Nitrogen emission and lactation characteristics of Holstein-Friesian dairy cattle fed low-protein diets with and without branched-chain volatile fatty acid supplementation.

This study aimed to assess the effects of branched-chain VFA (BCVFA) supplementation on milk and components yield and N emissions in dairy cows fed a low-protein diet, compared with a high-protein diet and a low-protein diet without BCVFA supplementation. In a 3 × 3 Latin square design, 48 Holstein-Friesian dairy cows (91 ± 15 DIM at the start of the experiment) were blocked in groups of 3 according to parity, lactation stage, and milk yield. Within each block, cows were randomly assigned to 1 of 3 barn units for independent emission measurements (16 animals per barn unit). The experiment lasted a total of 15 weeks, consisting of 3 treatment periods. Each treatment period lasted 5 weeks and included a 3-week adaptation period followed directly by a 2-week measurement period. The 3 dietary treatments were (1) a high-protein diet (HP) with a dietary CP content of 167 g/kg DM, (2) a low-protein diet (LP) with a dietary content of 154 g/kg DM, and (3) LP with Ca salts of BCVFA (LP+) targeting 253 and 109 mmol/cow/d of isobutyrate and 2-methylbutyrate, respectively. Both LP and LP+ resulted in a reduced NH3 emission by, on average, 21% compared with HP. Supplementation with BCVFA (LP+) did not improve milk and component yields or DMI, and did not affect CH4, CO2, NH3 and N2O emissions compared with LP. However, LP+ lowered DM and tended to lower OM excretion (21% and 23%, respectively) compared with HP. Decreasing dietary CP content (i.e., HP vs. LP or LP+) did not affect DMI but tended to decrease milk crude protein content and decreased milk urea content, and yields of milk, crude protein, and lactose. The emissions of CO2, CH4, and N2O were not affected by dietary CP content. Hence, it can be concluded that when the dietary CP content decreased from 167 to 154 g/kg DM, BCVFA supplementation did not counteract the negative effects of protein-limitation on milk and component yields. This may suggest that the low-protein diet may not have provided adequate rumen N to support increased microbial protein synthesis from BCVFA or that the effect of a lower metabolizable protein supply overruled any effect of supplemental BCVFA.