Evaluation of growth hormone response to insulin-induced hypoglycaemia in dairy cattle during a 670-day lactation
L. C. Marett A D , M. J. Auldist A , W. J. Wales A , K. L. Macmillan B , K. DiGiacomo C and B. J. Leury CA Agriculture Research and Development, Department of Environment and Primary Industries, Ellinbank, Vic. 3821, Australia.
B Department of Veterinary Science, University of Melbourne, Werribee, Vic. 3030, Australia.
C Melbourne School of Land and Environment, University of Melbourne, Parkville, Vic. 3010, Australia.
D Corresponding author. Email: leah.marett@depi.vic.gov.au
Animal Production Science 54(9) 1323-1327 https://doi.org/10.1071/AN14224
Submitted: 12 March 2014 Accepted: 21 May 2014 Published: 10 July 2014
Abstract
Plasma growth hormone secretion in response to insulin-induced hypoglycaemia was evaluated in cows undergoing an extended lactation. Twelve multiparous Holstein-Friesian cows that calved in late winter and were managed for a 670-day lactation by delaying mating. Four experimental periods of 40 days commenced at ~73, 217, 422 and 520 (±9.1; mean ± s.d.) days in milk (DIM) during which cows were individually offered a diet of perennial ryegrass (73 and 422 DIM) or pasture hay and pasture silage (217 and 520 DIM) supplemented with either 1 kg DM grain (CONT) or 6 kg DM grain (GRAIN). Cows were fitted with jugular catheters during the final week of each experimental period. Seven blood samples over a 24-h period were collected and an insulin tolerance test was performed on each cow using a dose of 0.12 µU insulin/kg liveweight at ~100, 250, 460 and 560 DIM. This resulted in an increase in plasma growth hormone concentration occurring at 100 DIM, an intermediate and delayed response at 250 DIM, and no response at both 460 and 560 DIM. Cows in the CONT treatment had higher basal plasma growth hormone concentrations and tended to have a greater peak growth hormone response than GRAIN cows. The growth hormone response at 100 and 250 DIM is likely a homeorhetic mechanism to support milk yield during early-mid lactation. These observations are consistent with the known actions of growth hormone to promote lipid mobilisation, hepatic glucose production and an overall state of catabolism. With further research, the insulin tolerance test may be a useful tool to identify cows with greater growth hormone secretory response and increased milk production or persistency.
Additional keywords: extended lactation, insulin tolerance test, lactation persistency, nutrient partitioning, somatotropin.
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