Factors limiting the intake of feed by sheep. II. Studies with wheaten hay

Abstract

A study was made of factors limiting the sheep's intake of a wheaten hay diet having an organic matter digestibility of approximately 57% and containing 4.4% crude protein and a mineral supplement. Voluntary consumption of chopped wheaten hay increased by 8–13% when protein was infused per abomasum, or when the protein content of the diet was raised to 7 or 15% by the addition of wheat gluten. A similar change in voluntary feed consumption (VFC) was observed in one of two experiments in which urea was infused per abomasum. The VFC response to protein infusion per abomasum persisted for at least 2 weeks after the infusion was terminated. Voluntary consumption of a diet of chopped wheaten hay + gluten (85/15) did not change when additional protein and micronutrients were provided. When the wheaten hay was ground or ground and pelleted, VFC increased and digestibility declined; the intake of digestible energy showed little change. The addition of protein to the ground and pelleted hay was accompanied by substantial rises in feed intake, digestible energy intake, and digestible fibre intake and in the quantity of organic matter transferred through the alimentary tract. The provision of protein and grinding and pelleting increased the intake of wheaten hay to 189% of that attained when chopped hay was offered. The intake of wheaten hay + gluten (85/15) was lower than that of lucerne hay when the diets were in either chopped or ground and pelleted forms; these diets had similar organic matter digestibilities and protein contents. The results were considered to support the conclusion that the primary factor limiting the intake of the wheaten hay was a deficiency of nitrogen, and that after this had been remedied, the next limiting factor was the resistance of the diet to removal from the rumen. The mechanisms whereby various factors limit intake are discussed, and it is suggested that digestibility is likely to be unreliable for predicting the VFC of non-pregnant, non-lactating ruminants.