|dc.description.abstract||In dairy animals, the transition period, which spans from 3 weeks before to 3 weeks after parturition, is the most stressful time in their productive lives. This period is characterized by drastic physiological, metabolic, and endocrine adaptations to accommodate parturition and lactogenesis. Goats unable to adapt to this challenging time are more susceptible to infections and metabolic diseases, which might have a substantial impact on maternal health and productive efficiency beyond the transition period. An in-depth understanding of the biology of the transition period is essential for developing optimized strategies that could enhance milk yield without compromising herd health and welfare. While there is ample published information regarding the transition period in dairy cattle, to date, the endocrine and metabolic status of periparturient dairy goats has only been vaguely described in the literature.
Therefore, the overall goal of this dissertation was to expand on previous knowledge of hormonal and metabolic regulation of energy metabolism during the transition period and to explore factors that might aggravate the metabolic burden of pregnancy and lactation in periparturient dairy goats. Thus, a series of studies were conducted in a large commercial goat dairy farm in Australia to 1) determine the effects of month of kidding, parity number, and litter size on lactation curves of dairy goats raised in intensive systems; 2) characterize temporal variations in circulating levels of selected hormones and metabolites involved in energy balance regulation during the transition period; 3) investigate the effects of level of milk production, parity number and litter size on maternal metabolic profile; 4) determine whether higher plasma concentrations of markers of negative energy balance are associated with inferior productive performance; and 5) determine whether differential productivity is related to differences in nutrient partitioning between high- and low-yielding goats.
In the first part of this study, an analysis of the production data revealed that goats kidding in spring, in third/fourth parity, or carrying multiple fetuses produce more milk than their counterparts. Interestingly, although the month of kidding had the most significant impact on the shape of lactation curves, the magnitude of such impact increased with increasing parity number. Also, based on the concentration of key biomarkers of energy metabolism analyzed during this time, it was possible to conclude that nutritional deficit was increased with increasing milk yield, parity, and litter size (listed in order of importance) and that both pregnancy and lactation were less able to elicit lipomobilization in primiparous compared with multiparous goats. Further, the likelihood of early removal from the milking herd was significantly increased in goats with elevated blood levels of beta-hydroxybutyrate (BHB). On the other hand, contrasting studies in dairy cows, a positive association was observed between blood levels of non-esterified fatty acids (NEFA) and milk yield.
Nevertheless, it was unclear what role, if any, the endocrine system played in the differential productivity in early lactation observed between high- and low-yielding goats. Therefore, in the second part of this study, goats of high and low milk yield were subjected to 3 metabolic challenges (glucose, insulin, and adrenocorticotropin hormone infusions) to determine if differential productivity is related to differences in some aspects of the regulation of nutrient partitioning in dairy goats. The results suggested that differences in milk yield, and overall production efficiency in early lactation, are primarily due to differences in insulin secretion and clearance rates rather than related to differences in peripheral tissue responsiveness to the effects of catabolic and anabolic hormones.
In summary, the research within this thesis provides the first comprehensive overview of both lactation performance and the metabolic status of Australian dairy goats. Collectively, the novel findings presented here contribute to further the current understanding of various aspects of the regulation of energy metabolism in periparturient dairy goats. Just as important, this study also provides the local industry with robust and relevant information on the effects of several factors on the productive and metabolic responses of dairy goats during the transition period. Such information can assist with the optimization of farming practices and breeding plans, thereby accelerating increments in the national herd productivity.||