Interactions of rumen-protected histidine and dietary starch in low-protein diets with adequate lysine and methionine: Effects on lactational performance and nitrogen utilization in dairy cows

The objective of this study was to investigate interactions between dietary starch amount and rumen-protected His (RPHis) on nutrient utilization, milk production, N partitioning, and plasma AA and metabolites concentrations in dairy cows fed MP-deficient diets (14.0% CP in DM, 84%-86% of MP recommendation). Twelve multiparous cows mean ± SD: 136 ± 53.1 DIM, 36.4 ± 3.0 kg/d milk yield (MY), and 750 ± 52.6 kg BW were assigned to a 3 4 × 4 Latin Square design with 21-d periods and a 2 × 2 factorial arrangement of treatments: (1) high-starch without RPHis (HS; 28.4% starch), (2) HS with RPHis (HSH; on average 32 g of RPHis/cow per day, supplying 16.5 g digestible His/d), (3) low-starch without RPHis (LS; 17.4% starch), and (4) LS with RPHis (LSH; on average 31 g of RPHis/cow per day, supplying 15.9 g digestible His/d). All diets were supplemented with a rumen-protected Met and Lys product to meet the recommended supply by the National Academies of Sciences, Engineering, and Medicine (NASEM, 2021). Measurements included analyses of feed, feces, milk, and urine, blood samples. Starch × RPHis interactions were detected only for MY, lactose yield, and milk fat content. With LS diets, RPHis decreased MY (29.7 vs. 30.5 kg/d) and lactose yield (1.42 vs. 1.47 kg/d), but increased milk fat content (4.54% vs. 4.40%). Under HS conditions, RPHis reduced milk fat content (4.36% vs. 4.49%) and tended to increase milk true protein:fat ratio (0.79 vs. 0.76), without affecting MY (32.2 vs. 31.7 kg/d). Across RPHis amounts, HS diets increased ECM yield (33.7 vs. 31.7 kg/d), milk true protein yield (1.08 vs. 0.98 kg/d), and OM intake (21.7 vs. 21.2 kg/d), while decreasing apparent total-tract digestibility of CP (58.7% vs. 65.0%), and NDF (43.0% vs. 51.4%). High-starch diets improved N use efficiency (NUE; 32.5% vs. 28.0%), by reducing N intake and urinary urea N excretion and increasing milk N secretion but increased fecal N excretion and resulted in a more negative N balance. Urinary purine derivative (PD) concentrations were greater with HS diets, indicating an enhanced microbial protein synthesis. Plasma His concentration was 9.4 μM (23%) greater in HS fed cows. Supplementing RPHis did not affect nutrient intake, digestibility, milk true protein yield, or NUE, but increased plasma His by 10.2 μM (26%), indicating improved His supply from the RPHis product. Overall, increasing dietary starch in low-protein, MP-deficient diet improved ECM and milk protein yield, and NUE, by enhancing ruminal energy-protein synchrony and shifting N excretion from urine toward milk and feces. In contrast, RPHis supplementation failed to improve ECM production or N utilization, likely reflecting His supply not being the primary limiting factor under the present conditions or that its utilization was constrained by the overall AA profile of MP with the low protein basal diet. Future work should quantify the in vivo bioavailability and metabolic utilization of RPHis and further examine how His interacts with energy supply and overall EAA balance to regulate productive responses in dairy cows fed low-protein diets in longer-term studies.