Most dairy cows exhibit different degrees of hypocalcaemia around calving because the gestational Ca requirements shift to the disproportionately high Ca requirements of lactation. Ca homeostasis is a robust system that effectively adapts to changes in Ca demand or supply. However, these adaptations often are not rapid enough to avoid hypocalcaemia. A delay in the reconfiguration of intestinal Ca absorption and bone resorption is probably the underlying cause of this transient hypocalcaemia. Several dietary factors that affect different aspects of Ca metabolism are known to reduce the incidence of milk fever. The present review describes the interactions between nutrition and Ca homeostasis using observations from cattle and extrapolations from other species and aims to quantitatively model the effects of the nutritional approaches that are used to induce dry cows into an early adaptation of Ca metabolism. The present model suggests that reducing dietary cation–anion difference (DCAD) increases Ca clearance from the blood by dietary induction of systemic acidosis, which results in hypercalciuria due to the loss of function of the renal Ca transient receptor potential vanilloid channel TRPV5. Alternatively, reducing the gastrointestinal availability of Ca by reducing dietary Ca or its nutritional availability will also induce the activation of Ca metabolism to compensate for basal blood Ca clearance. Our model of gastrointestinal Ca availability as well as blood Ca clearance in the transition dairy cow allowed us to conclude that the most common dietary strategies for milk fever prevention may have analogous modes of action that are based on the principle of metabolic adaptation before calving.

This paper reviews the acute and chronic effects of feeding, diet composition and daily variation on equine acid–base status. The dietary cation–anion difference (DCAD) has a marked effect on blood acid–base balance in both the short term and long term. In general, diets with a low DCAD generate an acute acidosis that develops into a persistent though mild systemic, strong ion acidosis with long-term feeding. In contrast, high-DCAD diets result in a mild, persistent strong ion alkalosis characterized by elevated plasma pH, [HCO3] and total CO2 (TCO2). The acute blood acid–base responses to feeding include alterations in plasma pH and plasma concentrations of HCO3, TCO2, Cl−, Na+, K+ and protein. The forage component of the diet appears to be primarily responsible for the acute responses. There is limited research examining abrupt alterations in DCAD and the influence of daily variation on equine plasma acid–base balance. It is concluded that the composition of the diet, duration of adaptation to the diet, timing of feeding and time of day need to be considered when examining acid–base balance in horses.