Management of Digitaria insularis in soybean pre-sowing desiccation by ACCase alternative herbicides and its impact on soybean carryover.

The objective of this study was to evaluate the use of alternative herbicides to ACCase inhibitors to control Digitaria insularis (sourgrass) during pre-sowing soybean desiccation. Two field experiments were conducted with two temporal replicates under different climatic conditions, with the following treatments: imazethapyr + glyphosate, clethodim + fluroxypyr, chlorimuron + glyphosate, imazapic + imazapyr, mesotrione + atrazine + glyphosate, tembotrione + atrazine + glyphosate, nicosulfuron + atrazine and imazamox + glyphosate, all of which were composed of a sequential application of glufosinate ammonium. The water stress conditions in experiment 1 compromised the efficacy of acetolactate synthase enzyme (ALS) inhibitor herbicides in the control of sourgrass. Even under water restriction conditions, the treatments containing mesotrione + atrazine + glyphosate and tembotrione + atrazine + glyphosate provided control greater than 80%. In experiment 2, the combination of ALS inhibitors with glyphosate proved to be a great alternative for the management of sourgrass control. All treatments showed no significant phytotoxicity in soybean for either experiment.

Light availability in the cultivation environment and the action of glyphosate on Digitaria insularis: physiological aspects and herbicide root exudation.

The root exudation decreases the susceptibility of some species to herbicides, which is still little studied in Digitaria insularis, popularly known as sourgrass, one of the main weeds of annual crops in the world. Thus, we sought to identify whether there is an occurrence of root exudation of glyphosate in D. insularis and the influence of this herbicide on physiological and control parameters of this species when cultivated under different light conditions. The experimental design was 2 x 5, with the first factor represented by environments: full sun and artificial shading. The second factor was represented by doses 0, 370, 740, 1110, and 1480 g ha-1 of glyphosate. The plants grown in shading showed more significant injury in the initial phase. The increase in the glyphosate doses reduced the photochemical efficiency of the photosystem II (ФPSII), electron transport rate (ETR), photosynthetic rate, stomatal conductance, transpiration rate, and water use efficiency of D. insularis regardless of the cultivation environment. The light restriction increased the ФPSII in D. insularis at three days after applying the herbicide (DAH); at 6 DAH, the shaded plants showed a more pronounced reduction in ФPSII. D. insularis did not show root exudation of glyphosate, and shading did not influence this process.

Effect of low glyphosate doses on flowering and seed germination of glyphosate-resistant and -susceptible Digitaria insularis.

BACKGROUND: Herbicide hormesis is characterized by stimulation of various growth and developmental parameters, such as biomass and height, at low herbicide doses. Other possible hormetic effects are earlier flowering, higher seed weight, more seeds, and a shorter plant life cycle, which could favor the propagation of the species. This study tested the early flowering in glyphosate-resistant and -susceptible Digitaria insularis biotypes under treatment with low glyphosate doses. RESULTS: Hormesis caused by low glyphosate doses occurred in all experiments. The hormetic effects were a decrease in time necessary for the formation of inflorescences and increased seed weight and germination speed. Higher glyphosate doses were required for the hormetic effect in the glyphosate-resistant than the -susceptible D. insularis biotype. CONCLUSIONS: Hormesis caused by low glyphosate doses in D. insularis may provide an advantage for the dissemination of this species, helping to alter the weed flora. As the doses that cause stimulation in glyphosate-resistant biotypes are higher than in glyphosate-susceptible biotypes, the selection of resistant biotypes may be favored in glyphosate-sprayed fields, increasing the rate of infestation of glyphosate-resistant biotypes.

Population genomics of Digitaria insularis from soybean areas in Brazil.

BACKGROUND: Digitaria insularis is a weed species that has gained considerable importance in Brazil's soybean production areas that rely on glyphosate-resistant cultivars. Herbicide-resistant weed populations of this species have been reported in many regions in Brazil, first in the south, followed by later reports in the north. We hypothesized that the spread of herbicide-resistant D. insularis is facilitated by movement of agricultural machinery from the southern regions of Brazil. RESULTS: Population genomics revealed a weak or no genetic structure (FST  = [0; 0.16]), moderate expected heterozygosity (HE  = 0.15; 0.44) and low inbreeding (FIS  = [-0.1; 0.1]) in D. insularis populations. Our data supported the hypothesis that herbicide resistance gene flow predominantly occurred in a south-to-north direction based on a migration analysis. We also found evidence of local adaptation of resistant populations in the northern soybean-growing regions of Brazil. CONCLUSION: Evidence in our work suggests that gene flow of glyphosate-resistant D. insularis is associated with movement of agricultural machinery, although local selection pressure seems to play an important role in the evolution of herbicide resistance throughout the country. Our results suggest preventive practices such as equipment sanitation should be implemented to limit the spread of herbicide resistant D. insularis. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.