SummaryTwo populations of perennial ryegrass (PRG) (Lolium perenne L.) were selected for low (LS) and high leaf shear breaking load (HS) in 1984 at DSIR, Palmerston North. Leaf shear breaking load for the LS group was c. 41% lower than for the HS group and the LS selection had significantly shorter, narrower leaves and smaller leaf cross-sectional areas than the HS selection. LS leaves were c. 27% less resistant to shear than HS leaves per unit of cross-sectional area. The lower leaf shear strength in the LS selection was associated with a significantly lower concentration of sclerenchyma tissues in the leaf cross-sectional area compared with the HS selection. The total shear load required to break down the same dry weight of leaves to 1 mm particle size (leaf index of masticatory load) was calculated and shown to be influenced by the differences in leaf morphology. LS leaves were estimated to have a 21% lower index of masticatory load than HS leaves when both were grown under optimum climatic conditions.