THE EFFECT OF HIGH GROWTH TEMPERATURE ALONE AND IN COMBINATION WITH WATER STRESS ON FORAGE YIELD AND QUALITY OF TWO TROPICAL GRASSES

Two tropical grass species were grown under controlled conditions to evaluate the effects of high temperature and water stress on forage growth and quality. Coastal bermudagrass (Cynodon dactylon (L.) Pers.) and Kleingrass 75 (Panicum coloratum L.) were cloned, established in pots and grown under day/night temperatures of 30/20, 35/25 and 40/30 C. Two water levels (-9 and -12 bars leaf water potential) and 3 stages or age of regrowth at harvest (fifth leaf stage, 21 days regrowth, and anthesis of Kleingrass) were used at each growth temperature. Forage yield, percent leaf, plant height and number of tillers per plant were determined at each harvest. Leaf and stem fractions were analysed for in vitro dry matter digestibility, and cell wall components. Digestibility coefficients for the neutral detergent fiber, hemicellulose and cellulose fractions were determined.;Water stress reduced dry matter yield approximately 38% primarily by a reduction in tiller number per plant. The percent leaf was greater for the water stressed plants than for the well watered plants but the leaf area was less due to reduction of growth and delayed maturation.;In vitro dry matter digestibility (IVDMD) significantly decreased as growth temperature increased from 30/20 to 35/25 C (day/night) then increased as growth temperature was further increased to 40/30 C. IVDMD means for 30/20 and 40/30 C were not significantly different. The changes in dry matter digestibility were associated with changes in percent lignin, and the digestibility of the hemicellulose component of the cell wall.;At the fifth leaf stage, water stress tended to decrease in vitro dry matter digestibility of the stem fraction of Kleingrass and the stem and leaf fractions of Coastal bermudagrass. This decrease was associated with a decrease in the percent and digestibility of cellulose, and an increase in the percent of lignin of the water stressed plants. At the more mature stages of regrowth, water stress did not alter IVDMD of the leaf or stem fractions of Coastal Bermudagrass or Kleingrass.;Increased growth temperatures significantly increased dry matter yield by accelerating maturation rate and development of individual tillers. Significant increases in leaf area, weight per tiller and plant height with increased growth temperature were observed, but increased temperature did not influence the percent leaf of the plants.