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

¹ Department of Animal Science, Macdonald Campus of McGill University, 21,111 Lakeshore Road, Ste. Anne de Bellevue, QC, Canada H9X 3V9

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 insulin plus glucose (12.5 IU·h^–1 + 50 g·h^–1). Blood was collected continuously during the last 24 h of each infusion period. Statistical analyses of data for energy balance, milk yield, and DMI were performed on the last 3 d of each infusion period. Production data before and after infusions (i.e., no recombinant bST) estimated that recombinant 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 affected by infusion, but glucose infusion resulted in higher BW loss than did saline infusion (2.33 vs.0.08 kg·d^–1, respectively), and insulin 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 glucose infusion increased milk protein 11 and 14% compared with response to saline and glucose infusions, respectively; no change occurred in the proportion of casein and whey proteins. Serum bST was increased 109% with exogenous recombinant bST. Serum IGF-I was lower for cows infused 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 µIU·ml^–1 and 39.4 mg·dl ^–1 for cows infused with saline, glucose, and insulin plus glucose, respectively.

The results of this study suggest that low concentrations of plasma insulin in early lactation may limit the IGF-I response to recombinant bST (uncoupling). Despite higher IGF-I, milk yield was lower, probably as a result of low blood glucose. These results suggest that, in early lactation, insulin is still anabolic because the BW gain of cows increased. However, milk yield was still higher than that for cows in late lactation with similar insulin concentrations.

Key Words: bovine somatotropin • insulin • glucose)

Submitted on July 28, 1995
Accepted on April 23, 1996

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