Starting in mid-lactation, goats were treated daily for 22 weeks with 0.15 mg recombinant bovine GH (bGH)/kg, or an equivalent volume of vehicle. One gland of each goat was milked thrice daily throughout treatment, the other twice daily. Mammary differentiation was studied in biopsy samples obtained before treatment and after 3 and 22 weeks of treatment, by determination of in-vitro synthesis rates of milk constituents and measurement of enzyme activities. Mammary growth was measured using a whole-body imaging technique (magnetic resonance imaging; MRI). bGH caused an immediate and sustained increase in milk yield of approximately 23% overall, whilst the glands milked thrice daily produced approximately 14% more than the control glands milked twice daily. The effects of the combined treatment were additive, but not synergistic. A synergistic effect of the combined treatment resulted in a significant improvement in lactation persistency. A stimulatory effect of milking frequency on mammary enzyme activities was evident only in bGH-treated goats at 3 weeks, but in both groups at 22 weeks. Synthesis rates of casein and lactose were increased at 3 weeks only by the combined treatment. Thus bGH accelerated or augmented the differentiative response to thrice daily milking. Mammary parenchyma volume, estimated by MRI, increased significantly during the first 12 weeks of bGH treatment and remained higher throughout the rest of the treatment period. Cell number was estimated from parenchyma volume and DNA concentration; this decreased significantly in the controls between weeks 1 and 22, but remained constant in the bGH group. In nine of the ten goats, parenchyma volume and cell number increased in the gland milked thrice daily relative to the control gland milked twice daily during the course of the experiment. Thus bGH stimulated growth of the mammary gland over and above that induced by the frequent milking. The absence of any detectable increase in thymidine incorporation suggests that this growth consisted of cellular hypertrophy rather than hyperplasia.

Work at The Hannah Research Institute has demonstrated the effects of an inhibitory (autocrine) protein, within the whey fraction of milk, on the efficiency of milk secretion and responses to changes in milking frequency. The regulatory mechanism involves both the concentration of the inhibitor and the site of milk storage (relative to its site of action, the secretory tissue). Milk storage characteristics influence responses to a number of management practices, most notably changes in milking frequency (Dewhurst and Knight, 1992; Knight and Dewhurst, 1992). There is, therefore, a need to understand phenotypic sources of variation in the sites of milk storage in order to define standard conditions and procedures for making cistern measurements.

Exposure to long day photoperiod is known to stimulate milk secretion in dairy cattle, but the mechanism has not been determined. Melatonin (MEL), which is produced by the pineal gland, is stimulated during darkness and has been known to mimic the short day signal when administered during long day exposure. Preliminary evidence has suggested that long term MEL treatment sensitizes lactating goats to subsequent long light exposure (Knight, 1993). The present study was undertaken to test if repeated short cycles of MEL would prime goats to subsequent long light effect on milk yield.