Leaf content of straw diets influences supplementation responses by sheep

Young sheep were offered separated barley straw fractions consisting predominantly of either leaf or stem components, either without or with one of the three supplements. Supplements consisted of urea and sodium sulfate (N/S), cereal grain mixed with urea and sodium sulfate (Bar/N) or fishmeal (FM) as a protein meal expected to provide digestible rumen-undegraded protein (DUP). Voluntary intake of stem-rich straw was only 29.7 g/kg W0.75 per day. Although intake of this stem-rich straw was not changed (P>0.05) by provision of any supplement, estimated metabolizable energy (ME) intake was increased up to 70% due to increases in diet digestibility and total DM intake. When no supplements were provided the voluntary intake of leaf-rich straw was similar to that of stem-rich straw. N/S and FM supplements increased intake of leaf-rich straw and of estimated ME by up to 112%, while Bar/N supplement did not change straw intake and increased estimated ME intake by 74%. All supplements increased (P<0.05, P<0.01 or P<0.001) liveweight (LW) change, feed conversion efficiency (FCE), nitrogen (N) retention and wool growth. However, the responses to the N/S and FM supplements were much greater with the leaf-rich than the stem-rich straw. LW change and wool growth was increased more by the FM supplement than could be attributed to the increased ME intake, suggesting an increase in the efficiency of utilization of ME when this supplement was provided. In conclusion, the experiment demonstrated that responses to supplements by young sheep offered cereal straw are dependent on the morphological composition of the straw.