Supply of a histidine-rich bypass protein source improves productive performance and N efficiency independently of metabolizable Met and Lys.

The milk protein equation of the Nutrient Requirements of Dairy Cattle 8th Revised Edition includes additive effects of the metabolizable (m-) supply of Met, Lys, Leu, Ile, and His. However, additive effects of those AA in dietary studies have not been reported. This study aimed to evaluate the relationship between dietary level of mHis and of mMet, mLys, and mLeu as a group (MKL) on lactation performance, N efficiency, and metabolism, with a 2 × 2 factorial arrangement of treatments. The 4 diets consisted of low (L) or high (H) mHis (HIS; 2.2 vs 2.7% MP) and L or H levels of mMet (2.4 vs 2.8% MP) and mLys (8.0 vs. 9.0% MP). Although mLeu was aimed to increase along with Met and Lys, higher than expected Leu content in the HIS diet resulted in similar increase for both factors. Thirty-two multiparous lactating Holstein cows (77 ± 17 DIM) were enrolled by DIM and parity in a 4 × 4 Latin square study replicated 8 times with 4 periods of 28 d. The 4 experimental diets contained the same inclusion level of corn silage, alfalfa silage, cottonseed, dry corn grain, fat supplement, urea, and vitamins and minerals, while varying inclusion levels of corn gluten meal, a blood meal-based protein mix, and rumen-protected Met and Lys to increase the metabolizable supply of targeted AA. The L-HIS/L-MKL diet was 15% MP deficient while meeting requirements for NEL. Data were analyzed with a mixed model containing fixed effects of HIS, MKL, HIS × MKL interaction, period, and square, and the random effect of cow within square. Both factors increased DMI to a similar extent (+1 kg/d), but no further increase was observed when combined, indicating antagonistic effects between factors. Accounting for DMI, H-HIS increased the supply of mHis and mLeu by 17 and 21 g/d, respectively, compared with L-HIS. Meanwhile, compared with L-MKL, H-MKL increased the supply of mMet, mLys, and mLeu by 15, 44, and 20 g/d, respectively. Both factors increased circulating concentrations of their respective AA in plasma, except for Leu, and decreased glucose to a similar extent (-2.7 mg/dL), without affecting the concentrations of insulin, nonesterified fatty acids, or triglycerides. The yields of milk, lactose, and > 16C fatty acids were increased only by HIS. The yields of ECM (HIS 1.7 kg/d, MKL 0.9 kg/d), true protein (HIS 74 g/d, MKL 45 g/d), fat (HIS 46 g/d, MKL a trend of 31 g/d), and < 16C and = 16C (a trend for HIS) fatty acids were additively increased by the 2 factors. Plasma and milk urea N concentrations, urinary urea N, and total urinary N outputs were increased by MKL. Meanwhile, HIS only increased MUN and tended to increase urinary urea N output. Under L-MKL, HIS maintained gross N use efficiency, while it recovered the drop induced by H-MKL. Despite non-additive effects on DMI, increasing mHis supply, in addition to mMet, mLys, and mLeu, increased milk protein yield and improved N use efficiency, contradicting the limiting AA theory.