Glucose does not stimulate milk protein yield of dairy cows when essential amino acids are in excess supply

Abstract Text:

To determine if glucose stimulates milk protein yield when essential amino acids (EAA) are supplied in excess, 5 early-lactation, rumen-fistulated dairy cows (78 ± 13 DIM) were abomasally infused for 5 d with EAA and glucose solutions in a 5 x 5 Latin square design. The 5 infusion treatments were saline, 844 g/d EAA in the profile of casein, 1126 g/d EAA, 844 g/d EAA + 1000 g/d glucose, or 1126 g/d EAA + 1000 g/d glucose.  Cows were fed a diet containing 6.96 MJ/kg NE_L and 12% crude protein on a dry basis.  Milk composition and yield during the last 2 d of each period and plasma metabolite concentrations during d 4 of infusion were subjected to ANOVA using PROC MIXED of SAS, where cow was a random effect.  EAA infusion increased essential and branched-chain amino acid concentrations in plasma 3- to 4-fold compared with saline (P < 0.001). Non-EAA concentrations decreased 11 to 17% during EAA infusions (P < 0.001). Addition of glucose to EAA infusates decreased essential and branched amino acid concentrations in plasma (P < 0.031) and had no effect on Non-EAA concentrations.  EAA infusion increased concentrations of non-esterified fatty acids (NEFA; P = 0.004) and urea nitrogen (P < 0.001) in plasma, but had no effect on glucose concentrations.  Addition of glucose to EAA infusates increased glucose concentrations in plasma 13 to 19% (P < 0.001), decreased NEFA 26 to 32% (P < 0.001)  and had no effect on urea nitrogen.  Dry matter intake was not affected by EAA infusion, while daily milk yield increased 16% and milk protein yield increased 27% (for an average of 262 g/d) at the highest level of EAA infusion compared with saline (P < 0.001).  Milk protein concentration increased from 2.9% with saline to 3.3% with EAA (P <0.001).  The addition of glucose to EAA infusates caused DMI to decrease 0.65 kg/d (P = 0.040), tended to increase milk yield (P = 0.057), had no effect on milk protein yields (P = 0.318), but tended to decrease milk protein concentration (P = 0.097) and decreased milk fat concentration (P < 0.001).  Thus, increased supply of glucose at high levels of EAA supplementation did not improve milk protein yields, but because of the decline in feed intake there was an increased efficiency of capture of dietary protein into milk protein.

Keywords: milk protein, essential AA