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.
The methods described in this report were part of a larger set of experiments led by our group at the University of Kentucky looking at how weed management practices and changing climates will affect marestail populations. It also includes field trials at other universities.
Objective 1. Weed seed was collected in the summer of 2018 from 30 individual plants from two populations in Kentucky and the other participating states; emergence time of eight populations was studied. The study area was established in a no-till field that was not cover cropped. 8” diameter PVC pipes (rings) were inserted into the soil to isolate the seed populations. The PVC pipe extended 3” above the soil surface to prevent seed dispersal via wind and water movement from other areas. Seeds from the eight populations and a control treatment to assess ambient emergence were assigned to a ring using a completely randomized design. Six replicate of each treatment were included for a total of 54 experimental units. Collected seeds were broadcasted into their respective ring to simulate seed rain. Once the seeds were sown, marestail emergence was counted twice a week until soil temperatures reached 52 degrees Fahrenheit at 2” depth, and resumed in spring once temperatures returned to the above the base temperature. Counted plants were removed. Soil moisture and temperature were collected hourly at 2” depth using sensors and dataloggers. Air temperature and precipitation were continually measured at a nearby weather station. To measure over-winter success, native populations were tracked by establishing one 16×32’ supplemental plot at each location. In this plot, 100 marestail rosettes were flagged. Diameter of these flagged plants were measured before winter and again in the spring. Height and diameter measurements were taken weekly starting the following spring and summer (of 2019) until maturity. Other plant species, not of interest, were removed from the surroundings to eliminate competition.
Objective 2. Plots were arranged into a randomized complete block design, with four replications and ten different treatments. Five treatments were planted with cover crops to be compared with the no cover cropped counterparts. Treatments also included fall or spring herbicide applications, some with residual activity. After the cover crop was established in October 2018 and a fall herbicide application was applied to the corresponding treatments, two small quadrants were established in each plot. Within these quadrants, marestail emergence was counted and removed from the time the field study began to the termination of the cover crop in the spring. Cover crop biomass measurements were also collected following any herbicide application. Soybeans were planted two weeks after the termination of the cover crop, and soybean yield measured at harvest.
The majority of marestail emergence from simulated seed rain occurred in fall of 2018. Weather conditions were warm and wet; these ideal conditions can potentially explain this high volume of marestail emergence. In KY, all populations that were sown exhibited a similar emergence time, regardless of where the seeds were collected. This suggests that local conditions influence emergence time more than specific population genes. However, more study years are necessary to ensure that findings are applicable to multiple weather conditions. Rosette size was between 0.6-3.2″ in diameter (15-80 mm), with an average diameter of 1.75″ (45 mm). 68% of these plants survived the winter, and larger rosettes had a greater chance of surviving the winter. It is likely that larger plants with more developed roots are better equipped to survive the freeze and thaw of winter. This is important for farmers considering that plants that overwinter successfully have a competitive advantage in spring. Tactics that can reduce plant size in the fall may increase the proportion that die over winter.
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.
Educational & Outreach Activities
July 2019, University of KY Pest Management Field Day, talk on cover crops for weed management
August 2019, Southern Cover Crop Conference, presented a poster that highlighted our research
No farmers participated in this experimental design, yet there were many farmers and extension agents who had the opportunity to see our results at the Southern Cover Crop Conference at the University of Auburn in July 2019. Farmers and educators both were very aware of the competitive pressure marestail is putting on our no-till cropping systems.
Five conference presentations (two at 2019 North Central Weed Science Society; one at 2018 North Central Weed Science Society; two at 2020 Weed Science Society of America)
Our results show a positive impact on suppressing marestail through the adoption of cover crops to an IWM program. Further agricultural research that attempts to show cover crops role in weed management should include a broader area of collaborators, and expand beyond only looking at marestail.
Our results provided a clear model for incorporating cover crops into their IWM programs, as well as a model herbicide program that uses a variety of chemistry in order to reduce the risk of resistant populations.