Summary
The Brazilian government is presently encouraging the rapid development of cattle pasture in large parts of the Amazon Basin through a massive program of financial and tax incentives. This has been encouraged by recent claims in Brazil that pasture improves soil quality, and therefore presumably could supply an indefinite yield of cattle to the ranchers that are responding to the incentives program. Calculation of beef productivity to be expected in areas such as Brazil's Transamazon Highway by a variety of means all lead to the conclusion that government expectations are overoptimistic. The present paper develops equations for prediction of cattle yields for a study area on the Transamazon Highway including the effects of both soil nutrients and competition from weeds. Low levels of soil phosphorus and invasion of inedible weeds soon reduce grass and cattle yields to very low levels. Results of a computer simulation of cattle yields based on the relationships developed linking yields to soil nutrients and weeds, and on the soil information gathered in the Transamazon Highway study area, are presented. The pasture simulation forms a part of a larger simulation of the agroecosystem of the Transamazon Highway colonists which is aimed at assessing the importance of various factors on the carrying capacity of the area for human populations. Although the carrying capacity study shows that agriculture based largely on annual crops is a risky business, it also casts doubt on the presumption of secure and sustainable yeilds from the pasture development schemes currently being promoted in the Brazilian Amazon.