Conservation of natural resources in Brazilian agriculture: Implications on weed biology and management

Natural resources conservation is a key national priority of Brazilian cropping systems, especially in the soybean and sugarcane agricultural areas of central/south and southeast/northeast, respectively. Society has influenced growers to adopt “conservation cropping systems”, which results in reduced damage to the agricultural environment (agro ecosystems) as a whole. These systems are based on plant residue covering the soil surface (cover crop or residue after harvest), with a minimum disturbance of the topsoil layer. These systems, when compared to the conventional cropping system, reduce diurnal soil temperature variation, decrease water evaporation from the soil surface, enhance organic matter content and microorganism activity in the topsoil layer, improve both the physical and chemical soil properties, and provide better soil erosion control. The widespread adoption of no tillage soybean production systems has achieved tremendous success in enhancing soil quality and preventing soil erosion. Plant residue covering the soil surface influences the weed population dynamics through chemical (carbon/nitrogen ratio and allelopathy), biological (parasitism and predation by microorganisms, insects and small mammals) and physical factors (less direct sunlight reaching the soil surface, reduced diurnal soil temperature variation and physical obstruction of weed emergence). Weed control by pre-emergence herbicides is also directly affected by the plant residue covering on the soil, since these residues may intercept the herbicide, preventing part of the sprayed herbicide from the soil by adsorption, therefore, influencing the efficacy of the herbicide. Overall weed management tactics need to be studied and reconsidered for this new “conservation cropping system” as compared to the conventional systems.