Seletividade a herbicidas e cultivo consorciado de trevo branco + milho

Abstract

Intercropping cash crops with perennial legumes can improve crop-livestock systems. However, there is a lack of knowledge on herbicide management to control weed into white clover (Trifolium repens L.) fields and to suppress it when grown intercropped with corn. Aiming to answer some of these questions, the work was divided into two parts: one carried out at greenhouse and one at the field. Glyphosate, 2,4-D, imazethapyr and paraquat + diuron were studied in greenhouse experiments at four application rates in order to evaluate their effects on white clover (Trifolium repens L.) growth and aerial shoot injury at three different phenological stages. At the field, interference between white clover living mulch and corn crop (Zea mays L.) was studied using a clover sward established after one and two growing season in relation to the usual system of corn grown over black oat straw aiming to study the relationships occurring in this intercrop, such as white clover herbicide suppression, competition and its nitrogen effects on corn yield as well as its potential to became perennial after corn growth. Field experiment was divided in first and second period of corn sowing. White clover suppression at the first experiment were: partial suppression achieved by mowing the plots at the corn seeding day; partial suppression achieved by mowing plus chemically broadcast application of 2.4- D and partial suppression achieved by broadcast application of glyphosate. At the second corn sowing period, partial suppression was achieved by a chemically broadcast application of Paraquat/Diuron + 2.4-D; glyphosate + 2.4-D and glyphosate + glyphosate. Cover plants management were established in the main plots, while different nitrogen fertilization levels (0, 60, 120, 180 kg ha-1) were established at the subplots (4.2 x 8 meters). Herbicide-induced foliar injury on white clover ranged from no symptoms to early chlorosis, leaf necrosis, stunted growth and death depending on herbicide rate and white clover phenological stage. Imazethapyr showed the highest selectivity at the three-leaf trifoliate stage. Seedlings treated with glyphosate at 1080 g a.i. ha-1, however, were dead at 21 days after application (DAA). White clover herbicide tolerance increased with plant age, and selectivity was found to be affected as herbicide rate increased. Paraquat + diuron caused the highest levels of white clover phytotoxicity and plant death. The studied herbicides showed potential to be used as selective products on white clover depending on their rates, as well as the timing of their application onto white clover. Clover with 1 growing season is easier suppressed and can be handled in corn intercropping without affecting grain yield, with ability to resume growth in the next growing season. Although, perennial clover sward has greater ability to compete with corn and show higher herbicide tolerance, which difficult its management in intercropping systems. Corn grain yields ranged from 3.0 to 12.3 Mg ha-1 and were greatest in the Gly + Gly treatment. Nitrogen input is an important element for the optimization of such systems, combined with herbicide application to minimize competition effects on the main crop. Nitrogen rates increased plant height, number of grains per row, grain per spike, thousand grain weight and grain yield, being these parameters optimized when white clover was suppressed with broadcast application of herbicides. Based on the results, it is possible to use white clover as living mulch in corn cropping systems with its full recovery after corn harvest without replanting.