Grassland Management Technology for Sustainable Agro-Pastoral Systems in the Subtropical Zone of Brazil

摘要

The purpose of this study is to develop grassland management technology for sustainable agro-pastoral systems in the Cerrado region of Brazil. Several pot experiments were conducted for understanding agronomic attributes of main forage species. Responses of three Brachiaria species to water deficit stress were examined in two pot experiments. In the water stress treatment, B. humidicola decreased water requirement and increased water use efficiency both in the two experiments, consistently. Effects of arbuscular mycorrhizal fungi (AMF) on dry matter production and P acquisition were also examined for four grass species (B. decumnens [BD], B. brizantha [BB], B. humidicola [HU], and P. maximum [PM]) at three soil pH levels in a pot experiment. Inoculation of AMF and soil pH affected significantly on dry matter production and P acquisition of the grass species. Effect of soil pH on the mycorrhizal responsiveness in P acquisition was different among the grass species: at soil pH 4.3, PM showed the highest responsiveness, while at 5.1 and 6.4, BB showed the highest values. In the grazing experiment at the National Beef Cattle Research Center (EMBRAPA Gado de Corte), N balance was examined in the two cropping sequences: PM pastures established after 4-year cultivation of soybeans (CS4-P4) and 4-year rotation of soybeans with winter millet (CSW4-P4). PM pasture in CSW4-P4 produced less dry matter than in CS4-P4, because larger amount of soil N was taken from the ecosystem as carcass and via ammonia volatilization from urine or dung during the winter millet grazing in the earlier 4-year rotation. Moreover, forage productivity was monitored during three years on the PM pastures (CS4-P4), BB pastures sown with corns simultaneously (CSW1-C1+P3), and the conventional BD pastures maintained with and without fertilizers. Dry matter productivity of PM pastures (CS4-P4) was higher than the other pastures, whereas the amount of litter decomposed was also highest among the examined pastures, because grazing pressure was insufficient comparing their productivity. On the other hand, forage productivity of BB pastures (CSW1-C1+P3) was less than the other pastures, presumably because the growth of BB was restricted by the competition with the corns at the seedlingestablishment. Thus, it was concluded that maintaining soil N fertility and adequate grazing pressure were keys for the sustainability of the agro-pastoral systems.