Seasonal Changes in Organic Acids, Water‐Soluble Carbohydrates, and Neutral Detergent Fiber in Tall Fescue Forage as Influenced by N and K Fertilization1

Abstract

AbstractThe seasonal trends of the major energy‐supplying constituents in tall fescue (Festuca arundinacea Schreb.) affect performance of beef cattle in the southeastern U. S. since this grass is usually grazed nearly year‐round.Concern has been expressed about the effect of fertilization on tall fescue that would be grazed during the late winter and early spring when incidence of grass tetany is highest. The objective of this research was to determine the effect of N and K fertilization on three major organic constituents: organic acids, water‐soluble carbohydrates (WSC), and neutral detergent fiber (NDF) in tall fescue. The seasonal trends of these constituents were evaluated in relation to the eriod of incidence of grass tetany in the late winter and early spring, and to seasonal forage quality. Forage was sampled monthly in two 1‐year field experiments near Knoxville, Tennessee, on a thermic, Typic Paleudults soil. The freeze‐dried forage was analyzed for ash alkalinity, an estimate of total organic acids, as well as for WSC and NDF. Fertilization treatments reported are 0 and 112 kg N/ha for the first year and 0 and 168 kg K/ha for the second year. In the first year, N generally increased organic acids but in the second year, K generally decreased them. The most pronounced peaks for organic acids were in spring and summer. The seasonal change in organic acids did not appear to be enough to influence incidence of grass tetany. Nitrogen and K fertilization effects on WSC and NDF generally were not significant. Peaks of WSC occurred in early spring and fall with levels generally higher in the cooler months. The peak of NDF coincided with stemmy material in May. Levels of NDF were generally higher in the warmer months. Highly significant negative correlations between WSC and NDF in both years were the most salient relationships found. These negative correlations are probably the reason for the general impression that fast‐growing fescue forage is associated with incidence of grass tetany. The months with high NDF and low WSC were those in which tall fescue digestibility is usually lowest. The benefit from increased yields with N fertilization would have to be considered in relation to the possible increased incidence of tetany from increased N, K, and organic acid levels.