Período crítico de interferência de plantas daninhas e seletividade de herbicidas para a cultura da mandioca na região noroeste do Paraná

Abstract

The determination of the critical period of weed interference under local conditions is very important to establish weed control strategies suitable to maximize crop rentability. After setting such period, it is also relevant to evaluate selective alternatives for chemical control for different varieties of regional importance. With this intent, the initial objective of this work was to estimate the critical period of weed interference (PCPI) in cassava under soil/climate conditions of northwestern Paraná. Weeds with the highest values of relative importance in experimental area were Cenchrus echinatus and Brachiaria decumbens. By accepting a maximum tolerance of 5% of crop yield decrease, the period before weed interference (PAI) was 18 days after planting (DAP), and the total period of prevention of weed interference (PTPI) was 100 DAP. Therefore, PCPI for cassava for local conditions was from 18 to 100 DAP. As a second objective, a field trail was performed to evaluate the selectivity of pre-emergence herbicides, for two important cassava varieties (Fécula Branca and Fibra) grown in Paraná. Herbicides and respective doses (g a.i. ha-1) applied were diuron (400 and 800), metribuzin (360 and 720), isoxaflutole (60), atrazine (720), smetolachlor (1920), ametryn+clomazone (1350+1900), ametryn+trifluralin (1500+1350), isoxaflutole+metribuzin (60+320) and isoxaflutole+diuron (60+400). Herbicides were compared to a no-herbicide check, and all treatments were kept free from weed interference by performing manual weeding during the entire crop cycle. Only treatments with s-metolachlor, for both cultivars, and with metribuzin, for cultivar Fibra, did not cause visual injuries in cassava plants. Atrazine decreased crop stand for cultivar Fécula Branca at 60 DAP, but no decrease in crop height was observed. Both atrazine (for both cultivars) and diuron applied at 800 g a.i. ha-1 (for cultivar Fécula Branca) decreased root yield. Therefore, atrazine was considered to be non selective for both cultivars ans the highest dose of diuron was also considered non selective for cultivar Fécula Branca. The are differences of tolerance between cultivars in this experiment, being cultivar Fibra, in general more tolerant to the herbicidal treatments evaluated.