an increase in energy absorption reduces it. Conversely, if protein is in excess an increase in protein absorption reduces wool growth, but it is stimulated by an increase in energy absorption. The practical significance of the results is discussed.