New perspectives on the mineral nutrition of livestock grazing cereal and canola crops

Abstract

The grazing of cereal and canola crops during winter is an increasing component of grazing systems in southern Australia. The capacity of such crops to meet the requirements of young livestock and reproducing animals for sodium (Na), magnesium (Mg), calcium (Ca) and potassium (K) is reviewed. The growth responses of young stock to Na and/or Mg supplements when grazing wheat are discussed. These responses differ from ‘literature expectation’ in that they are rapid in onset and involve no clinical signs. We demonstrate that more insight about the variability in supplement response, both between and between crops species, is obtained when mineral contents are viewed from the perspective of their frequency distributions, rather than their mean values. The pivotal importance of high K concentration, low Na and the resultant K : Na ratio of forage is stressed, as is the interaction of these minerals in relation to Mg absorption. We also present frequency distributions for a range of mineral indices that ‘capture’ mineral interactions, including the ‘tetany index’, forage K :  (Na+Mg) ratio and dietary cation–anion difference. It is concluded that the last two indices warrant a much closer investigation. Possible effects of forage aluminium and organic acid contents are briefly discussed; these also deserve closer examination. As research in this area continues, it would be prudent to provide Na and/or Mg supplements for livestock grazing wheat and possibly oats and barley, and calcium supplements for livestock grazing oats or for reproducing animals grazing all three cereals.