Effects of Temperature on Fiber Concentration, Composition, and In Vitro Digestion Kinetics of Tall Fescue1

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Abstract
High temperatures during the growth of forage grasses adversely affect forage quality, but there is little information concerning possible temperature effects on the digestion characteristics of the cell wall. The purpose of this study was to examine the influence of different temperature regimes on the amount, the rate, and the extent of in vitro cell wall digestion in tall fescue (Festuca arundinacea Schreb.). Vegetatively propagated tall fescue plants were grown in a controlled environment over a period of 315 days, during which time the temperature was changed sequentially from 13/10 (T1), to 20/18 (T2), 30/27 (T3), and back to 13/10 (T4)°C, day/night, for a 14‐h photoperiod. Plants were harvested three times at 28‐day intervals at each temperature, after which the temperature was changed. Leaf material from the second and third harvests at each temperature was oven‐dried and ground for analysis. The first harvest at each temperature was discarded in order to reduce the confounding of treatments. Concentrations of neutral detergent fiber (NDF), cellulose, and hemicellulose increased with temperature up to T3 and decreased at T4, while lignin concentration remained about the same. Digestible NDF represented a greater proportion of dry matter in all temperature treatments but showed less variation with treatments than did indigestible NDF. Concentrations of indigestible NDF increased each time temperature was raised and decreased when temperature was lowered. Indigestible NDF concentrations were equivalent to T1 and T4. Rates of digestion of potentially digestible NDF followed first order reaction kinetics, and there were no temperature‐related differences in digestion rate constants. No direct relationship was shown between lignin concentration and digestion. The results indicate that high temperature causes reduced cell wall digestibility by increasing the amount of indigestible cell wall without affecting the rate of digestion of the more labile fraction.