Effects of starch and rumen-protected histidine in a low-protein diet on production and nitrogen efficiency of dairy cows

This study aimed to investigate interactions between dietary starch level and rumen-protected His (RPHis) in lactating cows fed MP-deficient diets (85% of requirements) on nutrient utilization, milk production, and plasma AA concentrations. Twelve multiparous Holstein cows [(136 ± 53.1 days-in-milk, 36 ± 3.0 kg/d milk yield (MY)] were assigned to a replicated 4×4 Latin Square design with a 2 x 2 factorial arrangement: 1) high starch (HS; 14% CP, 26% starch, 38% NDF), 2) HSRPHis (HS + RPHis), 3) low starch (LS; 14% CP, 16% starch, 44% NDF); 4) LSRPHis (LS + RPHis). The RPHis (62% His DM-basis, 84% estimated bioavailability) was supplemented to meet NASEM (2021) targeted digestible His supply (59 g/d) in LSRPHis and HSRPHis diets, and the digestible His-to-NEL ratio (1.6 g/Mcal; Räisänen et al., 2023) in HSRPHis. Diets were balanced with RPHis coating and rumen-protected Lys and Met. Data were analyzed using linear mixed models in R. There were starch×RPHis interactions for MY (P = 0.03), lactose yield, and fat content (P < 0.01), but not for energy-corrected milk (ECM) yield, nutrient intake or utilization. Regardless of RPHis, HS diets tended to increase (P = 0.06) DMI and increased (P = 0.03) OM intake by 2%, but reduced CP and NDF intakes by 5% and 10.5%, respectively, with digestibility reductions of 10% and 16% (P < 0.01), likely due to starch-induced ruminal pH suppression. High-starch diets increased (P < 0.01) MY, ECM (+2 kg/d), and milk component yields, improving feed efficiency (+3%; P = 0.02), which were likely linked to elevated glucose availability. High-starch diets also enhanced N use efficiency (NUE, +17%), and reduced urinary N excretion (−29%), while fecal N excretion increased (+10%; P < 0.05). Irrespective of starch level, RPHis tended to reduce CP digestibility (−5%; P = 0.07) and tended to increase fecal N excretion (P = 0.08) without affecting milk N or NUE. Conversely, RPHis reduced MY (−0.8 kg/d) and lactose yield in LS diet (P < 0.05). Milk fat content increased with RPHis in LS diet (+3%) but declined in HS diet (P < 0.05). Plasma total EAA, His and Leu concentrations increased with HS (P < 0.05), whereas His concentration tended to increase with RPHis (50 vs. 44 μM; P = 0.06). Further, 3-methyl-His, Val, and Ile concentrations decreased with RPHis (P < 0.05). While starch enhanced ruminal N capture and glucose-driven milk synthesis, RPHis did not improve lactational performance. This may stem from overestimated RPHis bioavailability, labile His pools mobilization, and low supply of other EAA, in this short-term experiment. Results emphasize the role of starch in optimizing NUE and production in low-protein diets, whereas responses to RPHis require further validation and considerations of overall AA supply and optimization under sustained MP deficiency.