Efeito de resíduos culturais no residual e eficácia de herbicidas pré-emergentes a base de s-metolacloro no controle de plantas daninhas na cultura da soja

Abstract

Pre-emergent herbicides are fundamental tools in integrated weed management. However, straw from the previous crop can act as a physical barrier, hindering the transport of these herbicides to the soil, reducing their availability in the solution and, consequently, their effectiveness in control. Thus, the objective of this study was to evaluate the influence of Zea mays L. (corn) straw intercropped with Urochloa ruziziensis cv. ruziziensis (R. Br. ex A. Rich.) R.D. Webster and Sorghum bicolor (L.) Moench (sorghum) straw on the availability of s-metolachlor in the soil, as well as the efficiency of preemergent herbicides in controlling Amaranthus hybridus L., Eleusine indica (L.) Gaertn., Digitaria horizontalis Willd., and Cenchrus echinatus L., on the establishment and productivity of soybeans. The experiment was conducted in randomized blocks, with four replicates, in a 5 × 3 + 2 and 5 × 3 factorial design. Factor A consisted of five preemergent herbicides (s-metolachlor, s-metolachlor + fomesafen, s-metolachlor + metribuzin, s-metolachlor + flumioxazine, and s-metolachlor + sulfentrazone) and two herbicide-free controls, while factor B consisted of three straw conditions (no straw, corn straw intercropped with Urochloa ruziziensis cv. ruziziensis, and sorghum straw). All herbicides showed high efficiency (>90%) in controlling A. hybridus, D. horizontalis, and C. echinatus. The presence of straw increased control by an average of 14% for D. horizontalis, C. echinatus, and E. indica, and 9% for A. hybridus, compared to the absence of cover. The concentration of s-metolachlor in the soil showed similar behavior between straw conditions, which may be associated with precipitation of 28 mm at 2 DAA, sufficient to promote the transport of the herbicide to the soil. Up to 10 DAA, there was an 87.6% reduction in the total applied, coinciding with the period of highest rainfall. The differences between the s-metolachlor formulations were attributed only to the doses applied, and the higher doses did not prolong the residual period in the soil. The types of straw did not interfere with the concentration of s-metolachlor or the efficiency of the herbicides, due to favorable weather conditions for transport. No visual symptoms of phytotoxicity were observed, although there was a reduction in plant stand. The straws provided better conditions for the initial establishment of soybeans, especially corn straw intercropped with Urochloa ruziziensis cv. Ruziziensis. It is concluded that corn straw intercropped with Urochloa ruziziensis cv. ruziziensis and sorghum straw are effective tools in integrated weed management when associated with preemergent herbicides, without interfering with the concentration of s-metolachlor in the soil under adequate precipitation conditions.