YIELD, COMPOSITION AND QUALITY OF TALL FESCUE AS INFLUENCED BY N FERTILITY AND SOIL-WATER AVAILABILITY

Yield, quality and composition of tall fescue as influenced by water stress under 0, 100 and 400 kg N .cntdot. ha-1 during autumn in the southeastern USA was evaluated. Four replicates of each treatment were imposed in a completely random design in the field. The experiment was conducted in 2 successive years with treatments applied to an adjacent area to eliminate residual or carry-over effects. Water status was verified by gypsum block, tensiometer and determination of leaf stomatal resistance. Plant tissue was harvested 3 times during autumn and again in the following spring. Forage yield total was not significantly affected by soil water stress, apparently due to compensatory growth during recovery periods. Soil water appeared to be more readily depleted under 400 kg N .cntdot. ha-1 than other N treatments. Increasing N fertilizer significantly increased leaf area and leaf number, and was inversely related to the number of senesced leaves. Stomatal resistance increased under water stress conditions when N was limited (zero-N) but not when 400 kg N .cntdot. ha-1 was applied. Plots receiving 400 kg N .cntdot. ha-1 continued to transpire even during periods of limited soil water, but transpired less during subsequent stress periods. Kjeldahl and NO3-N were increased by application of fertilizer N and only NO3-N was significantly increased by water stress. Although high input of fertilizer N minimized the impact of mild water stress on the physiological response of tall fescue, eventual stand loss occurred. Water stress had no effect upon nonstructural carbohydrates, in vitro dry matter digestibility or the elements P, K, Ca, Mg, Cu and Zn. Consequently, the factors associated with the expression of tall fescue toxicosis during autumn are more subtle in expression, and do not appear to be associated with alterations in major nutritive components.