Identification of effective cover crop varieties and integrated management practices for weedy and invasive plant suppression in the Western US

Progress report for GW20-215

Project Type: Graduate Student
Funds awarded in 2020: $25,000.00
Projected End Date: 07/31/2022
Host Institution Award ID: G157-21-W7902
Grant Recipient: Utah State University
Region: Western
State: Utah
Graduate Student:
Major Professor:
Dr. Corey Ransom
Utah State University
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Project Information


Weedy and invasive plant species are one of the major causes of yield losses in agricultural systems (Flessner et al. 2019, Soltani et al. 2016, Soltani et al. 2017, Soltani et al. 2018, WERA-77). The arid Western US is lacking in cover crop research and the development of best management practices for weed suppression utilizing cover crops and integrated strategies. Producers have identified this need for understanding the competitive ability of cover crops against weeds, including most effective seeding rates and timing, and their incorporation into an integrated approach for Western US cropping systems (Cann et al. 2019). In this proposed two-year study, we will identify the types of cover crops and best-integrated strategies for the use of these practices in the Western US. Types of cover crops will be tested within different cropping systems using a range of management approaches. Integrated weed management tactics of seeding dates, rates, and cover crop types will be tested in an irrigated wheat system for suppressing weeds, specifically Kochia scoparia. We will measure cover crops competition with weeds and cash crops and also examine their potential future weed threat. The outcome of these studies will be the identification of cover crops best suited for practical use alone and in integrated weed management in the Western US. Results will be disseminated through fact sheets, educational meetings, the development of a regional cover crop selection tool, and partial budgets accounting for these practices. Surveys and interviews will be conducted to discern the likelihood of practice adoption.

Project Objectives:

The objectives are to 1) test cover crops types in terms of weed suppression, cash crop interference, and potential weediness and identify their optimum planting date in small grains crops and 2) identify optimal seeding rates for cover crops that can be used in an IWM approach in wheat.

For both objectives the following methods were used:

The determination of the cover crops with the greatest efficacy in weed suppression in arid regions of the Western US was measured through percent cover and biomass comparisons. The density and composition of weed species were measured once monthly across the growing period (May-July) each year by taking percent cover measurements. At the time of cash crop harvest (August), shoot growth from the plots was collected for biomass comparison. This was repeated a month later for comparison of post-harvest competitive suppression due to cover crop-weed interaction.

These metrics have also been applied to the cash crop to show any influence on cash crop development. Additionally, cash crop yield data has been calculated each year from the biomass samples taken. This has allowed for the identification of any cover crop influence on crop yield.


Objective 1


  • August: Establishment of plots across all participating farms, the exact date depends on producers. Harvest data collection.
  • September: Monthly data collection or harvest data collection
  • October-November: monthly data collection or harvest data collection; if previously harvested then post-harvest biomass collection; data processing
  • December: data processing


  • January- February: data analysis; posters and brochure production, drafts of publications and fact sheets
  • March-May: Establishment of plots across all participating farms, exact date dependent on the crop. Monthly sampling in later months at any early planted plots.
  • June- July: monthly sampling
  • August- September: monthly data collection or harvest data collection
  • October- December: harvest and post-harvest data collection depending on the crop; data processing


  • January- February: data analysis; educational material production
  • March-May: field survey for germination
  • June- August: finalize work on selection tool and partial budgets and publications

Objective 2


  • August-September: Monthly data collection or harvest data collection
  • October: post-harvest data collection; data processing
  • November: data processing
  • December-February: data analysis; posters and brochure production, draft publications and fact sheets


  • Late March to Early April: plot establishment and early planting
  • Three weeks post first establishment: late planting
  • May-July: Monthly data collection at Evans Research Farm and Cache Junction Research Farm
  • August: Monthly data collection if not mature and if mature yield data collected and plots harvested
  • September- Harvested if not previously at this time for yield data and weed
  • October- post-harvest data collection through subsamples cut and collected for weed and crop data; data processing
  • November-December: data analysis; educational material production, publications, and fact sheets


  • January- February: data analysis; educational material production, posters and brochures, drafts of publications and fact sheets
  • March-May: survey for germination of any escaped cover crop seed
  • June- August: finalize work on the cover crop selection tool and partial budgets educational materials and meetings


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Materials and methods:

