Title : Long-term effect of crop-rotations on weed dynamic and glyphosate consumption in the south of Buenos Aires province, Argentina
Abstract:
An estimated 80% of the crops are carried out under no-till systems in the south of Buenos Aires province (Argentina). Most agroecosystems have been characterized by undiversified cropping systems, where soybean is the main crop since 2004. This agricultural model tends to increase the evolution of herbicide-resistant weeds. In the current context, it is necessary to carry out an integrative weed management that includes cultural method as cover crops and rotations, in addition to chemical weed control. Diversified crop rotations break the weed cycle and prevent the weed adaptation. Moreover, these practices conduce to combine herbicides of several modes of action reducing risks of resistance. In the Argentinean Pampa, the temperate climate and rainfall distribution enable the production of pastures and crops in winter and summer. The aim of the current work was to evaluate the effect of three 6-year cycles of diversified crop-rotations on weed dynamic. In this sense, a long-term experiment was carried out in CEI Barrow (MAIBA-INTA), where was determined the effect of five crop sequences on weed species. Weed surveys were performed at the last year of every 6-year cycle. A randomized block factorial design (3 x 5) with four repetitions was applied. After every cycle, the effects of five crop rotations were evaluated: CR1- Conservationist agriculture; CR2- Crop-pasture rotation; CR3-Crop rotation for poor soil quality; CR4- Crop-pasture rotation including cover crops (oat-vetch) and CR5- Intensive crop rotation: two crops by year. A significant interaction was detected between cycles and rotations on weed density (pl.m-2), Shannon diversity index and floristic richness. The analysis of all weeds showed that the rotation with cover crops (CR4) had the lowest weed density (12 to 42 pl.m-2) every cycle, while was 19.4 pl.m-2 in the last year. The interaction between the last 6-year cycle and CR2 and CR3 showed the highest weed density, both treatments were significantly different from the rest. Weed diversity was higher in rotations of the first cycle than the last cycle. In contrast, floristic richness and weed diversity were similar in the sequence CR4. After 18 years of rotations, 16 weed species remained: Sonchus oleraceus and Polygonum aviculare were the most common, Avena fatua disappeared from all rotations and Lolium spp. was not registered in CR1, CR2 and CR4. Twenty six applications of glyphosate were carried out on CR4, while CR5 was associated to the highest consume of this herbicide (45 applications). No weeds have shown herbicide insensitivity under any rotation during 18 years.
