Fitness costs on herbicide-resistant Digitaria insularis (L.) Fedde germination

Digitaria insularis poses a significant challenge in weed control due to its perennial habit, dense clumping growth, and the widespread presence of herbicide-resistant biotypes. Our research investigates whether single or multiple herbicide resistance biotypes of D. insularis experience fitness costs, specifically affecting their germination. To determine the resistance factor, a dose-response curve was employed using glyphosate and haloxyfop-P-methyl herbicides separately in a completely randomized design (CRD) with four replicates per dose (nine doses total). Shoot dry mass was measured at 100 days after herbicide application (DAA), with control assessments performed at 14, 28, and 42 DAA. Subsequently, a separate CRD experiment examined the germination rate as a function of temperature and photoperiod for each biotype. This factorial scheme tested six temperatures across three biotypes (susceptible and two resistant types) under three light exposure periods (0, 8, and 12 hours). Germination percentage and the germination speed index (GSI) were calculated for 14 days, with counts of healthy seedlings recorded daily. Statistical analysis confirmed the resistance/susceptibility of the biotypes based on the dose-response curve. For the susceptible and simple resistance biotypes, the most favorable temperatures for germination were 20, 30 and 40 °C, at which the highest germination percentages and a higher germination speed index were observed. On the other hand, for the biotype with multiple resistance, the temperatures of 25, 30 and 35 °C were more favorable, promoting superior results in both parameters studied.