Objective 1: Determine how species diversity within crop mixtures can influence arthropod communities, particularly weed seed predators. Research has established that increased plant species diversity maintains a more stable community, is more productive, and provides additional habitats for animals. These additional niches allow greater diversity of arthropod species to inhabit crop mixtures compared to monocultures. I expect arthropod abundance and diversity, specifically weed-seed predators, to increase as crop mixture diversity increases, and increase weed suppression. If weed-seed predators show no preference for broadleaf and grass weeds, I expect weed functional groups to be suppressed equally and increased crop species diversity in mixtures will only increase the rate of consumption, but not alter the composition.
Objective 2: Determine how functional groups within crop mixtures can shift or suppress weed composition. I will test functional groups (grasses, broadleaves) in mixtures of forage crops to determine their influence on the weed community. I will measure both composition, the proportion of grass and broadleaves, and suppression, the total number of weeds. Because of planting density, herbicides and optimal timing and depth for rapid germination, I expect the forage crops to outcompete, and therefore suppress, weed populations of the same functional group. Consistent with my preliminary results, I expect grass forage crops to better suppress grass weeds, shifting the weed community to comprise more broadleaf weeds and the opposite for broadleaf forage crops.
Objective 3: Determine how an increase in weed seed predators and selective suppression of functional groups ultimately influences the number and composition of weeds in forage crop mixtures. I aim to quantify interactions of the first two objectives to determine the ultimate result of weed community suppression and composition. The dominant functional group of the forage mixture, regardless of the number of species, is likely to shift the weed composition. However, increased number of forage species is expected to increase the rate of consumption by weed-seed predators and therefore suppress overall weed abundance. By evaluating these interactions, I will identify which pressure, the functional group or the number of crop species, drives the weed community.
Objective 4: Quantify productivity of forage mixtures and tradeoffs among input costs, yield and quality. I will quantify forage yields to compare biomass, quality and determine gross profit. Input costs, including herbicides, will be used to calculate net return, quantify economic value of weed control and compare input and net return for each forage mixture.
This is important to understand how species diversity in forage systems can help sustain beneficial arthropods in forage systems. This may have great potential in managing weeds, attracting weed seed predators and influencing weed composition. At the end of this research we hope to better understand the ecology of how weeds are suppressed by both species diversity in the cropping system as well as by preference of weed seed predators (arthropods). We also aim to predict potential needs for weed suppression and document economic value in reduced chemical controls of weeds in forage systems.
We planted the first year of the 2-year field study with seven of the eight crop diversity treatments. One treatment (soybean/sunflower) had planting issues and did not take. We also used the split-plot design for herbicide application to compare our control (no herbicide) to plots that were treated conventionally with a recommended herbicide program. We harvested weed biomass and quantified the number and functional group of weeds in a 0.25m2 micro plot at 4- and 8-weeks post-planting. We surveyed the arthropod community using sweep netting, pitfalls traps and sentinel prey traps. Crop biomass at harvest did not go as planned due to machinery issues. The silage chopper kept getting clogged with material, likely due to the “vine” nature of the forage soybeans and the higher moisture in sunflower and sorghum. We aim to try again next year with the possibility of just using a hand-harvest.
Finally, we conducted three replications of our greenhouse component comparing weed biomass (and potential for suppression) among different proportions of soybean and corn (ie, different functional groups). We recorded weekly number of emerged weeds and weighed biomass after 6 weeks. The setaria spp. grass weed seed had very poor germination rates. We aim to conduct an additional 2-3 replications using different seed this year.
In the field experiments, there was a higher abundance of arthropods in the control plots (no herbicide) compared to plots sprayed with herbicides. Additionally, predatory ground beetles (Carabidae and Staphylinidae) were more abundant in control plots, but made up less than 10% of collected insects from pitfall traps. Instead, spiders made up about 30% of collected ground arthropods. There was no trend of ground arthropod abundances or diversity between different crop types.
There was a slightly higher abundance of foliar arthropods in the control split-plots compared to the herbicide plots. The 4-species mixture had the greatest arthropod abundances in both herbicide and control plots, whereas corn and corn/soybean mix had the least in both split-plots.
Both weed biomass and abundances were higher in control plots, as expected. There were nearly 5-times as many weeds (both broadleaf and grasses) at 4 weeks. Similarly, there were 7-times more grass weeds and 4-times more broadleaves at 8 weeks post-planting. Grass weed biomass was 6-times greater and broadleaf weeds biomass was 2-times greater in control plots by 4-weeks post planting. Grass weed biomass at 8 weeks was similar in control and herbicide-sprayed plots, but broadleaf weed biomass was 5-times greater in control plots.
Considering the effect of crop type, or treatment, on weed abundances and biomass, corn and sorghum had much fewer weeds overall, indicating that they may be superior competitors. However, sorghum plots had a relatively high weed biomass at 8 weeks, whereas corn had low abundances and low weed biomass. Finally, broadleaf weed abundance overall was higher in all plots compared to grass weed abundance, rather than a trend correlated with the type of crop grown, as we expected.
We expect to see a clearer trend among crop treatment type with the second (upcoming) year of data. However, we did see slightly higher abundances in both ground and foliar arthropods in control plots.
Finally, for the greenhouse experiment, germination rates of grass weed seeds were nearly 0% for all treatments. We hope to complete 3 more replications using better grass weed seed in this next year.
Education & Outreach Activities and Participation Summary
In 2019, education and outreach related to this project related primarily to working with undergraduate students. We had 3 undergraduate students helping from May-September and 2 students working presently. I was able to use the project to help show weed identification to the students and we facilitated several open discussions about the hypotheses and ideas behind the project. Furthermore, using pitfall traps and sweep netting, the students were able to see and help identify several different species of insects down to family. This project allowed for direct training of both weed and arthropod ID, followed by several opportunities to practice field-based ID.
Additionally, one of the undergraduate students, Hayden Bock, had an interest in exploring soil-related components (compaction) among different crop types and arthropod weed seed predators. Both John Tooker and I worked with Hayden to help him develop an independent research project to explore his interests in soil compaction, weed seed arthropod predators and forage species mixtures. We used this SARE project as a starting point to help Hayden design his own objectives.
We’ve only been able to measure preliminary environmental impact, which indicated herbicides may impede some predatory arthropods, including those that may eat weed seeds. However, weeds were much more suppressed, both in number and abundance, by herbicides than by potential weed seed predators.
We had some logistical issues with planting (skips throughout one full treatment) and harvesting (severe compaction in the chopper) which led to limited harvest data. We hope to overcome some of these limitations next year by altering our machinery or techniques associated with the machinery.
Additionally, abundances were higher for both ground and foliage arthropods in control plots compared to those sprayed with herbicides. We expected to see greater differences between crop treatments, which wasn’t apparent. We hope to see trends more clearly with a second year of data.