Factors governing faecal nitrogen wastage in sheep

Abstract

Dietary factors controlling faecal nitrogen loss in sheep have been examined in selected metabolism data. Two factors, total nitrogen and dry matter intake, accounted for a large part of the variation in faecal nitrogen. The first group of data consisted of 44 observations from sheep fed five "nitrogen- free" diets. This approach is commonly used to estimate metabolic faecal nitrogen (M.F.N.). Analysis showed that the regression of M.F.N. on intake of dry matter, for the combined data, had a significant intercept of +0.70 g/day. Hence the common expression of X.F.N. estimates in terms of unit intake will lead to errors. These would be important with the low levels of intake frequently associated with unpalatable "nitrogen-free" diets. When in addition an allowance is made for the small quantities of nitrogen contained in these diets, there is excellent agreement between observations from different sources. The second group of data consisted of 99 observations, from two sources of pasture diets, covering a wide range of nitrogen intakes. High correlations (r = +0.900 and + 0.936) were recorded between total faecal nitrogen and intake of dry matter. However, at high and low levels of total nitrogen intake there was evidence of departure from a common linear relationship. A positive effect of nitrogen per se was shown where total nitrogen intake was less than 8 or more than 46 g/day. The significance of these findings is discussed with reference to: (1) The applicability of faecal nitrogen index methods which have been proposed for the calculation of grazing intake. (2) The assumption that the metabolic component of total faecal nitrogen is constant.