Nitrogen mineralization from sheep faeces can be predicted from the apparent digestibility of the feed

Abstract

It is difficult to predict plant availability of N in faeces because most faecal N is bound in organic form. In this study the influence of diet and faeces composition on mineralization of sheep faeces in soil were investigated. Net mineralization of C and N from 16 different samples of sheep faeces originating from sheep fed different known diets was studied after incubation in a sandy soil. After 4 weeks net mineralization of N ranged from −41 to 9% of faeces N and after 12 weeks −28 to 43% was net mineralized. Mineralization was related to different feed and faeces characteristics (apparent digestibility, NDF, ADF, crude fibre, lignin, C/N ratio and N concentration) by regression analysis. The two single factors showing the highest correlation with N mineralization after 12 weeks were the log transformed N concentration of faeces and the C/N ratio (r=0.84 and r=−0.87). The N concentration and C/N of faeces were closely related to apparent digestibility of the feed (r=0.88 and r=−0.93), and apparent digestibility was the feed characteristic showing the highest correlation with faeces N mineralization after 12 weeks (r=0.81). The estimate of faeces N mineralization was not improved when chemical characteristics of the feed were combined in a multiple linear regression analysis. Mineralization of sheep faeces C showed the highest correlation with NDF of the feed (r=−0.89) and N concentration of faeces (r=0.88). The study showed that it is difficult to make reliable predictions of the mineralization of sheep faeces N in soil based on chemical analyses of the feed. However, when using a biological measure of the feed quality (apparent digestibility) a robust prediction of faeces N mineralization was possible.