EFFECTS ON NUTRIENT AND HORMONAL PROFILE OF LONG-TERM INFUSIONS OF GLUCOSE OR INSULIN PLUS GLUCOSE IN COWS TREATED WITH RECOMBINANT BOVINE SOMATOTROPIN BEFORE PEAK MILK-YIELD

Ten Holstein cows were treated with 30.9 mg . d(-1) of recombinant bST from 15 to 41 d of lactation. The Latin square design included three infusion periods of 6 d each with 3 d of rest between infusion periods . Infusions were physiological saline, glucose (50 g . h(-1)), and ins ulin plus glucose (12.5 IU . h(-1) + 50 g . h(-1)). Blood was collecte d continuously during the last 24 h of each infusion period. Statistic al analyses of data for energy balance, milk yield, and DMI were perfo rmed on the last 3 d of each infusion period. Production data before a nd after infusions (i.e., no recombinant bST) estimated that recombina nt bST increased milk yield of cows infused with glucose and saline by 3.1 and 3.6 kg . d(-1), respectively. Net energy intake was not affec ted by infusion, but glucose infusion resulted in higher BW loss than did saline infusion (2.33 vs. 0.08 kg . d(-1), respectively), and insu lin plus glucose infusion resulted in BW gain (0.65 kg . d(-1)). Milk yield was 39.9, 39.6, and 37.6 kg . d(-1) for cows infused with saline , glucose, and insulin plus glucose, respectively. The insulin plus gl ucose infusion increased milk protein 11 and 14% compared with respons e to saline and glucose infusions, respectively; no change occurred in the proportion of casein and whey proteins. Serum bST was increased 1 09% with exogenous recombinant bST. Serum IGF-I was lower for cows inf used with glucose than for those infused with saline (21.03 vs. 27.44 ng . ml(-1)) and increased to 46.55 ng . ml(-1) for cows infused with insulin plus glucose. Serum concentrations of insulin and glucose were 13.7 and 56.7, 18.5 and 61.9, and 30.5 mu IU . ml(-1) and 39.4 mg . d l(-1) for cows infused with saline, glucose and insulin plus glucose, respectively. The results of this study suggest that low concentration s of plasma insulin in early lactation may limit the IGF-I response to recombinant bST (uncoupling). Despite higher IGF-I, milk yield was lo wer, probably as a result of low blood glucose. These results suggest that, in early lactation, insulin is still anabolic because the BW gai n of cows increased. However, milk yield was still higher than that fo r cows in late lactation with similar insulin concentrations.