Nutritional management of the transition cow in the 21st century - a paradigm shift in thinking

The transition period is defined as the 6-8 weeks encompassing late pregnancy and early lactation, involving coordinated changes across multiple tissues and an enormous increase in nutrient requirements. Failure to transition successfully can result in reduced DM intake, milk production, delayed oestrus, failure to conceive and increased incidence of metabolic and infectious diseases, many of which are inter-related. Modern technologies have enabled the measurement of transcriptional changes in genes involved in multiple biochemical pathways across the transition period, enabling a better understanding of the implications of management and nutritional changes on cow health and productivity. Most recent research efforts have focussed on the association between pre-calving energy intake and postpartum health and productivity, with a general recognition that the positive relationship between pre-calving energy intake (and relevant circulating metabolites) and postpartum health and productivity is, for the most part, not causative (i.e. responses are very likely to reflect the same metabolic perturbation, but one is not necessarily the cause of the other). This effect is consistent in both grazing systems and in systems where cows are fed total mixed ration in confinement. These results require a paradigm shift in the extension message to farmers. Because of the focus on energy nutrition, there has been only limited recent research on the requirements of cows for protein, with recommendations based largely on predicted requirements rather than measured responses. That said, metabolisable protein is unlikely to be a limiting nutrient for late-gestation dairy cows grazing up to 50% of their diet as high-protein forages, but could potentially be limiting prepartum mammary development in animals on lower protein diets, such as total mixed rations formulated for dry cows. The physiological role of fatty acids, in addition to the role of fat as an energy source, is an emerging and important research area, with increasing evidence, at least in vitro, that specific fatty acids regulate metabolic processes. Knowledge gaps and future research areas that should be prioritised are identified and discussed.