Abomasal infusions of essential and nonessential amino acids to evaluate productive efficiencies, metabolism, energy, and amino acid utilization in lactating dairy cattle.

The use of MP in diet formulation incorporates both EAA and nonessential amino acids (NEAA). Further, under conditions of high metabolic demand, NEAA synthesis might be energetically unfavorable and could be a limiting step if other substrates are limited. The objective of this study was to evaluate production outcomes and metabolism of cows fed a diet meeting the ME requirement but MP-limited with abomasal infusions of NEAA and EAA. Twelve multiparous Holstein cows were randomly assigned to 1 of 5 treatments in a replicated 5 × 5 Latin square crossover design: (1) water to meet 90% of EAA and NEAA (90AA; 0 g/d); (2) EAA to meet 100% of EAA and 90% of NEAA (100EAA; 188 g/d); (3) 90% of EAA and 100% of NEAA (100NEAA; 158 g/d); (4) 100% of EAA and NEAA (100AA; 346 g/d); or (5) 100% of EAA and 110% of NEAA (110NEAA; 504 g/d). Continuous infusions were conducted for 18 d, and sampling occurred on d 15 to 18 for milk, blood, diet, refusals, and feces. Dry matter intake and milk yield were recorded daily. Data were analyzed by fitting a mixed-effects model and Tukey pairwise comparison using R (version 4.2.0; R Core Team). Body weight, DMI, milk yield, protein, and lactose were not different among treatments. Milk fat yield and concentration trended lower and milk total fatty acid (FA) content was lowest in the 110NEAA treatment. Preformed FA in milk were lower for the 110NEAA treatment compared with the 100EAA, whereas the opposite was observed for mixed FA. Plasma insulin concentrations trended to be lower for the 100EAA and 100NEAA compared with the 110NEAA treatments. Although total plasma AA did not differ among treatments, plasma NEAA and EAA responded to the respective infusions. Overall, infusing NEAA above the MP requirement tended to increase insulin, which is related to lower lipolysis, and this was supported by a significant decrease in preformed milk FA yield and concentration.