SUMMARY Twenty-one Holstein bull calves were randomly assigned at birth to 3 groups. Two groups (each of 7 calves) were raised as follows: fed a milk diet alone or fed milk with grain supplementation after 2 weeks of age; studies were done when calves reached 4 weeks of age. The 3rd group was fed on milk with grain supplementation until weaning after which the calves were maintained on grain and pasture. These calves (older calves) were studied at 12 weeks of age. Either propionate (0.28 mmol/kg) or glucose (0.56 mmol/kg) was injected iv in a random order. Samples of blood were obtained from the calves before and immediately after injections were done and at 2, 5, 10, 15, 30, 45, and 60 minutes after secretagogue injection. Plasma was examined for glucose by a glucose oxidase procedure and for immunoreactive insulin (iri) and glucagon (irg) by radioimmunoassay. The iri response to the injection of glucose was greater in older calves (P < 0.02). Patterns of iri secretion, as determined by heterogeneity of regression, showed age differences for both secretagogues (P < 0.05). Base-line irg was greater in milk/grain-fed calves than in milk-fed calves (P < 0.05). Mean irg response to propionate injection was higher (P < 0.05) in milk/grain-fed calves than in milk-fed calves. Plasma glucose concentration increased in older calves, but decreased in milk-fed calves after propionate injection. The data indicate that maturation in the ruminant is accompanied by altered regulation of insulin and glucagon secretion. These data also indicate that there may be a role for glucagon in the development of hepatic gluconeogenic mechanisms in the ruminant.
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Owens et al. (1986) studied this question.