Effects of plasma insulin concentrations on milk yield and composition in dairy cows

The objective of this trial was to determine the effect of plasma insulin concentration on different blood metabolites and the resulting impact on lactation performances and milk fatty acid ( FA ) profile in lactating cows. Four Holstein cows were fed ad libitum once daily a total mixed diet containing 16.9% CP, combined with abomasal administration of casein (500 g/d) as a source of supplemental amino acids. The experiment was arranged as a crossover design where cows were submitted to a hyperinsulinemic euglycemic clamp twice at two different rates of insulin infusion. Each experimental period consisted of an initial 2-d interval to establish baseline measurements, followed by a clamp period that was maintained for 4 days. The cows received the intravenous infusion of insulin, providing an increase in circulating concentration of the hormone by two and a half- ( 2.5× ) and five-fold ( 5× ). The glucose infusion rate required to maintain euglycemia (±10% of baseline) was 1.8 times greater ( P = 0.07) for cows on 5× as compared with 2.5×. The concentration of IGF-1 was greater at 5× compared with 2.5× ( P = 0.08). Total plasma essential amino acids were lower at 5× than at 2.5× ( P = 0.03). DM intake was lower in cows receiving the 5× compared to those receiving 2.5× ( P = 0.08). No significant differences were observed in total energy intake (diet + glucose) or milk yield between treatments. The yield of fat-corrected milk ( P < 0.01) and milk fat yield ( P = 0.01) were significantly lower at 5×, whereas milk fat concentration tended to be lower ( P = 0.15), compared with 2.5×. Changes in milk fat composition in response to different rates of insulin infusions were characterized by a lower yield of preformed FA (C18; −55 g/d, P = 0.01), which contributed to 58% of the overall decrease in milk fat yield (−95 g/d) at 5× compared with 2.5×. Of lower magnitude, the tendency for reduced yield of de novo synthesized FA (C6 to C14; P = 0.13) contributed to 9% of the decrease in fat yield. Milk protein concentration and yield did not differ between treatments. Milk N efficiency was 12.9% greater at 5× than at 2.5× ( P = 0.04). In conclusion, greater plasma insulin led to lower availability of preformed FA for milk fat synthesis. Reduced availability of plasma essential amino acids, despite postruminal supply of casein, may have limited the response to greater plasma insulin regarding milk protein synthesis.