- Agronomic: soybeans
- Crop Production: cover crops
- Pest Management: weed ecology
Problem and solution pursued: Application of sustainable weed management practices in agronomic crops will be one of the largest challenges farmers face in the future. Marestail is a problematic weed species that can reduce soybean yields, especially in no-till systems. This species has evolved resistance to many common herbicides. This, combined with an overall desire to reduce herbicide use, leads to the need for integrated management practices that reduce weed density and growth. In Southern states, marestail can emerge in both the fall and the spring, and plants must be managed prior to planting soybean to minimize yield loss. Predicting marestail emergence time will help growers determine optimum control strategies -- such as, when cover crops should be planted or when herbicides should be applied for maximum efficacy. This project studied marestail emergence timing in Kentucky and three other states. Survival of fall-emerging plants was also studied to determine whether winter mortality can contribute to reducing the number of plants present at soybean planting. Lastly, this project examined a broad range of management strategies (including cover cropping) to determine which were most effective in reducing marestail density and biomass to minimize competitive losses and provide a regional model for managing this weed.
Research approach: This project included a field experiment in a no-till soybean system where different management strategies were applied. These strategies included treatments with cover crops (over-wintering and winter-killed), herbicides (fall or spring applied, with or without residual activity), and combinations of herbicides plus cover crops. Cover crops and fall herbicides were applied in the fall of 2018 after corn harvest. Spring herbicide treatments and cover crop termination occurred the following spring. Soybean was planted in May 2019 on 15" rows. Marestail density and plant growth were measured in all treatments starting in the fall and continuing through soybean harvest in October 2019; soybean density and yield were also measured. To assess marestail emergence time, seeds from two different Kentucky populations were planted in September 2018. Emerged seedlings were counted and pulled twice per week. Plant growth and survival was also measured within a separate population of flagged specimens.
Research conclusions: Cereal rye cover crop biomass in the management-focused experiment averaged 2500 lb/acre by termination in April 2019; it was terminated at approximately Feekes 8 growth stage. Very few marestail plants emerged, despite additional marestail seed being added to all plots used in this experiment. The number of plants emerged from cover crop planting in the fall until soybean planting the following spring ranged from 0.2-1.52 plants per square foot (2-17 plants per square meter), and no differences were detected between treatments. Soybean yield was similar across all cover crop and herbicide treatments and averaged 27 bu/acre. August and September precipitation were unusually low in 2019, likely contributing to reduced yield. Similar to the field experiment, mostly fall emergence was noted in the experiment designed to measure emergence timing. Greater than 90% of the marestail emergence in KY occurred shortly after seeds were planted in the fall of 2018, regardless of location where seed was collected. Fall weather conditions in 2018 were ideal for emergence, with warm temperature and precipitation soon after planting. Emergence time may vary with different weather conditions, and analysis of similar experiments across our multi-state region will allow for broader inference. Larger marestail rosettes were more likely to survive the winter. Taken together, these results suggest that (1) given adequate fall weather conditions, marestail can emerge in the fall; (2) cover crops can reduce marestail density at planting to similar levels as herbicide use; and (3) smaller plants are less likely to survive the winter. Management practices that can delay and suppress emergence, including cover crops, can contribute to managing this species.
Objective 1: Characterize marestail biology to better inform how cover crops can be used to manage this species. We will primarily answer the questions: when does it emerge and how well does it survive the winter?
Objective 2: Determine how well the cover crops will suppress marestail emergence and growth.