Effect of increased intake of milk replacer in young calves on growth and the neonatal mode of action of IGF-I.

Abstract Background Optimizing growth and rumen development of calves in the preweaning period can lead to reduced costs of raising replacement heifers, bulls and fattening beef cattle. In the neonatal period, the quantity and quality of milk replacer consumed by calves directly impacts their growth, development and maturational changes in the somatotropic axis. This study evaluated the effects of plane of nutrition in neonatal calves on growth, development of the somatotropic axis and mode of action of IGF-I. Eight Holstein bull calves (3–4 days old) were fed either a low (LM, average 0.63 kg DM/d) or high (HM, average 1.15 kg DM/d) level of a 28% crude protein milk replacer until slaughtered at 41–42 days old.Results Compared to LM calves, calves on the HM diet had heavier (P < 0.01) body, carcass and semitendinosus muscle weights at slaughter. At three weeks of age, plasma insulin-like growth factor I (IGF-I) concentrations rose more rapidly in HM calves and were significantly higher at three, four and six weeks of age compared to LM calves. IGF-I mRNA levels were higher (P < 0.01) in liver, and lower (P < 0.01) in longissimus dorsi muscle in HM calves compared to LM calves. Growth hormone and IGF-I receptor mRNA levels in liver and muscle were not affected by plane of nutrition.Conclusions Increasing intake of a high-protein milk replacer in suckling calves improved growth rate in the preweaning period. The higher plane of nutrition promoted a shift towards an endocrine mode of IGF-I action and a decline in the autocrine/paracrine IGF-I control of muscle growth.