Abstract:
Two tropical grasses, Panicunl maximum var. trichoglume and Setaria spliacelata cv. Nandi were compared with two cultivars of Lolium perenne, S.24 and Kangaroo Valley, grown in controlled environment at day/night temperatures of 15.6/10, 21 .1/15.6,26.7/21.1, and 32.2/26.7�_C. The plants were harvested when still vegetative. The tropical grasses grew faster than the L. perenne cultivars at all but the lowest temperatures, and had a generally higher relative water content. The in vitro dry matter digestibility of the L. pereiine cultivars appeared inherently higher than that of the tropical grasses, but for each species digestibility Mtas modified by the environment and was highest at the lower growth temperatures. The tropical grasses were lower in alcohol- and cold water-soluble sugars, and higher in starch and structural (cell wall) carbohydrates than the L. perenne cultivars. For all grasses, as temperature increased from 15.6/10 to 26.7/21.1�_C the concentration of the structural carbohydrates increased whilst that of the total non-structural carbohydrates changed relatively little. A further increase in temperature to 32.2/26.7�_ resulted in a marked decline in the concentration of simple sugars and structural carbohydrates whilst polymeric sugars increased. At this latter temperature the starch content in Setaria and Panicurn increased to as high as 16 and 13% of dry weight respectively. The differences in digestibility appeared to be associated with the differences in the proportions of the different carbohydrate fractions.