Selectivity of pre-emergent herbicides in cassava (Manihot esculenta Crantz) and its relationship with the stem cutting planting system

Cassava (Manihot esculenta Crantz, Euphorbiaceae), an important source of food for millions of people in the world, is distributed mainly in Asia, Africa, Latin America, and the Caribbean. Weeds are important biotic constraints on cassava production, and herbicide application can provide effective weed control in this crop. However, the different possibilities of herbicide selectivity (factors associated and not associated with the plants) must be studied before implementation. The aim of this study was to evaluate the effects of the combination of different pre-emergence herbicides and two stem cutting planting systems (vertical and horizontal) on phytotoxicity and several agronomic traits of the cassava crop. The experiment was conducted through a factorial arrangement in a completely random design in Corrientes, Argentina. The factors studied were planting systems (horizontal and vertical position), weed control methods (hoeing or application of diuron, S-metolachlor, pendimethalin or clomazone, 625; 1920; 900 and 1000 g a.i. ha−1, respectively), and cropping seasons (2018–2019 and 2019–2020). The following variables were measured: phytotoxicity, plant height, relative plant stand, total weight of tuberous roots per plant, starch concentration of tuberous roots, and starch production per ha. The highest precipitations occurred in the 2018–2019 cropping season. The vertical planting system in combination with herbicides showed lower phytotoxicity values and higher values of plant height, starch concentration, and starch production per ha than the horizontal planting system. The highest phytotoxicity values were recorded with the application of diuron combined with the horizontal planting system and in the 2018–2019 season, with a 33.33% decrease in relative plant stand compared to the use of hoeing. Starch concentration remained unchanged with the use of clomazone and pendimethalin, whereas the effect of these herbicides on starch production varied with the cropping season, decreasing only in 2019–2020. Starch production was significantly lower with the use of S-metolachlor (up to 55.29% in 2018–2019) and of diuron (up to 60.57% in 2019–2020) than with the use of hoeing. In conclusion, phytotoxicity values were higher for the horizontal planting system than for the vertical system. The herbicide with the highest phytotoxic effect on the cassava crop was diuron; the effect increased when combined with the horizontal planting system in the cropping season