Stimulated growth hormone release in juvenile cattle genetically selected for high and low milk yield

Abstract

The purpose of the present study was to test if plasma growth hormone (GH) concentrations in juvenile male and female cattle before or after intravenous stimulation with secretagogues was affected by selection for high (H) vs. low (L) milk yield in lines of Norwegian cattle. In the first of two experiments (A), 32 yearling heifers (16 H and 16 L, at 307–424 days of age) were tested by use of four doses of growth hormone releasing factor (GRF); 0.02, 0.10, 0.50 and 2.50 μg/kg live weight, on 4 consecutive days. The animals were fed ad libitum on a silage-based ration before and during the experiment. Growth hormone was assayed in plasma from blood samples taken at −15, −5, 0, 5, 10, 15, 20, 30, 45 and 60 min from stimulation. Plasma GH concentrations were log transformed before statistical analyses. Response variables were; PRIOR (mean of −15, −5 and 0 min samples) and PEAK (mean of 10, 15 and 20-min samples). In experiment B, 37 calves (19 H+18 L, 22 males and 15 females, age 114–259 days) were subjected daily to one of three intravenous stimulation tests (GRF, 0.10 and 0.50 μg/kg or thyrotrophin releasing hormone (TRH) at 0.20 μg/kg live weight) on each of 3 consecutive days. Feeding was restricted to cover estimated maintenance requirements only. Rations were given once daily during the test days and 3 days prior to first test. Blood sampling and variables followed those of experiment A. Selection line did not significantly affect GH variables in experiments A or B at any dose of GRF or TRH. GH response increased with increasing dose of GRF up to 0.50 μg/kg. At the highest GRF dose, the response was delayed and persisted longer. Doses giving intermediate to large response increased repeatability of GH measurements. It is concluded that GH secretion in juvenile cattle can be accurately assessed using GRF based stimulation tests combined with restricted and controlled feeding, but it is not affected by selection for milk yield in Norwegian cattle.

Keywords:

• calves
• challenge test
• growth hormone
• growth hormone releasing factor (GRF)
• repeatability
• thyrotrophin releasing hormone (TRH)

Acknowledgments

We express our thanks to a number of people who have taken part in this study through care of animals, blood sampling, processing and assaying, and commenting upon the text: Inger-Johanne Jørgensen, Borghild Tveit, Claes Gøran Fristedt, Jeton Starova, Ellen Helø, Tore Bolstad, Jørn Våge, Preben Boysen, Thorsten Olesen, Hanne Møller Purup, Inge-Lise Sørensen, Jens Bech, Ryszard Skrzypek and Mark Henryon. We also acknowledge financial support from the Norwegian Cattle Association (NRF).

Notes

Løvendahl, P. and Klemetsdal, G. (Department of Animal Breeding and Genetics, Danish Institute of Agricultural Sciences, P.O. Box 50, Foulum, DK-8830 Tjele, Denmark; Department of Animal Science, Agricultural University of Norway, P.O. Box 5025, N-1432 Ås, Norway). Stimulated growth hormone release in juvenile cattle genetically selected for high and low milk yield.

Additional information

Notes on contributors

Peter Løvendahl *

Løvendahl, P. and Klemetsdal, G. (Department of Animal Breeding and Genetics, Danish Institute of Agricultural Sciences, P.O. Box 50, Foulum, DK-8830 Tjele, Denmark; Department of Animal Science, Agricultural University of Norway, P.O. Box 5025, N-1432 Ås, Norway). Stimulated growth hormone release in juvenile cattle genetically selected for high and low milk yield.