Influence of Climatic Factors and Forage Age on the Chemical Components of Ryegrass Related to Grass Tetany

Climatic factors are known to influence several soil-plant-animal relationships, including many criteria used to evaluate the grass tetany potential of forages. Specific climatic influences on these criteria are not well understood. Our objectives were to evaluate effects of growth temperature, soil moisture level, length of flooded period, and forage age on the following forage chemical parameters that have been associated with the incidence of grass tetany: N, K, Ca, Mg, AI, and total water-soluble carbohydrate (TWSC) concentrations plus ratios of N/TWSC and K/(Ca+Mg). Annual ryegrass (Lolium multiflorurn Lam.) grown in growth cabinets at two daylnight temperatures (21/13 and 5/0 °C), two soil moisture levels (flooded and optimum), and three flooded periods (7, 14, and 21 days) was harvested 35,42, and 49 days after planting. Regrowth tissue was harvested on Day 49 from plants harvested on Days 35 and 42 (14 and 7 days regrowth, respectively). Temperature and soil moisture variables were initiated 21 days after planting in Tensas silty clay (Vertic Ochraqualf) collected from a tetany-prone pasture. Ryegrass exposed to 5/0 °C for 2 weeks consistently contained less than the critical Mg level of 2.0 g kg⁻¹, while that grown at 21/13 °C always contained > 2.0 g kg⁻¹ Mg. Concentrations of Mg, Ca, K, and N were higher in younger tissue than older tissue and higher in regrowth than in first growth tissue. Regrowth tissue produced at low temperatures contained only 2.0 g kg⁻¹ Mg, but over 34 and 51 g kg⁻¹ N and K, respectively, and had K/(Ca + Mg) ratios exceeding 3.0. Concentrations of TWSC were also very low in regrowth tissue. Although soil moisture effects were small, Mg concentrations were lower under flooded soil conditions. Forage AI concentrations were generally < 50 µg g⁻¹, regardless of environmental conditions imposed or age of forage at harvest, indicating that AI level was not a factor in determining the grass tetany potential of ryegrass under these conditions. The study shows that grass tetany potential of ryegrass is higher after cold, wet periods and that young regrowth tissue can increase that potential