Weed management practices and benefits in Conservation Agriculture systems

Abstract

1 Introduction. When growing crops, the eradication of plants from a field has only a temporary effect and, soon after, it is readily recolonized from the soil seedbank, vegetative structures in the soil and by seeds transported to the field by wind, run-off water, animals, soil mass movement or even agricultural practices (irrigation, harvesting, manuring, tillage etc.). We must be grateful for the tenacity that plants show to occupy most terrestrial environments, as it allowed the evolution of terrestrial animals and ultimately us. But, in agricultural fields, most plants are out of place and will compete with crops for water, nutrients, light and space, may diminish the quality of pastures, may impair the harvest, may be a repository of diseases or pest insects and so on. From the moment humans moved from nomad gathering communities and began producing their own food through agriculture, they conceived a multitude of methods to manage weeds based on the technologies and knowledge of its time. In an everchanging world methods of weed control that seem very promising at a given moment sooner or later will be questioned by their impact on the environment or the development of new, more efficient alternatives.Conservation Agriculture (CA), as defined by FAO (FAO, 2017), means farming systems that aim at sustained crop production by the compliance with three principles: no or minimum mechanical soil disturbance, permanent organic soil cover (by both crop residues and cover crops) and species diversity (through rotations, sequences and associations). These farming systems were practised on circa 180 Mha worldwide in 2015/16 and the expansion rate since 2008/09 has been calculated at 10 Mha/year (Kassam et al., 2015, 2019). The growing adoption of CA systems worldwide shows that farmers experience benefits when converting to CA. However, lack of soil disturbance, permanent soil cover and crop rotation/association under CA are likely to have an effect on weeds’ dynamics, both in terms of diversity/composition and size. In high input systems, heavy reliance on herbicides for weed control often means that integrated weed control strategies are disregarded by the farmer. Unfortunately, this is true in many no-till systems that purely focus on minimum soil disturbance but disregard the other two pillars of CA: diverse crop rotations and cover crops to revegetate the fields in fallow periods. Herbicide-resistant weeds, withdrawal of some herbicides from the market, more restrictive environmental and health regulations and policies that promote the reduction/ banning of pesticide input (e.g. organic farming promotion), all converge to question if alternative weed control methods and practices under CA are feasible and whether integrated weed control strategies tailored to reduce herbicide inputs provide the economical results farmers need to maintain their livelihood.