POTASSIUM EFFECTS ON MINERALS AND ORGANIC-ACIDS IN 3 COOL-SEASON GRASSES

Cattle frequently develop grass tetany when grazing cool-season forages, and the incidence is increased when K concentrations in plants are high. High concentrations of organic acids may also increase the incidence of grass tetany by complexing Mg and Ca. The objective of this experiment was to study the effects of K fertilization on the concentrations of major cations, inorganic anions, and organic acids in cool-season forages. An experiment was conducted in a controlled environmental growth chamber using a mixture of 50% silt loam soil and 50% coarse sand. There were two K levels (0 and 125 mg K kg-1 soil mixture) and three species of cool-season grasses: Linn perennial ryegrass (Lolium perenne L.), Nordan crested wheatgrass (Agropyron desertorum Schultes), and Lincoln smooth bromegrass (Bromus inermis Leysser). Potassium fertilization significantly increased K concentrations and the K/(Ca + Mg) ratios (on a mol(c) kg-1 dry wt. basis), but significantly decreased the concentrations of Mg and Ca in shoots of all species. The addition of K significantly increased the concentrations of aconitic acid in shoots of crested wheatgrass and bromegrass for both harvests. The concentrations of malic acid in shoots of all species, at the first harvest, were markedly increased by K fertilization. However, at the second harvest, the earlier addition of K did not significantly affect the malic acid concentrations in shoots of any of the three species. The concentrations of total organic acids (whether measured directly or indirectly) in shoots of all species were significantly increased by K fertilization at the first harvest, when added K was high in the soil-sand mixture. When cool-season grasses are fertilized with high levels of K and NO3-N, the forage produced is more likely to cause grass tetany in grazing animals.