Seasonal changes and the effect of temperature and leaf moisture content on intrinsic shear strength of leaves of pasture grasses

The intrinsic shear strength of leaves of 6 temperate pasture grasses was measured using a guillotine. The intrinsic shear strength of the leaves was represented as the force required to shear a leaf corrected for the length of cutting blade in contact with the material. When green leaves were maintained at 100% relative humidity, their moisture content was maintained and their intrinsic shear strength remained constant over a 24-h period. Shear strength increased when leaves were allowed to dehydrate. Shear strength varied significantly over a growing season, with strength generally higher in summer and lower in winter. When grown in a constant regime with day/night temperatures ranging from 14/6 degrees C to 34/26 degrees C, no consistent effects of temperature on shear strength were observed across species; however, higher temperatures did increase lignin content of the leaves and decreased in vitro dry matter digestibility. Any one chemical constituent of the leaves (e.g. lignin, cellulose) did not appear to contribute significantly to their strength. The intrinsic shear strength of senescent leaves of Phalaris aquatica was significantly higher than that of green leaves, but there was no correlation between the strength of green leaves and that of senescent leaves. Hence, mature forages during the summer period will exhibit increased intrinsic shear strength at a time of the year when the digestibility of the feed also declines and the quality of feed begins to limit animal production.