A goat is not a sheep: physiological similarities and differences observed in two ruminant species facing a challenge of calcium homeostatic mechanisms

Abstract

As research to investigate calcium (Ca) homeostasis in ruminants is often done using sheep or goats as models for the dairy cow, it was the aim of the present work to give an overview of similarities and differences between small ruminant species observed in recent studies. In both species, ruminal Ca absorption and renal Ca excretion were not affected by dietary Ca supply. But while sheep kept on dietary Ca restriction showed decreased plasma Ca and increased phosphate (P) concentrations, goats were able to compensate for the low Ca availability. This might have been caused by the greater stimulation of vitamin D-dependent intestinal Ca absorption observed in goats. As a response to dietary Ca restriction, sheep had a smaller increase in plasma calcitriol but a greater increase in the circulating concentration of a bone resorption marker. Species differences were also found in respect to Ca and P concentrations in ruminal and abomasal fluids as well as in saliva. We could demonstrate a 2-fold greater salivary P secretion for goats compared with sheep. However, the physiological relation of this finding to Ca homeostasis has not yet been clarified. Fundamental differences in the contribution of gastrointestinal Ca absorption and bone mobilisation to the maintenance of Ca homeostasis were also observed in the peripartal period and when lactating and non-lactating animals of both species were compared.