Nitrogen Fertilization Effects on Seasonal Ca, Mg, P, and K Levels of Western Wheatgrass and Green Needlegrass¹

Abstract

Nitrogen fertilization has been found to increase forage production of range grasses in the northern Great Plains; however, little information is available as to how N fertilization may affect the mineral composition of the forage. Our objective was to determine the effects of N fertilization on the levels of P, K, Ca, and Mg, and the grass tetany proneness of the vegetative and floral tillers of western wheatgrass (Agropyron smithii) and green needlegrass (Stipa viridula) throughout the growing season. Nitrogen fertilizer (0, 40, 80, 160, 320, and 640 kg N/ha) was applied in a completely randomized design to each grass species in a calcareous silt loam (Typic Haploboroll). Plant samples were collected on 10 and 11 sampling dates in 1973 and 1974, respectively. Nitrogen fertilization generally increased Ca, Mg, K, and ash alkalinity and generally had little effect on the P levels of the forage; however, the absolute changes caused by N fertilization were very small and of little practical significance. Except for K and P, the level of each nutrient generally increased significantly in each grass as the growing season progressed from April to October. Season or growth stage had more effect than did N fertilization on forage nutrient content. Floral tillers had lower levels of each nutrient than did the vegetative tillers. The Mg and P contents of both the floral and vegetative tillers of both grasses were often < 0.2%. Therefore, supplementation of Mg and P to grazing livestock may be required. The potential for grass tetany, according to the K/(Ca + Mg) ratio of the forage and blood serum Mg estimates, was highest during April and May for both grasses. Western wheatgrass tended to have a greater potential for causing tetany than green needlegrass early in the growing season. Time of harvest and percentage of vegetative and floral tillers can greatly affect the concentration of K, P, Mg, and Ca in the forage. In general, the earlier the harvest, the higher the nutritional value of the forage.