Measurements of physical strength and their relationship to the chemical composition of four species of Brachiaria

Shear strength and grinding resistance of leaves of four species of Brachiaria collected at 4 and 6 weeks of re-growth were determined using modifications of techniques described in the literature. Physical attributes of strength were correlated with analyses of plant cell wall constituents and in vitro digestibility (IVDMD). Methodologies were compared to determine which was best able to describe the physical strength of the samples. Both the grinding resistance and the shear strength technique were able to detect differences between Brachiaria species and ages of re-growth. However, the shear strength technique was more sensitive for identifying physical strength differences at the species level. Both techniques identified the same species (Brachiaria ruziziensis) as the softest, but subsequent ranking of species by the shear strength technique depended on the leaf morphological characteristic used to express the results. Shear strength measurements were correlated to the cell wall components and IVDMD of the samples. The highest correlations were obtained for acid detergent fibre (ADF), cellulose and lignin with shear strength measurements expressed per unit of leaf width (kg/cm), per unit of linear density (kg/g cm) and for the raw shear strength data (kg). Grinding resistance was not correlated to the chemical composition and IVDMD of the samples. Preferential use of the shear strength technique is suggested since it provides a sensitive measure of the physical strength of forage leaf tissue and is a suitable indicator for identifying nutritive quality differences between Brachiaria species.