Twenty-four bull calves, eight Hereford × Friesian, eight Aberdeen Angus × Friesian and eight Friesian, were reared from birth to slaughter at 13 weeks of age solely on a milk substitute diet given ad libitum.
Samples of jugular blood were taken at weekly intervals from 5 days to 89 days of age for determination of concentrations of prolactin, growth hormone (GH), insulin and total thyroxine (T4) in plasma, tri-iodothyronine (T3) uptake, and thus free T4 index (FTI).
The Friesian calves had a higher mean body temperature and lower mean T3 uptake than did either of the beef crosses.
The Aberdeen Angus cross had a higher mean dry-matter intake per unit of metabolic body size (DMI/W0·73), heart rate and plasma insulin concentration than did the Hereford cross, and a higher insulin concentration but not a higher heart rate than did the Friesian. Live-weight gain and DMI/W0·73 for the Friesian was intermediate between that of the Aberdeen Augus cross and the Hereford cross.
DMI/W0·73 was positively correlated with insulin: GH ratio, prolactin: GH ratio and T4: GH ratio.
The increase in heart rate per unit DMI/W073, a measure of the heat increment of feeding, was negatively correlated with the plasma insulin concentration, insulin: T4 and insulin: GH ratios.
Prolactin concentration, GH concentration and T3 uptake declined with age, whereas insulin concentration, T4 concentration and FTI increased with age. The relationship was linear for prolactin and insulin concentrations, but curvilinear for the other variables. The Friesian breed had a higher GH concentration than did the beef crosses during the first 12 days of life. The large increase in insulin concentration for the Aberdeen Angus × Friesian occurred from about 47 days of age. The peak T4 concentration and the lowest T3 uptake of the breeds occurred between 33 and 42 days of age.
Possible mechanisms for the control of dry-matter intake by preruminant calves and the difficulties associated with prediction of performance from plasma metabolic hormone concentrations obtained at a young age are discussed.