Efeito de subdoses de 2,4-D na produtividade do algodão e suscetibilidade da cultura em função de seu estádio de desenvolvimento

This research was aimed at evaluating the effects of sublethal rates of 2,4-D on cotton development and yield. Two field experiments were carried out with cv. IAPAR-95, using two fold-checks. For the first trial, applications were accomplished at the beginning of cotton flowering (stage F1), by spraying rates of 0.84; 1.68; 3.36; 6.72; 13.44 and 26.88 g of acid equivalent (a.e.) per hectare, equivalent to drifts of 0.125; 0.25; 0.5; 1.0; 2.0 and 4.0% of a 670 g of a.e. ha^-1 rate. In the second field trial, treatments were composed by the factorial combination of two rates (6.72 and 13.44 g a.e. ha^-1) and three stages of crop development at the time the herbicide was applied (C1, C3/C4 e C6). Analysis of the set of data from the first experiment showed that rates higher than 3.36 g of a.e. ha^-1 (0.50%) applied during crop flowering decreased yield, being the floral buds drop the most harmful symptom concerning cotton yield. In the second trial, a strong increase in crop tolerance was observed as plants get older. The only treatment decreasing crop yield was the rate of 13.44 g of a.e. ha^-1 (2.0%) applied at C1 stage. Therefore, cotton sensibility to 2,4-D dropped substantially after boll formation.