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 m easured 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 maint ained 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 stren gth varied significantly over a growing season, with strength generally hig her 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 cons istent effects of temperature on shear strength were observed across specie s; however, higher temperatures did increase lignin content of the leaves a nd decreased in vitro dry matter digestibility. Any one chemical constituen t of the leaves (e.g. lignin, cellulose) did not appear to contribute signi ficantly to their strength. The intrinsic shear strength of senescent leaves of Phalaris aquatica was s ignificantly 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 sh ear 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.