AN EVALUATION OF INTERSEEDED WINTER WHEAT (TRITICUM AESTIVUM) FOR THE SUPPRESSION OF COMMON WATERHEMP (AMARANTHUS TUBERCULATUS) IN SOYBEAN (GLYCINE MAX)

Author

Madison Renea Decker, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2023

Degree Name

Doctor of Philosophy

Department

Agricultural Sciences

First Advisor

Gage, Karla

Abstract

Due to the widespread distribution of herbicide resistant common waterhemp [Amaranthus tuberculatus (Moq.) Sauer (syn. rudis)], chemical weed control options have become limited. Intense selection of herbicide resistant biotypes will result in a higher demand for alternative weed control approaches. Living mulches have been utilized in row crops for the suppression of weeds, but this practice has not been widely adopted in corn and soybean production in the Midwest. Therefore, five studies were conducted to investigate the implementation, efficacy, and practicality of interseeded winter wheat in soybeans for the suppression of common waterhemp. The goal of these studies was to provide regional growers with information that can be utilized first-hand. An improved understanding of the use of integrated weed management (IWM) approaches may reduce the gap between the theoretical aspects of IWM programs and the practical implications. Expanding resources for regional growers could help increase weed management diversity, increase adoption of sustainable practices, and slow the selection of herbicide resistant biotypes. Studies were conducted in the 2019, 2020, and 2021 growing seasons at the Agronomy Research Center, Horticultural Research Center, and Belleville Research Center. The tested interseeded system was compared to a standard soybean-only weed management program to quantify weed control and crop competition. Results from field studies indicate that on average planting winter wheat without the use of herbicides reduced common waterhemp counts by 83% in fall plantings, 81% in April plantings, 12% in early-May plantings, and 14% in late-May plantings compared to the soybean-only comparison. Similarly, common waterhemp biomass was reduced 78% in fall plantings, 94% in April plantings, 30% in early-May planings, and 37% in late May plantings when compared to the soybean-only comparson. Planting winter wheat earlier in the spring (April) provided the most consistent winter wheat biomass production. In some instances, planting winter wheat in April ahead of soybean planting reduced soybean grain yield compared to the soybean-only standard comparison. Soybean grain yield reductions in the April planted treatment compared to the soybean-only pre-emergent followed by post-emergent treatment (standard comparison) was upwards of -27% when herbicides were applied, to -82% when no herbicides were applied. However, in the same treatment, soybean grain yield was also as high as 28% over the soybean-only standard comparison. The variability in outcome is likely correlated with the weather and precipitation events corresponding with the growing season. Terminating the winter wheat by soybean growth stage V3 could avoid significant soybean grain yield reduction. In seasons with increased moisture during the soybean reproductive stages, waiting to terminate the until as late as V6 did not result in soybean grain yield reduction compared to the soybean-only standard comparison. Interseeded winter wheat did not deter the incorporation of post-emergent applied residual herbicides. The use of residual herbicides in the post-emergent application did not always result in better end of season common waterhemp efficacy. Additionally, endomycorrhizal fungi were incorporated as a seed treatment in the interseeded system to observe competitive alleviation by microbial species. In these studies, no differences were detected in soybean growth or grain yield. Deduction in height was noted in greenhouse studies in interspecies pots. The introduction of additional species in the treatments had a negative impact on soybean growth characteristics and those negative impacts were not alleviated by the introduction of endomycorrhizal fungi. Large-scale on-farm interseeded trials were successful in reducing the height and number of emerged common waterhemp and Palmer amaranth. When comparing the height of common waterhemp and Palmer amaranth in the soybean-only post-emergence only treatment prior to the post-emergence application, the pre-emergent program and the interseeded program preformed similar with a 91% reduction in weed counts. Similarly, when observing weed heights, the pre-emerent herbicide reduced weed height by 48% while the interseeded treatments reduced weed heights by 69%. This could indicate that when interseeded winter wheat is properly established, interseeding may extend time for the grower to make an on-label herbicide application. It is not uncommon for growers to struggle with timeliness of post-emergent herbicide application given the rapid growth of common waterhmp and Palmer amaranth. Implementation method of this interseeded system did not reduce efficacy of the interseeded winter wheat program. Soybean grain yield was not significantly reduced by the interseeded winter wheat at any location in the on-farm trials. This dissertation provides evidence that when implemented successfully, interseeding winter wheat in soybean is an effective IWM strategy for common waterhemp suppression.