Determination of Cover Crop Competitiveness in Semi-Arid Regions

We set up variety trials in 2021 across 4 Utah State Research farms to determine which cover crops are most effective at suppressing weed species in small grain crops in a randomized block design. The crop was planted according to current standard practice, typical seeding rates and dates were used for crop planting. Plots 3m x 9m were used, including borders (2’ or 0.66m), at all sites in a randomized complete block design with five replications. The cover crop types tested were: white yarrow (Achillea millefolium), frosty berseem clover (Trifolium alexandrium), fixation balansa clover (Trifolium michelianum), cicer milkvetch (Astragalus cicer), and medium red clover (Trifolium pratense). Plots were randomly assigned a cover crop type to be seeded at the recommended planting rate of the cover crop or as a non-seeded control. At Evans Research Farm and Cache Junction Research Farm cover crops were inter-seeded after germination and establishment of the grain crop, when the crop was approximately at the 12 to 13 stage according to the Zadoks growth scale. At Kaysville Research Farm and the Field 17 research field cover crops were inter-seeded directly after drill seeding of the grain crop.

Percent cover by species as a measure of competition was measured for each cover crop type, grain crop, and weeds. Quadrats (0.25m2) were used to sample once a month commencing after planting through to harvest. At harvest, quadrats were used to collect three biomass samples across each plot. Shoot growth of all species present within the quadrat was cut at the soil surface, placed in a labeled bag, and stored at 4°C. These samples were sorted by species, dried, and weighed for biomass. The yield of the cash crop will be estimated from the samples collected. Data will be analyzed using an ANOVA in R.

Cultural Weed Management for Invasive and Weedy Plants in irrigated Spring Wheat Productive System

We established a field experiment to determine an optimal strategy cover cropping application for the suppression of Kochia scoparia and other weed species in irrigated spring wheat. We used a completely randomized block design with three replications at two locations: Cache Junction Research Farm and Evans Research Farm in Logan, UT. Plots, 3m x 9m, were planted at the recommended seeding rate for irrigated spring wheat, 100lbs/ac (Brown and Walsh 2016). Each plot was assigned one of three seeding rates of the cover crop, and one of five cover crop types or a none cover cropped control. At USU's Cache Junction Research Farm planting occurred on April 10, 2020, April 23, 2021, and at Evans Research Farm planting was done on April 20, 2020, and April 29, 2021. Seeding rates used were based on documented seeding rates of each cover type. The three levels for seed rate were as follows: 75% of the recommended seeding rate (light), the recommended rate (recommended), and 125% of the recommended rate (heavy). The cover crops used were dynamite red clover (Trifolium pratense), hairy vetch (Vicia villosa), Utah sweetvetch (Hedysarum boreale utahensis), strawberry clover (Trifolium fragiferum), and rose clover (Trifolium hirtum). 

Percent cover by species, as a measure of competition, was recorded similarly to the previous description. We used a quadrat to sample once a month after planting until the crop harvest. At harvest, quadrats were used to collect three random samples spaced equidistantly per plot. Shoots within each quadrat were cut at the soil surface and placed in labeled bags. These were stored in a cooler at 4°C. Samples were sorted into species dried and weighed for biomass. The yield was estimated from the grain yield of the harvest biomass samples. In the fall after the first cash crop harvest, biomass samples were collected again to establish the cover crop’s ability for suppression on the field after harvest has occurred. Samples were processed as described previously. Data was analyzed using a MANOVA in R.

Research results and discussion:

Cultural Weed Management for Invasive and Weedy Plants in irrigated Spring Wheat Productive System


Table 1 illustrates the average wheat stand count for 2020. There was no significant influence of location, cover crop, or rate of seeding of the cover on the average wheat stand count for 2020. This would indicate that the use of the inter-seeded cover did not interfere with the establishment of the desired cash crop. Average percent cover for wheat through the growth period, through May, June, and July, was only significantly influenced by the location of planting (p= 1.52e-8; p= 0.0012; p= 0.0038). As seen in Figure 1, the average percent cover of wheat was consistent across each location, with Cache Junction having better coverage and thereby more competitive wheat stands. This is expected due to the site histories, Cache Junction Research Farm was able to produce more vigorous stands of wheat. This would also support that the inter-seeding of cover crops did not negatively impact the growth of wheat throughout the season. However, as discussed below many of the cover crops seeded did not establish well or at all. This could have influenced the lack of significant impact of the cover crop type or its seeded rate on the desired cash crop.

