a1 ARC Animal Breeding Research Organisation, Dryden Field Laboratory, Roslin, Midlothian EH25 9PS
a2 National Institute for Research in Dairying, Shinfield, Reading RG2 9AT
a3 ARC Unit of Statistics, University of Edinburgh, Edinburgh EH9 3JZ
Plasma hormone concentrations and metabolite responses to changes in energy equilibrium were measured in experiments designed to compare a) Hereford × Friesian with Friesian calves and b) Friesian calves sired by bulls with a high level of Improved Contemporary Comparison with those sired by bulls of a low level. The concentration of thyroxine and triiodothyronine tended to be greater in calves of higher dairy merit. The concentration of growth hormone was largely unrelated to dairy merit in the circumstances studied. The concentration of insulin was greater in Hereford × Friesian than in Friesian calves following feeding and following the injection of propionate. Responses to the injection of insulin and of growth hormone (as measured by changes in the plasma concentration of urea nitrogen, free fatty acids, glucose or the other hormone) were little affected by the level of Improved Contemporary Comparison of a calfs sire.
It was concluded that the thyroid system and the response of insulin to perturbation of energy balance may provide criteria of genetic merit for milk production which are neither age- nor sex-limited.
(Received June 18 1982)
(Accepted February 14 1983)
§ Present address: Veterinary Research Institute, Peradeniya. Sri Lanka.