Urea-N Recycling in Lactating Dairy Cows

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This study was designed to determine the effect of rumen degradable protein (RDP) and rumen undegradable protein (RUP) on urea-N recycling and microbial N flow. Eight mid-lactation Holstein cows were assigned to a repeated 4 x 4 Latin square. The diets were isoenergetic with RDP and RUP concentrations arranged in a factorial design (10.0 and 12.5% RDP and 5.6 and 8.1% RUP as a percentage of DM).

The 10.0% RDP diets resulted in greater milk yield and lower milk protein concentration than the 12.5% RDP diets. High RUP diets tended (P = 0.1) to increase bacterial N flow in the liquid fraction. The NRC 1989 model predicted flow of microbial N and total N from the rumen more accurately than the NRC 2001. The NRC 2001 model predicted a higher RDP requirement and a lower RUP requirement for all four diets compared with the NRC 1989 model. Both models reflect the dietary changes that were intended by increasing the RDP and RUP in a factorial manner. There was no effect of RUP or RDP on the g/d of urea-N transferred from the blood to the gut or returning to the ornithine-urea cycle. However, plasma urea-N (PUN) incorporated into rumen microbial protein tended (P = 0.14) to increase with the low RDP diets. The rate of transfer of PUN to the gut appeared to be independent of PUN concentrations. The gene expression for urea transporters (bUT-B2) in the rumen did not change due to diet. As dietary protein intake increased, a constant amount (g/d), or decreasing fraction (g/g), of PUN was transferred to the gut. The apparent saturation of urea transporters in the gut prevented excess PUN from recycling to the gut on high-protein diets sparing the energy cost for N excretion that would have resulted from a constant percentage of PUN being recycled.