Manipulation of ruminant lactation using photoperiodic and endocrine treatment
This thesis examines some aspects of milk yield manipulation utilizing some factors that can effect the function of the mammary gland. In the first part of study, the effect of photoperiod on lactation performance in the goats was studied, and particularly to investigate if this response can be potentiated by prior exposure to short periods of short days elicited by melatonin treatment. Long light did not produce clear stimulatory effect on milk yield, but a small response was seen in autumn and only in goats that were not treated by melatonin. Repeated short cycles of melatonin did not sensitize lactating goats to subsequent long light effect to milk yield regardless of stage of lactation or commencement time of year. Indeed, this treatment might produce a detrimental effect on milk yield when applied in early lactation. The second part of the study was to determine the maximum metabolic capacity of cows from different genetic merit. We adopted a multiple galactopoietic stimuli, increasing milking frequency, bovine somatotropin and thyroxine, applied in additive stepwise fashion at peak yield to cows from high and low genetic merit. This approach was successfully drove the cows into what we believe their maximum metabolic capacity. Milk yield was increased in an additive fashion at each stimulus. The increase in milk yield capacity was associated with mammary growth which was detected during the maximum stimuli. There was no significant difference in the response to the galactopoietic stimuli between cows from different genetic merit which did not suggest that high genetic merit cows are milking closer to their maximum capacity and, therefore, at greater risk of collapse of metabolic control than low genetic merit cows.