Effects of tannins and additional rumen-protected protein on nitrate responses in dairy cows: Lactational performance, enteric methane emissions, nitrogen utilization, and blood metabolites

The objective of this study was to evaluate the effects of nitrate and Acacia mearnsii bark extract (AMT) supplementation; the latter with and without additional rumen-protected rapeseed meal (RPR), on performance, enteric CH₄ emissions, and N partitioning in dairy cows fed a low-CP diet. The additional RPR aimed to counteract tannin-caused depressions of protein digestibility and potential reductions in milk production. Following a 2-wk covariate period, 18 multiparous Holstein cows were subjected to a split-plot design, with 2 main plot treatments (0.67% nitrate in dietary DM or isonitrogenous urea) and 3 subplot treatments arranged in a 3 × 3 Latin square: (1) a corn-silage-based control diet (CON; average 14.3% CP in DM), (2) CON supplemented with 3% of DM AMT containing 60% tannins (AMT), and (3) the AMT diet with an additional 6.4% of DM RPR (AMT+RPR). Measurements included gas emissions using a GreenFeed system and analyses of feed, milk, feces, urine, and blood. Nitrate supplementation improved nitrogen use efficiency (NUE; milk protein N/N intake) by 11% compared with urea, driven by a trend toward increased milk protein N secretion without increasing N intake. It also reduced total N excretion and CH₄ intensity (g/kg ECM) by 12% without affecting CH₄ yield (g/kg DMI) or daily production. Nitrate also increased milk yield by 12% (28 ± 0.6 vs. 25 ± 0.6 kg/d) without affecting ECM or milk composition, and did not increase plasma methemoglobin (1.5% ± 0.42% vs. 1.4% ± 0.44% as % of hemoglobin, for nitrate, and urea, respectively). In contrast, AMT did not affect CH₄ production, yield, or intensity, but reduced ECM yield (-8%) and NUE (-4%) across urea and nitrate groups. It reduced milk N output by 9% and shifted urinary N excretion to fecal N (+44%; +80 g/d), contributing to a more environmentally favorable N partitioning profile. In addition, AMT decreased digestibility of OM (-17%), CP (-30%), and NDF (-39%) in the urea-fed cows; these reductions were less pronounced (8%, 17%, and 23%, respectively) when nitrate was included. Additional RPR increased dietary CP intake (+10% with nitrate; +6% with urea) but failed to mitigate the adverse effects of AMT on digestibility, milk yield, or NUE. In addition, NUE declined by 10% with AMT+RPR versus AMT due to increased N intake without improvement in milk N output, suggesting inefficient utilization of the additional protein and potential energy dilution (-4.7% starch in AMT+RPR diets). Additional RPR reversed the reductions in MUN and BUN induced by AMT and elevated urinary (+12%) and fecal (+11%) N excretions. In conclusion, nitrate at 0.67% of dietary DM improved NUE, increased milk yield, and reduced CH₄ intensity without adverse effects on blood methemoglobin or milk nitrate accumulation. Supplementing AMT can be expected to reduce the N emission by redirecting urinary to fecal N losses, but at the cost of NUE and digestibility. With this diet type, additional RPR appears to have been unable to compensate for AMT's negative effects. These findings support nitrate as a promising strategy for improving NUE and reducing CH₄ intensity in dairy systems, as well as highlight the need for further research on tannin-protein interactions in low-CP diets.