The Principles and Practice of Weed Control for Notillage Soyabean in Southern Brazil Using the Bipyridyl Herbicides

Abstract

An ICI survey of the arable area of southern Brazil in 1971 confirmed the urgent need for some form of conservation tillage system for soyabean wheat primarily to control soil erosion. A practical system of no-tillage soyabean was developed for N. Parana, with weed control based on careful crop residue management and sprays of paraquat mixed with either linuron or metribuzin. The marked superiority of these mixtures over paraquat alone for the control of vigorous annual grasses was noted. This effect is, in part attributed to the capacity of metribuzin and linuron (along with other herbicides) to inhibit photosynthesis, thereby facilitating the translocation of paraquat. Where residual herbicides are not used, diuron at low rates in suitable formulation proved the best additive for paraquat. Problem broadleaved weeds are controlled by adding 2, 4-D to paraquat. Temperature, rainfall, the presence of fallows and the interval between harvest and the next planting, all affect pre-plant weed control for no-till soyabean. A key issue is to prevent excessive stands of weeds. Control within the crop can be provided by shielded, directed sprays of paraquat for which tractor mounted sprayers were developed. Recent discoveries of pre- and post-emergence selective herbicides provide improved and alternative in-crop weed control options. Crop diversification and further general no-till research show promise of further rational, chemical and cultural methods of weed control.