Potential Productivity of Temperate and Tropical Grassland Systems

Summary Grasses of temperate regions of the world belong primarily to the subfamily Festucoideae, whereas tropical regions are populated by tribes of the Panicoideae subfamily. Temperate grasses have a C₃ biochemical pathway of photosynthesis in contrast to tropical grasses, which have a C₄ photosynthetic mechanism. Both tropical and temperate legumes are C₃ plants. Compatible mixtures of legumes and grasses have long been successful in many temperate regions, but combining C₄ grasses and C₃ legumes as mixtures in the tropics has been only moderately successful. Maximum net primary productions of temperate and tropical grasslands are in the order of 25 and 80 tons/ha/yr, respectively. Thirty tons/ha/yr is considered an exceptionally high yield in the wet-dry tropics, and no more than 10 to 15 tons/ha/yr can be expected at 40° N latitude. Rates of conversion of net primary production to animal yield in temperate and tropical grasslands are quite different. Although net primary production in tropical grasslands may be several times that of the most productive temperate grasslands, a much smaller proportion of the production may be recovered by the grazing animal. Animal yields/ha/yr on high-producing cultivated temperate and tropical pastures seldom exceed 1,000 kg. Failure of tropical pastures to give higher yields of meat and milk commensurate with their greater net primary production can be partially accounted for by much greater losses in the grazing process. Other factors are lower daily intake of energy above maintenance, which results in a much larger proportion of the energy consumed/ha being expended for maintenance than for performance. It is concluded that more nearly maximum yields of animal product for any particular environmental circumstance will be attainable when management schemes are devised to match feed supply with demand and to recover maximum net primary production mutually beneficial to both pasture and animal. Other strategies include improving the quality of consumed forage by genetic manipulation, developing legume and grass cultivars that will coexist in mixtures, conserving forages, and supplying the mineral requirements of the grazing animals.