Effects of shade on some factors influencing nutritive quality of green panic and Siratro pastures

Abstract

The effect of illumination at 100, 60 and 40% sunlight over 4 months on herbage quality was studied in the legume, Siratro, and the grass, green panic, grown in small plots in the field in pure and 50 : 50 mixture swards, defoliated every 4 (D4) or 8 (D8) weeks. Laminae and stems of defined ages were harvested periodically from the D8 treatment, and after 8 and 16 weeks the D4 and D8 canopies were sampled in 10-cm strata for analysis of dry matter digestibility (DMD). Shading to 60 and 40% sunlight decreased the DMD of the green panic and the mixed swards by up to 10-12 percentage units in the D8 treatment. The shade effect on DMD was smaller in the D4 treatment. Shading increased leaf bulk density in the upper strata of the grass and mixed swards. The shade effect on sward DMD was due to a decrease in DMD of lamina and stem of green panic and, in the lower canopy, also to a decrease in leaf : stem ratio of the grass. The lower DMD of green panic was associated with lower total soluble carbohydrate and higher lignin in the shaded tissues. The concurrent decrease in cell wall content of green panic under shade (as a balance to higher crude protein) indicates that shade must have greatly reduced the digestibility of the cell wall material. The chemical composition and DMD of Siratro was largely unaffected by shade.