Minerals in pastures—are we meeting the needs of livestock?

Abstract

In Australia, ruminants rely on introduced pastures or native vegetation for most or all of their nutritional requirements. Recent pasture selection and breeding programs have focused on improving or facilitating the establishment, persistence and growth of plants, with little emphasis on nutritive value or mineral composition. In some cases, such as selection for phosphorus (P) utilisation efficiency, mineral supply from plants may even decrease. Currently, a significant proportion of pasture plants contain less calcium (Ca), P, magnesium (Mg), sodium (Na), sulfur, copper, iodine, zinc, selenium or cobalt than is required for growth and reproduction, with significant genetic variation among and within legumes and grasses. Young crops and shrubs are now also an integral part of grazing systems. Many young crops contain concentrations of Ca, Mg, Na and potassium (K) that are low or imbalanced for ruminants. Conversely, many shrubs contain minerals at levels higher than required by livestock. Livestock requirements may have changed in recent years with animals selected for more efficient feed conversion, and flock and herd structures changed to increase productivity. New studies have indicated that higher mineral supply may be beneficial during periods of oxidative stress related to growth, reproduction, and external stresses such as heat and parasites. These results indicate that mineral supply from pastures is not sufficient to support high levels of production for at least part of the year and that designing grazing system to incorporate the complementary benefits of grasses, legumes, crop forage and shrubs may improve the mineral status of grazing ruminants.