Insulinemic energy and amino acids supplied postruminally independently regulate the performance and metabolism of peak-lactation dairy cows

Increasing AA sequestration in milk is essential for improving N efficiency in dairy cows and to reduce the environmental impacts of dairy production. Isoleucine, Leu, and Met are promising AA to improve milk protein yield and N efficiency due to their signaling roles in mammary epithelial cells. Glucogenic diets can also stimulate milk protein synthesis and, through their effect on insulin, potentiate the signaling effects of AA. Our objective was to evaluate the effects of Ile, Leu, and Met on production performance and N metabolism, and the role of energy source (ES; glucose or acetate) on lactating cows' responses to AA. The experiment consisted of a 4 × 4 Latin square design replicated 4 times with 4 periods of 21 d and 4 treatments arranged as a 2 × 2 factorial. Sixteen ruminally cannulated, multiparous, Holstein cows were housed in tiestalls and fed the same experimental diet with 4% energy and 15% MP deficiency. The 4 treatments were continuously infused into the abomasum and consisted of isoenergetic amounts (2.86 Mcal/d) of glucose (GLC) or acetate (ACE), without AA as negative control, or with a mix of 12 g/d of Ile, 50 g/d of Leu, and 20 g/d of Met (ILM). Experimental periods consisted of 2 wk of adaptation and 1 wk of sampling, in which feed, urine, milk, and blood samples were collected. Data were analyzed with a mixed model in R, containing the fixed effects of period, square, AA infusion, ES, and ES × AA interaction, and the random effects of cow within square. Overall, ES did not alter the response to AA on production or N metabolism (i.e., no significant interactions). Infusion of ILM, regardless of ES, increased circulating insulin concentration, milk protein and fat contents, and yields of de novo and mixed fatty acids, as predicted by Fourier-transform mid-infrared spectroscopy, but decreased lactose yield and concentration and tended to decrease milk yield. Meanwhile, ILM did not alter ECM or N efficiency for milk production. Compared with ACE, GLC increased DMI, circulating concentrations of glucose and insulin in plasma, and yields of milk and milk protein, but decreased circulating levels of acetate, BHB, and nonesterified fatty acids, as well as yields of presynthesized fatty acids, total milk fat, and fat-corrected milk. Similar to AA infusion, ES did not affect ECM yield nor N efficiency for milk production. In summary, compared with ACE, GLC did not alter the performance response to ILM but independently affected production and metabolism, consistent with its insulinemic potential. Cows responded mildly to AA infusion, most remarkably increasing de novo milk fatty acid production and milk protein content without impairing N efficiency, while also decreasing milk lactose concentration and yield.