Magnesium, Nitrogen Form, and Root Temperature Effects on Grass Tetany Potential of Wheat Forage

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Abstract
Grass tetany, a Mg deficiency syndrome, is a disorder of ruminants grazing grass or cereal forages under certain environmental conditions. A solution culture experiment was conducted to gain a clearer understanding of the nutritional and environmental factors affecting Mg concentrations in winter wheat (Triticum aestivum L.) forage. Effects of N form (NH4/NO3 in mM—10:0, 5:5, or 0:10), root‐zone temperature (RZT) (10,15, or 20 °C), and Mg supply (0.4 or 4 mM) on growth and mineral composition of winter wheat forage were investigated. Shoot and root yields were highest for the mixed‐N form, and increased linearly with increasing RZT. Increasing the Mg supply significantly increased Mg concentrations, and decreased Ca concentrations, in both shoots and roots. Concentrations of K and Ca, and estimates of total organic acids significantly increased with increasing proportion of NO3. There was no significant effect of N form on shoot Mg concentrations at the high Mg level. However, at the low Mg level, increasing the proportion of NO3 significantly decreased shoot Mg concentrations, producing a forage more likely to cause grass tetany. Ratios of Mg accumulated in shoots to Mg in whole plants were negatively correlated with K concentrations in roots. Correlations with K concentrations in shoots were much lower. Apparently, increasing the K concentration in roots depressed the rate of net Mg translocation from roots to shoots. When Mg in the root zone is low and K is high, increasing the proportion of NO3 may decrease shoot Mg concentration and thus increase the likelihood of hypomagnesemia.