Differential susceptibility of morning glory (Ipomoea and Merremia) species to residual herbicides and the effect of drought periods on efficacy

The plants commonly known as morning glory belong to the genera Ipomoea and Merremia and are among the most important weeds in Brazil. Greenhouse studies were conducted in 2016/2017 to compare the susceptibility of different morning glory species - Ipomoea quamoclit, I. nil, I. hederifolia, I. triloba, Merremia aegyptia, and M. cissoides to soil-applied herbicides and to evaluate the effectiveness of these herbicides after periods of drought. In the first experiment, to evaluate the efficacy of each herbicide at pre-emergence stage, a fully randomized 6 (doses) x 6 (species) factorial experimental design with six replicates was used. The herbicides tebuthiuron (3,200; 1,600; 800; 400; 200 and 0.0 g a.i ha-1), amicarbazone (2,100; 1,050; 750; 375; 187 and 0.0 g a.i ha-1), sulfentrazone (1,200; 600; 300; 150 75 and 0.0 g a.i ha-1), mesotrione (240; 120; 60; 30; 15 and 0.0 g a.i ha-1), saflufenacil (200; 100; 50; 25; 12 and 0.0 g a.i ha-1), and imazapic (360; 180; 90; 45; 22 and 0.0 g a.i ha-1) were applied. Ipomoea hederifolia, I. nil, and I. quamoclit were most susceptible to saflufenacil; I. triloba and M. cissoides were most susceptible to tebuthiuron; and M. aegyptia was most susceptible to amicarbazone. Therefore, the best herbicide treatment depends on the species of morning glory. In the second experiment, to determine the effectiveness of herbicides after periods of drought, the treatments were arranged, for each herbicide, in a fully randomized 6 x 5 factorial design with four replicates, six species of morning glory, and five sowing times after periods of drought (0, 15, 30, 45, and 60 days after application (DAA) of the herbicides) relative to the application of the amicarbazone (1,050 g a.i ha-1), imazapic (180 g a.i ha-1), mesotrione (120 g a.i ha-1), saflufenacil (100 g a.i ha-1), sulfentrazone (600 g a.i ha-1), and tebuthiuron (1,600 g a.i ha-1). Herbicide effectiveness after periods of drought began to decrease after 15 days for saflufenacil; 30 days for imazapic, mesotrione, and tebuthiuron; 45 days for amicarbazone; and 60 days for sulfentrazone. Pre-emergence application of amicarbazone and sulfentrazone exhibited consistent efficacy during the drought periods, satisfactorily controlling the species of morning glory.