Mg, Ca, and K Status of Silage Corn and Wheat at Periodic Stages of Growth in the Field

The aim of this study was to compare the Mg and Ca content of a winter forage — wheat (Triticum aestivum) with a summer forage — corn (Zea mays L.) on several levels of N, P, and K fertilization. Specifically, the effect of soil fertility level and growth stage of the plants on the Mg content of the plant was studied with its connection to potential hazard of grass tetany (hypomagnesemia). Knowing the intrinsic differences between species of forage and the trend in Mg content changes during the growing season might help in preventing grass tetany. In two separate years corn and wheat were grown on a permanent plots fertilization experiment at the Bet Dagan experimental farm, Israel. In this field, five levels of N and P and three levels of K are checked in a factorial design. In the corn experiment the factorial combination of three levels of N; 0, 200, and 400 kg N/ha; three levels of P; 0, 80, and 150 kg P/ha; and two levels of K; 0 and 310 kg K/ha; were selected for detailed study. Plant samples were taken every week. In the wheat experiment five levels of N; 0, 30, 60, 120, and 240 kg N/ha on three levels of P; 0, 16, and 64 Kg/ha, and K at 270 kg/ha were sampled every 2 weeks. The plant material was dried, weighed, and analyzed. Magnesium and Ca uptake by wheat were found to be highly correlated (r = 0.986 for Mg and r = 0.982 for Ca) with N uptake and relatively unaffected by P fertilization. The absolute amounts of Mg and Ca taken up per ha of crop produced were considerably higher in corn i.e., 33.0 kg Mg/ha and 53.9 kg Ca/ha, than in wheat, i.e., 13.8 kg Mg/ha and 22.5 kg Ca/ha, when averaged over all treatments, exclusive of Kₒ. The antagonistic K-Mg effect was readily observed in corn. Thus, 64.3 kg Mg/ha taken up in the N₂₀₀P₁₆₀K₀ treatment was reduced to 39.1 kg Mg/ha in the N₂₀₀P₁₆₀K₃₁₀ plots at equal dry matter production. With the exception of one stage of growth in wheat 4 weeks after emergence, Mg concentrations were below 0.2%, a level considered critical in the prevention of hypomagnesemic disorders in grazing cattle and sheep. Lowest Mg concentrations in Wheat, i.e., 0.06% to 0.07%, were observed in February just prior to and in the early stages of heading, 2 to 4 weeks prior to maximum dry matter production. Because of its lower uptake of Mg and Ca, wheat silage should be supplemented by Mg additives to prevent hypomagnesemic disorders. Based on this study, it is recommended that special attention should be paid to add Mg salts to milking cows in the mid-winter months when grass pastures are used.