The feeding value of cereal straws for sheep. III. Supplementation with minerals or minerals and urea

Abstract

The effects of supplementation with a complete mineral mix or with the mineral mix and urea and sulphate on the intake and digestion of three straws by sheep was examined in six experiments. The straws differed in chemical composition and physical characteristics, their in vitro organic matter digestibility being about 29% for a wheat straw designated as WS4, 45% for WS1 and 52% for a rice straw, M7. The mineral supplement had no effect on intake or digestibility of any of the straws. While it reduced or tended to reduce rumen ammonia levels with the two more digestible straws, there were no significant effects on non-ammonia nitrogen (NAN) flows from the abomasum or on amounts of crude protein apparently digested in the intestines (DCPi). The urea supplement had no effect on the intake and digestion of WS4; despite increased rumen ammonia levels there were no effects on NAN flow or DCPi. However, with the most digestible straw (M7), urea increased intake, rumen ammonia level, NAN flow and DCPi. Urea with WS1 increased intake (although this was only significant when expressed per unit liveweight) and it increased rumen ammonia levels, NAN flow and DCPi. It appeared that with WS4, the uptake of rumen-degradable nitrogen by rumen micro-organisms was limited by the availability of energy. The amount of digesta dry matter (DM) accommodated in the reticulo-rumen increased linearly with DM intake across all diets, while DM flow through the abomasum increased linearly with DM intake or amount of DM accommodated in the reticulo-rumen. In addition, NAN flow and DCPi were related linearly to digestible organic-matter intake. These results are discussed in relation to the regulation of feed intake by sheep given cereal straws.