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The paper by Dove and Kelman (2015b) in this issue of Animal Production Science continues a series of Australian publications addressing the need for mineral supplements for sheep grazing dual-purpose wheat (Dove 2007; Dove et al. 2007, 2012; Dove and McMullen 2009). Responses have been variable but overall assessment indicates there are positive growth responses to sodium (Na) and magnesium (Mg) supplements. The specific causes of the responses have not been easy to interpret due to a multiple mineral imbalance. The wheat plants are very low in Na (often 10–20% of requirement) and marginal in Mg and calcium (Ca); the situation is further complicated by high potassium (K) (800–1000% of requirement) (Dove and Kelman 2015b). The high K and low Na potentially reduce Mg absorption and the high K may also interfere with Ca metabolism by causing a high, positive dietary cation–anion difference (Masters and Thompson 2015). It is worthwhile examining these responses in more detail. In early experiments, significant liveweight responses (30–50%) were measured following supplementation with a mixed mineral supplement (containing Mg, Na and Ca) or to supplements that contained Na, Na + Ca or Na + Ca + Mg (Dove and McMullen 2009). The authors concluded that the responses were most likely due to Na and Mg supplementation. They went further in suggesting that the response may be primarily due to increased Mg supply to the sheep, either as a result of specific Mg supplementation or as a consequence of Na supplements lowering the K:Na ratio and increasing Mg availability. These conclusions are similar to those of previous studies where liveweight responses in young sheep were measured in both Na- and Mg-supplemented sheep (Dove et al. 2007). In a subsequent study, young sheep grazing dual purpose wheat did not respond to supplements of Na and Mg (Dove et al. 2012). Importantly, there do not appear to be any cases where responses to Mg + Na supplementation have markedly exceeded those to Na supplementation alone. This means the responses are not additive and therefore unlikely to be mutually exclusive

Mineral nutrition of sheep: new insights into interactions when grazing vegetative crops