Milk Urea Concentration in Dairy Sheep: Accounting for Dietary Energy Concentration

In dairy sheep milk urea concentration (MUC) is highly and positively correlated with dietary crude protein (CP) content and, to a lesser extent, with protein intake. However, the effect of dietary energy and carbohydrate sources on MUC of lactating ewes is not clear. Thus, the objective of this study was to assess the effects of diets differing in energy concentration and carbohydrate sources on MUC values in lactating dairy ewes. Two experiments were conducted (experiment 1, E1, and experiment 2, E2) on Sarda ewes in mid and late lactation kept in metabolic cages for 23 d. In both experiments, homogeneous groups of five ewes were submitted to four (in E1) or three (in E2) dietary treatments, consisting of pelleted diets ranging from low energy (high-fiber diets: 1.2-1.4 Mcal of net energy for lactation (NE_L)) to high energy (high-starch diets: 1.7-1.9 Mcal of NE_L) contents, but with a similar CP concentration (18.4% dry matter (DM), on average). Each diet had a different main ingredient as follows: corn flakes, barley meal, beet pulp, or corn cobs in E1 and corn meal, dehydrated alfalfa, or soybean hulls in E2. Regression analysis using treatment means from both experiments showed that the best predictor of MUC (mg/100 mL) was the dietary NE_L (Mcal/kg DM, MUC = 127.6 - 51.2 × NE_L, R² = 0.85, root of the mean squared error (rmse) = 4.36, p < 0.001) followed by the ratio CP/NE_L (g/Mcal, MUC = -14.9 + 0.5 × CP/ NE_L, R² = 0.83, rmse = 4.63, p < 0.001). A meta-regression of an extended database on stall-fed dairy ewes, including the E1 and E2 experimental data (n = 44), confirmed the predictive value of the CP/ NE_L ratio, which resulted as the best single predictor of MUC (MUC = -13.7 + 0.5 × CP/NE_L, R² = 0.93, rmse = 3.30, p < 0.001), followed by dietary CP concentration (MUC = -20.7 + 3.7 × CP, R² = 0.82, rmse = 4.89, p < 0.001). This research highlights that dietary energy content plays a pivotal role in modulating the relationship between MUC and dietary CP concentration in dairy sheep.