No abstract

The effects of administering bovine growth hormone (bGH) on milk production in cattle were examined in two experiments.In experiment 1, a Latin-square design, three groups of four Friesian cows were injected daily with one of two doses of bGH (6 ng/kg and 30 ug/kg; 0·82 unit/mg) or with saline (control) throughout three successive 21-day periods in mid-lactation. At 30 ug/kg daily bGH stimulated a significant increase in milk yield (P < 0·001) and this response increased with the duration of treatment. Changes in the yields of milk fat, protein and lactose closely followed the changes in milk production. The concentrations of milk fat (P < 0·05) and protein (P < 0·05) rose and fell respectively during the 1st week of injections.' A daily dose of 6 ug/kg bGH failed to stimulate an increase in milk yield but milk lactose content was reduced (P < 0·05) during the 2nd and 3rd weeks of treatment. The proportion of milk serum samples containing detectable quantities of the hormone was increased by the higher dose of bGH.In experiment 2 the milk yields of four Friesian cows were measured when bGH (21·0 to 37·9 mg/day) or saline was infused intravenously for 6 days. The infusions produced up to eight-fold increases in plasma bGH. These were within physiological limits, and remained at elevated levels throughout the 24-h period. Three of the cows demonstrated a clear increase in milk production.

As part of an ongoing investigation of ovine neuroendocrine mechanisms relating to photoperiod and eventually to wool growth, the diurnal variation of depilation force (an index of the strength of attachment of wool fibres to the skin of sheep) was investigated in Merino wethers and Border Leicester Merino ewes. Circadian rhythms were demonstrated in depilation force in both ewes and wethers. Further experiments were performed to investigate circadian endocrine rhythms which may have some bearing on the observed rhythm in depilation force. Circadian rhythms 180° out of phase with the depilation force rhythm were observed in plasma cortisol concentrations and in pineal serotonin N-acetyltransferase activities in Merino wethers.