Beef Species Symposium I: Cattle Adapted to Tropical/Subtropical Environments
Warm-season perennial grass, also called C4, are the predominant forages for ruminant production in tropical and subtropical regions. In general, C4 grasses have greater herbage production in warm-climate regions due to a more efficient carbon fixation pathway than cool-season forages (C3) and anatomical characteristics results in C4 plants with less concentration of CP and soluble carbohydrates, and greater concentrations of cell wall components such as cellulose and hemicelluloses than C3 grasses. Several factors may contribute to the differences in nutritive value in C4 grasses, including species, cultivars within species, maturity, fertilization, and conservation practices. Routine forage testing programs only using NDF and ADF may often provide unacceptable values of DM intake and digestible DM for C4 grasses. Forage quality is a function of nutritive value and intake; however, it is challenging to estimate intake on grazing animals and DM intake is not well correlated to NDF concentration in C4 grasses. The vertical heterogeneity in sward canopy composition and the amount and accessibility of leaf are major canopy characteristics associated with intake. Green herbage mass or green leaf proportion in grazed horizon usually shows positive relationship with bite weight and intake is a function of bite weight and bite rate. Leaf density, plant-part composition and nutritive value of the upper canopy are also important to explain the variations in intake. Besides the plant-intrinsic characteristics, animal genetics and environmental factors may affect intake as well. Grazing is the most used method of C4 grasses utilization in tropical and sub-tropical regions; however, a small proportion of the forage produced is conserved as hay or silage. Greater forage production during the growing season may coincide with greater rainfall and inadequate climactic conditions for harvesting and baling forage in tropical and subtropical regions.