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Effect of a Nitrification Inhibitor on the K, Ca, and Mg Composition of Winter Wheat Forage¹

Winter wheat (Triticum aestivum L.) is an important forage for cattle in much of the Southern Great Plains. However, death losses in wheat pastures due to grass tetany of nursing and pregnant older cattle, and frothy bloat of younger cattle, often occur. Forages from fields moderately or heavily fertilized with N are generally more hazardous than forages from unfertilized fields. As regards grass tetany, high rates of N fertilization are particularly hazardous when soil K is high. The objective of the present study was to determine the effect of the nitrification inhibitor nitrapyrin (N-serve) on the relative uptake of K, Ca, and Mg by winter wheat growing in pots in the greenhouse. When nitrapyrin was added, the plants took up less NO^–3-N because nitrapyrin inhibited nitrification of the added NH⁺⁴. The plants also had lower concentrations of K, Ca, and Mg. However, the depression effect was greater for the divalent cations Ca and Mg, than for the monovalent cation K, so the K/(Ca + Mg) ratio was increased. Therefore, when nitrapyrin was added, the forage was more likely to cause grass tetany. Nitrapyrin also decreased plant organic acid concentrations and this probably decreased Mg and Ca uptake. The concentration of K was higher and Mg and Ca was lower during periods of rapid growth. Therefore, the forage was more likely to cause tetany during these periods. Nitrapyrin was effective in delaying nitrification, and this could be useful for supplying N to wheat later in the growing season. However, other steps might have to be taken to avoid an increased incidence of grass tetany.

Key Words: Aconitic acid • Ash alkalinity • Carbohydrates • Grass tetany • Hypomagnesemia • N • Organic acids • Triticum aestivum L. • N-Serve

² Soil scientists, USDA-SEA-AR Conservation and Production Res. Lab., Bushland, TX 79012; and soil scientist, USDA-SEA-AR, U.S. Plant, Soil, and Nutrition Laboratory, Ithaca, NY 14853.

Received for publication February 17, 1981.