Feed intake, rumen fermentation and performance of dairy cows fed diets formulated at two starch concentrations with either conventional urea or slow-release urea

This study evaluated the effects of high and moderate starch diets by substituting corn grain with wheat and rice bran in diets containing slow-release urea (SU, protected into lipids matrix microencapsulation) or conventional urea (CU) on the performance of high producing dairy cows. Twenty-four multiparous (days in milk = 83 ± 14; milk yield = 47 ± 4.5 kg/d) and sixteen primiparous (days in milk = 83 ± 12; milk yield = 37.5 ± 2.5 kg/d) Holstein dairy cows were randomly assigned to 1 of 4 treatments in a 2 × 2 factorial arrangement of treatments with 2 dietary starch concentrations [moderate starch (MS; 260 g/kg of dry matter, DM) vs. high starch (HS; 320 g/kg of DM)] and two sources of urea (SU vs. CU). The experiment lasted 52 days (d), consisting of 10 d for covariate period, 21 d for adaptation, and 21 d for sampling. In a preliminary in vitro gas test, the gas production was higher for SU than CU at 0.5, 1, 2, 6, and 24 h post-incubation (P < 0.01). There were interactions between urea source and starch level for dry matter intake (DMI, P < 0.01), propionate proportion (P = 0.04), milk urea nitrogen (MUN, P = 0.09) and rumen NH₃-N (P = 0.07) concentrations; the DMI (P = 0.01) was higher and propionate proportion (P = 0.05), MUN (tendency, P = 0.08) and rumen NH₃-N (tendency, P = 0.06) were lower with high than moderate starch level in CU diet, but it was not different between the two starch levels in SU diet. Furthermore, the urea source and starch level interaction was significant for butyrate proportion (P = 0.04); the butyrate proportion (P = 0.03) was lower with high than moderate starch level in SU diet, but it was not different between the two starch levels in CU diet. Cows fed CU had greater milk fat (P < 0.01) and protein (P = 0.04) yield and milk fat content (P = 0.05) and tended to have a greater 35 g/kg fat corrected milk yield (P = 0.09) than cows fed SU. The total volatile fatty acids concentrations (P = 0.04) and milk protein content (P < 0.01) were greater in HS-fed cows than in MS-fed cows. Blood urea nitrogen concentration was higher at 2 (P = 0.02) and 4 (P = 0.03) h after feeding in CU-fed cows than in SU-fed cows. Apparent digestibility of starch was greater in HS-fed cows than in MS-fed cows (P < 0.01) and in CU-supplemented cows than in those who received SU (P = 0.05). Overall, the SU product used in this study reduced the rate of ruminal nitrogen release compared to CU; however, no advantages are observed with the addition of SU when compared with CU on high-production dairy cow performance. Moreover, the results suggest that the performance of high-producing dairy cows can be maintained when wheat and rice bran are used instead of corn grains at 150 g/kg of DM.