摘要
Excessive intensification of dairy and beef cattle production systems in the industrialised countries has led to serious problems of pollution of water resources and the atmosphere. In order to develop an appropriate alternative, a few studies have been made by various research teams in Brazil, using low fertiliser inputs and modest animal stocking rates. The objective of this present study was to evaluate the effect of different stocking rates of beef cattle, and the introduction of a forage legume (Desmodium ovalifolium (Prain) Wall.), on the long-term sustainability of pastures of Brachiaria humidicola (Rendle) Schweick established in the Atlantic forest region of Brazil in the extreme south of the State of Bahia. Annual maintenance fertilisation was restricted to additions of 11 and 6 kg ha−1 of P and K, respectively (and no N). Live weight gain (LWG) of Zebu steers was evaluated for stocking rates of 2, 3 and 4 head ha−1 during six grazing periods from 1988 to 1997. Forage intake and the proportion of legume in the acquired ration was determined using steers fitted with oesophageal fistulae. The bolus samples were analysed manually in 1988–1989, and using the 13C natural abundance technique in 1995. There was no significant response of LWG to the presence of the legume in the acquired ration. LWG in the final grazing period (1995–1996) was similar to that recorded in 1988–1989 at all stocking rates, suggesting that this management regime resulted in long-term sustainable production even in the absence of the legume or of a N fertiliser input. This was confirmed by the soil fertility analyses for 1988 and 1997, where only levels of P showed a significant decrease. The net aerial primary productivity (NAPP) of the pasture was determined for 1995, the largest component being deposited litter (21–33 Mg DM ha−1 year−1), followed by forage intake (6.4–12.2 Mg DM ha−1 year−1).