Red clover isoflavones plus urea for partial soybean meal substitution: Implications for productivity and nitrogen...

Red clover extract (RCE) is rich in isoflavones, which have been reported to improve nitrogen utilization in dairy diets. Therefore, the objective of this study was to evaluate the effects of partially replacing soybean meal (SBM) with RCE plus feed-grade urea on lactation performance and nitrogen metabolism in lactating Holstein cows, relative to a slow-release urea (SRU) nitrogen source formulated to provide a similar amount of dietary nitrogen. In a randomized complete block design, 600 cows received one of 3 diets for 35 d (n = 200 per diet): SBM (10.81% SBM), SRU (0.65% SRU replacing 37.74% of SBM), or RCE (0.20% RCE + 0.57% urea replacing 37.74% of SBM). Diets were formulated to be isoenergetic and isonitrogenous. Data were analyzed using a mixed model with diet as a fixed effect and block and pen nested within diet as random effects; the pen was considered the experimental unit. Milk yield and milk composition did not differ between RCE and SBM. The DMI and feed efficiency were similar between these treatments, whereas SRU was associated with greater DMI and lower feed efficiency relative to SBM. Compared with SBM, RCE increased urinary purine derivatives and estimated microbial N, whereas concentrations of urea N in milk, plasma, and urine did not differ. Apparent NDF and ADF digestibility were similar between RCE and SBM but were lower with SRU. Differences among diets were also observed in selected blood biochemical and endocrine variables. These findings indicate that, during the 35-d experimental period, partial replacement of SBM with RCE plus urea maintained lactation performance relative to SBM and was associated with higher estimated microbial N compared with SBM, whereas responses to SRU differed in several nitrogen-related measures.