SUB- AND SUPER-OPTIMAL RAINFALL EFFECTS ON SOIL-APPLIED (GROUP 15) HERBICIDE EFFICACY

Author

Bradley Jacob Decker, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2025

Degree Name

Master of Science

Department

Plant, Soil, and Agricultural Systems

First Advisor

Gage, Karla

Abstract

Site of action group 15 herbicides are a group of soil applied herbicides that require rainfall for incorporation and control many troublesome small seeded broadleaves and grasses. Climate change prediction models for southern Illinois suggest a potential rainfall shift to create wetter springs, a higher potential for summer drought, and an increase in the frequency of rainfall events of over 2 inches. The direct impacts of these changes in climate have not been thoroughly studied on soil applied herbicides. We conducted studies to look at variable rainfall effects on soil applied herbicides, with a focus on site of action group 15 chemistries (very long chain fatty acid inhibitors). The objectives of this research are to investigate the interactions of rainfall amount, rainfall timing, and soil residue on group 15 soil residual herbicides. Four total projects will be conducted at the Horticultural Research Center and the Agronomy Research Center. The objectives of each project include: 1) examining the influence of delayed incorporation and cover crop residue on the efficacy of acetochlor, 2) examining the influences of delayed incorporation on the efficacy of six group 15 herbicides, 3) determining the maximum rainfall allowable for incorporation of soil residual herbicides, and 4) determining the minimum and maximum rainfall required for waterhemp control. Results from project one suggest that in situations of delayed incorporation, acetochlor that sat on the soil surface for 14-21 days until receiving an incorporating rainfall generally resulted in a significant reduction in efficacy of common waterhemp [Amaranthus tuberculatus (Moq.) Sauer]. Acetochlor that sat on the cereal rye biomass containing pots generally saw a reduction in efficacy of common waterhemp when incorporation was delayed until 21-28 days after application. When incorporation occurred at 0-14 days after herbicide application, numeric and sometimes statistical disadvantages were seen from the presence of cereal rye biomass on the control of common waterhemp by acetochlor when compared to their bareground counterparts. When incorporation occurs 28-112 days after herbicide application, the inverse is seen, showing that the presence of cereal rye showed numeric and sometimes statistical advantages to the control of common waterhemp by acetochlor. These data suggest that while negative effects of herbicide tie up from cereal rye exist, and are measurable, cereal rye has the potential to extend the utility of acetochlor in situations of extreme delayed incorporation. Results from project two were mixed across trial years, likely due to differences in greenhouse conditions, but suggest that dimethenamid-p and unencapsulated acetochlor are the least stable of tested herbicides in situations of delayed incorporation suggesting significant reduction in control of common waterhemp when incorporation is delayed until 14-28 days after application. S-metolachlor, pyroxasulfone, microencapsulated acetochlor, and an experimental microencapsulated acetochlor all exhibited reduced efficacy when incorporation was delayed until 21-28 days after application. Results from project 3 and 4 combined to suggest that pyroxasulfone generally exhibited reduced efficacy of common waterhemp with rainfall events of 8 cm, s-metolachlor generally exhibited reduced control with rainfall events of 4 cm, and dimethenamid-p generally exhibited reduced control with rainfall events of 2 cm. The trial design differences lead to high disparity between the acetochlor-containing treatments, likely because of encapsulation degradation differences between greenhouse and field scenarios, so only trial by trial conclusions were made from these herbicides. No conclusions were made about minimum rainfall requirements; I suggest trial redesigns to better optimize weed emergence in low rainfall scenarios. Overall this thesis provides a baseline of data that suggests situations in which variable rainfall patterns could cause diminished herbicidal efficacy as well as compares each of these herbicides to each other to provide farmers with valuable information revolving around pre-emergent herbicide decisions.