Addition of glycerol to lactating cow diets stimulates dry matter intake and milk protein yield to a greater extent than addition of corn grain.

The objective of this study was to determine if the addition of glycerol to the diet of dairy cows would stimulate milk protein yield in the same manner as the addition of corn grain. Twelve multiparous lactating dairy cows at 81 ± 5 d in milk were subjected to 3 dietary treatments in a replicated 3 × 3 Latin square design for 28-d periods. The diets were a 70% forage diet considered the basal diet, the basal diet with 19% ground and high-moisture corn replacing forages, and the basal diet with 15% refined glycerol and 4% added protein supplements to be isocaloric and isonitrogenous with the corn diet. Cows were milked twice a day and samples were collected on the last 7 d of each period for compositional analysis. Within each period, blood samples were collected on d 26 and 27, and mammary tissue was collected by biopsy on d 28 for Western blot analysis. Dry matter intake increased from 23.7 kg/d on the basal diet to 25.8 kg/d on the corn diet and 27.2 kg/d on the glycerol diet. Dry matter intake tended to be higher with glycerol than corn. Milk production increased from 39.2 kg/d on the basal diet to 43.8 kg/d on the corn diet and 44.2 kg/d on the glycerol diet. However, milk yield did not differ between corn and glycerol diets. Milk lactose yields were higher on the corn and glycerol diets than the basal diet. Milk fat yield significantly decreased on the glycerol diet compared with the basal diet and tended to decrease in comparison with the corn diet. Mean milk fat globule size was reduced by glycerol feeding. Milk protein yield increased 197 g/d with addition of corn to the basal diet and 263 g/d with addition of glycerol, and the glycerol effect was larger than the corn effect. The dietary treatments had no effects on plasma glucose concentration, but plasma acetate levels decreased 27% on the glycerol diet. Amino acid concentrations were not affected by dietary treatments, except for branched-chain amino acids, which decreased 22% on the glycerol diet compared with the corn diet. The decreases in plasma acetate and branched-chain amino acid concentrations with glycerol and the larger effects of glycerol than corn on milk protein and fat yields suggest that glycerol is more glucogenic for cows than corn grain.

Decline in mammary translational capacity during intravenous glucose infusion into lactating dairy cows.

The objective of this study was to determine effects of glucose on milk protein yield and mammary mammalian target of rapamycin (mTOR) activity in dairy cattle in early lactation. Eight multiparous cows at 73 ± 8 d in milk were randomly assigned to 2 treatments in a crossover design for two 6-d periods. Treatments were jugular infusion of either saline (Sal) or 896 g/d glucose (Glc). All cows were fed a total mixed ration with 42% neutral detergent fiber, had free access to water, and were milked twice a day. Within each period, blood samples were taken (d 5) and mammary tissue was collected by biopsy (d 6) from each hindquarter for Western blot analysis. In addition to Sal and Glc treatments, on d 6, rapamycin dissolved in 50% dimethyl sulfoxide was administered via the teat canals into the left quarters, with a control solution administered into the right quarters. Rapamycin had no effect on milk protein yields or phosphorylation state of mTOR signaling proteins. Infusions of Glc significantly increased milk yield but only tended to increase milk protein yields. Milk fat tended to be decreased in cows infused with Glc, whereas lactose yields were significantly increased. Glucose infusion did not increase plasma glucose levels, but insulin and nonessential AA concentrations increased by 21 and 16%, respectively, branched-chain AA concentrations decreased 24%, and essential AA concentrations tended to decrease by 14%. Infusion of Glc significantly decreased abundances of both phosphorylated and total ribosomal S6 kinase 1 (S6K1) in mammary tissue by 27 and 11%, respectively. Abundance of phosphorylated eukaryotic initiation factor 4E-binding protein 1 (4EBP1) decreased significantly by 25%, whereas total 4EBP1 exhibited a tendency to decrease by 16%. We conclude that the mTOR signaling pathway is not the only regulator of milk protein synthesis. Decreases in essential AA concentrations in plasma suggest that protein synthesis was stimulated in nonmammary tissues of the body, presumably skeletal muscle.