Selectivity and effectiveness of herbicides in the corn and grain sorghum crop

Abstract

Weeds play a key role in the competition for resources with maize and sorghum crops. This has negative effects, especially with regard to crop yields. The selectivity and efficacy of herbicides are the basis for the success of chemical weed control in agricultural production. This paper was divided into two articles. In the first article, the objective was to evaluate the efficiency in weed control and herbicide selectivity applied in pre and post emergence in the culture of sorghum. Seven treatments with four replications were evaluated in a randomized complete block design. The treatments used were: 1. Manual weeding; 2. S-metolachlor (1440 g ha-1 a); 3. S-metolachlor (1440 g ha-1 a.i.) + atrazine (2000 g ha-1 a.i.); 4. atrazine (2000 g ha-1 i.a.); 5. atrazine (3000 g ha-1 a.i.); 6. atrazine (2000 g ha-1 i.a.) + mineral oil (0.25%) and; 7. atrazine (2000 g ha-1 i.a.) + mineral oil (0.5%). It was verified that post-emergence atrazine was efficient in the control of weeds, besides being selective to the sorghum crop, not affecting productivity, except in mixture with mineral oil (0.5%). S-metolachlor did not present pre-emergence selectivity for the cultivars tested, because it was not selective, promoting a reduction in plant numbers and productivity. In the second article, the work was carried out in two stages. The first one was to study the effects of herbicides (tembotrione and nicosulfuron) associated with nitrogen fertilization periods on maize hybrids productivity. The experimental design was a randomized block design, in a 4 × 2 factorial scheme, with 4 replicates. The treatments were composed of four levels of the factors control modes [weeding; nicosulfuron + atrazine (20 and 32 + 1250 g ha-1 a.i.); and tembotrione + atrazine (75.6 ± 1250 g ha-1 ia), and two levels of the nitrogen fertilization time factor (0 and 7 days after the control), on maize hybrids: P30F53Leptra, DKB 230PRO3 and KWS 9004PRO2 in two growing seasons (1st and 2nd crop), agricultural year 2016/2017. In the second step, the selectivity of eight corn hybrids at different doses of the herbicide nicosulfuron was evaluated. The experimental design was a randomized complete block design, in a subdivided plot scheme, 3 × 8, with three replications. The main plot corresponded to the doses of nicosulfuron (0, 20 and 60 g ha-1 a.i.) and the subplot, to the maize hybrids, applied in two experiments according to the phenological stages V4 and V8. All herbicides are more effective in weed control in the 1st crop due to favorable weather conditions. Nitrogen fertilization can be carried out on the same day as the application of the herbicides nicosulfuron and tembotrione, with no reduction in productivity for the hybrids P30F53Leptra, DKB 230PRO3 and KWS 9004PRO2. There are tolerance differences between maize hybrids in relation to the dose of the herbicide nicosulfuron and phenological stage of application. Application of nicosulfuron alters the arrangement of the rows in the spikes of the hybrid P30F53Leptra.