Effect of dietary supplementation of slow-release urea on milk production, nutrient digestibility, rumen parameters, and blood metabolites of Holstein dairy cows

The current study assessed the effects of slow-release urea (SRU) supplement on milk yield and composition, rumen fermentation, protozoa community, blood and hematological biomarkers, and nutrient digestibility of early-lactating Holstein dairy cows. 32 multiparous lactating Holstein cows were randomly assigned to treatments in a replicated 4 × 4 Latin square design. The treatments were as follows: (1) CON diet (no urea supplement); (2) CON diet + 0.38% uncoated urea (U38); (3) CON diet + 0.43% SRU (SRU43); and (4) CON diet + 0.86% SRU (SRU86) on DM basis. Diet supplemented with U38 had lower DMI (26.2 kg/d) than CON (27.2 kg/d), SRU43 (27.1 kg/d), and SRU86 (26.8 kg/d; p = 0.04). Yields of milk, fat, and lactose were not affected by the treatments, whereas ECM yield (p = 0.09) and milk protein concentration (p = 0.07) tended to be lower for U38 compared to CON. Apparent digestibility of NDF (p = 0.02) and OM (p = 0.03) for SRU43 diet was higher compared to CON group. Concentration of NH3–N was numerically higher for U38 compared to the CON group (16.6 vs. 13.3 mg/dL, respectively; p = 0.12). The U38 diet raised (p ≤ 0.02) the concentration of plasma TG and urea compared to the other diets. The treatments did not affect plasma hematological indicators. The results of the current study demonstrated that dietary SRU43 supplement, contrary to uncoated urea, did not decrease DMI, while enhancing total-tract nutrient digestibility with no impact on milk yield and milk composition of lactating Holstein dairy cows.