Practical implications of mineral and vitamin imbalance in grazing sheep

Abstract

Rapid progress in research on mineral functions in biological systems over the past 20 years has provided new and unexplored implications for health and production in grazing livestock. The strong interaction among calcium (Ca), magnesium (Mg), phosphorus (P), sodium (Na), potassium (K) and vitamin D influences the absorption, utilisation and status of these nutrients, particularly Ca and Mg. An imbalanced intake of Ca, Mg, P, Na, K and vitamin D is common in ruminants grazing pastures and vegetative crops. The interaction may modify susceptibility to clinical hypocalcaemia, hypomagnesaemia and pregnancy toxaemia, or to subclinical and chronic deficiencies of Ca, Mg and Na. However, hypocalcaemia is also associated with endocrine failure. Whether this failure is susceptible to nutritional modulation or even whether supplements of Ca, Mg, Na and vitamin D reduce or exacerbate the risk of metabolic disease in sheep is unclear. Selenium, manganese, zinc, copper, sulfur, vitamin E and vitamin A play synergistic roles in the antioxidant defence mechanism and modulate the consequences of oxidative stress. In extensive grazing systems combined low intakes of these antioxidants are not unusual, particularly in seasonally dry environments and at a time coinciding with the increased oxidative stress that naturally occurs through the reproductive cycle. This oxidative stress is accentuated by heat stress and parasite infection. Oxidative stress in the short-term influences growth, reproduction, offspring survival and health. Long-term, oxidative damage to embryo DNA and changes in RNA expression, may influence lifetime performance of offspring. The high cost of providing mineral supplements to grazing sheep is a deterrence to addressing the implications of multiple mineral deficiencies. New herbaceous and shrub options to increase the botanical and nutritional diversity within pasture and crop-grazing systems may allow livestock to select a heterogeneous diet providing a more balanced mineral intake.