Effect of rumen undegradable protein and forage composition on nitrogen utilization of lactating dairy ewes

Grazing dairy ewes consume diets high in crude protein (CP), but may lack sufficient rumen undegradable protein (RUP) to maximize milk and milk protein yield. Increasing dietary RUP may increase milk yield and nitrogen utilization in dairy sheep.;In a trial based on stored feeds, increasing RUP from 7.6 to 9.7% of dry matter (DM) increased milk, milk fat, and milk protein yield by 14, 14, and 13%, respectively. This increase was noted in both low and high producing ewes. In diets with greater RUP (% of CP) milk nitrogen efficiency increased by 11%.;Two subsequent trials (cut-and-carry and grazing) were conducted to evaluate the effects of RUP supplementation and forage composition on milk yield and N utilization of ewes consuming fresh forage. In both trials, there was no interaction of RUP supplementation and forage composition. In the cut-and-carry trial, RUP supplementation tended to increase milk yield. In both trials, there was a linear effect of alfalfa inclusion level on milk yield, milk protein yield, and DM intake. In both trials, increasing alfalfa increased milk urea N and decreased N efficiency.;The utilization of rumen degradable protein by rumen microbes is related to the availability of dietary energy. The final trial evaluated the effects of protein degradability and energy level on nitrogen utilization. Dietary energy level did not affect milk yield, but ewes on high energy diets may have experienced subclinical rumen acidosis. There was no consistent effect of RUP on milk yield or milk protein yield. All diets contained RUP levels above 7% of DM, suggesting that all experimental diets may have provided sufficient RUP to support milk and milk protein yield.;The results of these trials suggest that supplying RUP at 9.7% of DM to lactating ewes will increase milk yield and milk protein yield, but further research is needed to identify specific RUP requirements for dairy ewes. Finally, forage composition and availability are determinants of milk yield in grazing ewes. Pastures should be managed to allow 50% legume and total DM intake levels of 3.4 to 3.7% of body weight/ewe/d.