Changes in rumen absorption processes during transition

Transition is defined as the period from 3 weeks before parturition until 3 weeks postpartum and is characterized by changes in endocrine status and by major alterations in the requirement for nutrients, vitamins and minerals. At parturition, the dry period diet is replaced by an ad libitum offered lactation diet, which causes changes of rumen fermentation because of its higher energy density and the overall increase of dry matter intake. Hence, ruminal fermentation parameters such as pH, volatile fatty acid (VFA) concentrations and osmotic pressure are changed. This causes alterations in the rumen epithelium generally termed adaptation: the modulation of enzyme activities, an increase in the number and size of papillae and enhanced transport rates of ions. These effects are well documented when sheep or cows are experimentally and independent of parturition shifted from a low energy (roughage) to a concentrate diet. Only a few studies have been conducted in cows during transition and, thus, data on diet dependent effects on postpartum rumen acidosis are absent and information regarding number and size of papillae is inconsistent. Furthermore, investigations of gene expression related to VFA absorption or metabolism have not shown diet dependent alterations. However, evidence from in vitro and in vivo studies exists that the stimulation of fermentation before parturition enhances the transport rates of Na, Cl and VFA across bovine rumen epithelium. The underlying mechanisms for epithelial adaptation to dietary alterations are poorly understood at present. Luminal factors such as increased VFA concentrations (particularly butyrate) and the hormonal effects of insulin and of insulin like growth factor 1 have partly been confirmed to be involved in the regulation of this adaptation process. More research is needed in order to determine the factors causally inducing adaptational responses and the amount of dietary change at which the adaptational responses apparently level off. Despite these uncertainties, an increase of energy and protein intake undoubtedly improves the barrier and transport functions of the epithelium. Consequently, at least for 2 weeks before parturition, a diet should be offered that is fermented more easily and more rapidly than the diet in the early dry period. Furthermore, efforts should be directed towards an improved voluntary food intake before and after parturition, with special attention being paid to the mechanisms enabling the epithelium to adapt to and handle the regular diurnal variations in pH, VFA concentration and osmotic pressure, mechanisms that seem to fail in the pathophysiology of subacute rumen acidosis.