RELATIONSHIPS BETWEEN SEASONAL FORAGE QUALITY PATTERNS AND STRUCTURAL CARBOHYDRATES OF WARM-SEASON GRASSES AND ENVIRONMENTAL FACTORS

Three warm-season grass species were grown to evaluate relationships between forage quality and environmental conditions. Two selections of kleingrass (Panicum coloratum L.), Kleingrass 75 and Kleingrass 75-25, green sprangletop (Leptochloa dubia (H.B.K.) Nees), and plains bristlegrass (Setaria macrostachya H.B.K.) were transplanted to the field in 1978. Forage yield, percent leaf, and in vitro digestibility of leaf and stem fractions were determined at three harvest frequencies during the 1978 and 1979 growing seasons. Correlations of percent leaf and the in vitro digestibility of leaf and stem fractions of each grass at each harvest frequency with the environmental factors rainfall, temperature, relative humidity, day length, and soil moisture were determined. In vitro digestibility of leaf and stem fractions was negatively correlated with temperature and day length and positively correlated with measures of moisture conditions (rainfall, relative humidity, and soil moisture). Relationships between percent leaf and the environmental factors measured were extremely consistent indicating that the observed seasonal patterns of forage quality were associated with changes in digestibility of the leaf and stem portions of the plants rather than just changes in plant leafiness.;Neutral sugar composition of kleingrass leaf and stem holocellulose was determined by gas chromatography. Statistically significant relationships were observed between the proportions of these monosaccharides and in vitro digestibility. The arabinose:xylose ratio decreased in stem material as digestibility decreased in association with internode elongation. In leaf samples, the proportions of xylose and glucose were associated with digestibility. Increased proportions of xylose and decreased proportions of glucose were associated with decreased digestibility. The variation observed in monosaccharide composition of leaf cell walls may have been associated with secondary cell wall development. Variation in stem neutral sugar composition was related to environmental conditions during plant growth, but statistically significant relationships were not obtained between leaf neutral sugar composition and environmental factors.;Multiple regressions of percent leaf, leaf digestibility, and stem digestibility on the environmental factors monitored were performed. These analyses indicated that relationships existed between the measures of forage quality and environmental conditions. However, the amount of variation in forage quality measures accounted for by the models used was extremely variable. Trends in forage quality could be anticipated from environmental conditions during plant growth, but sufficient regression relationships were not obtained to allow prediction of specific digestibility values from environmental conditions during growth.