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

Shearing strength as an additional selection criterion for quality in Brachiaria ecotypes

Leaves of 12 accessions of Brachiaria sp. (three accessions from each of four species) were subjected to tests of shearing strength using a Warner Bratzler meat shear. Morphological measurements of these leaves were also taken in an attempt to standardize shearing strength measurements per unit of leaf tissue. The technique was evaluated for its capacity to detect differences between accessions of the same species. All samples were also analysed for dry matter (DM), organic matter (OM), neutral detergent fibre (NDF), acid detergent fibre (ADF), lignin, crude protein, silica and digestibility in order to relate measures of physical strength to chemical components. Results demonstrated the presence of a clear relationship between shearing strength and structural (cell wall) components. Correlations between shearing strength and NDF, ADF and lignin were 0·74, 0·82 and 0·80 respectively. The technique identified clear differences between accessions of the same species (P < 0·05). These measurements can thus be used as a powerful tool for identifying forage plants of superior nutritional quality within selection programmes. Measurements of strength standardized for morphological characteristics illustrated that the ranking of the samples varied considerably depending on which morphological character was used