Table 1: Average wheat stand count for 2020 for both locations across all cover crop types and seeding rates.


Wheat Stand (Average plants/m row)


Seed Rate

Cache Junction


Dynamite Red Clover










Hairy vetch










Rose Clover










Strawberry Clover










Utah sweet vetch















Figure 1

Figure 1: Average percent cover of wheat across the growing season (May, June, and July) of 2020 for both Cache Junction and Evans Research Farm.


Average percent cover of the cover crops was seen to be influenced by all factors and interactions (location, cover crop, and seeding rate) in both May of 2020 (site p= 0.031; cover p= 9.49e-7; rate p= 2.77e-3; cover rate interaction p= 5.78e-3; location cover interaction p= 7.713e-3). When reviewed for individual cover crop comparisons to the non-cover cropped controls hairy vetch was the only cover crop seen to have significantly different coverage than the controls (p=9.05e-6). Percent cover was observed to be significantly different for the recommended (p=2.59e-4) and the light (p=3.73e-4) seed weight. The interaction of location with hairy vetch was significant as well (p=1.37e-3).

June held the same trend where significance was seen for all treatments and interactions (location p= 7.61e-5; cover p= 1.07e-9; rate p=1.25e-4; cover rate interaction p= 1.24e-3; site and cover interaction p= 3.78e-3). When reviewed individually both dynamite red clover and hairy vetch were seen to have significantly different percent cover than the non-cover cropped control (p= 2.29e-7; p= 2.73e-6). The low (dynamite red clover p= 2.18e-4; hairy vetch p= 0.01) and recommended (dynamite red clover p = 3.322e-3; hairy vetch p= 4.76e-4) rates differed significantly from the high rate of seeding. The average cover present for both hairy vetch and dynamite red clover were seen to be significantly different between each location (p= 9.48e-3; p=2.48e-4). Evans Research Farm had higher average percent cover for the cover crops than that seen at Cache Junction. This could be due to our observed increased weed presence at Cache Junction either competing with the cover crop or obscuring the view of the cover.

For July average percent cover was significantly different between locations (p=5.207e-5) and cover crop (p=8.42e-7), as well as their interaction (p=5.01e-4). There was significantly more percent cover seen at Evans than at Cache Junction. When reviewed for individual comparisons to the non-cover cropped control only dynamite red clover and hairy vetch were seen to significantly differ (p=0.015; p=0.018). Hairy vetch seeded at the recommended rate differed from the non-treated control (p=0.008). For both dynamite red clover and hairy vetch there was a significance to their interaction with the location (p= 6.62e-4; p= 3.92e-4) with Evans having the greater average percent cover (Figure 2).

Figure 2: Average percent cover of the cover crops, weeds, and B. scoparia (kochia) present in July 2020 for both Cache Junction and Evans Research Farm.

Overall, percent cover of the cover crop was low in this study. Only in July did cover for hairy vetch and dynamite red clover near or exceed half of the sampled area (Figure 2). Also, only in the late growing season at Evans was there much cover coming from the cover crops rose clover, strawberry clover, and Utah sweet vetch. This would indicate that these species may not be the best suited for use in the Western US. However, conditions were not optimal for the germination and establishment of these species. At Cache Junction water was cut to half of the recommended irrigation rate for the area. This could have contributed to the difference in cover observed between the Evans Research Farm and Cache Junction Research Farm site.

Further testing would be needed to see if these covers can be used successfully here. Testing germination and establishment across a variety of climate conditions under more controlled conditions such as in a greenhouse or germination chamber would be recommended.



Throughout the growth period in 2020 (May, June and July) average percent cover of weeds and B. scoparia was seen to only be significantly influenced by the location (weeds: p= 1.25e-4; p= 0.018; p= 2.1e-7; kochia: p= 6.72e-7; p= 2.78e-5; p= 3.99e-6). Again, this is expected as discussed above only two of the five cover crops employed in our inter-seeding consistently established (Figure 2). These results would suggest that overall, the use of cover cropping did not aid in the suppression of weeds as weed cover was not significantly different between the covered plots and the non-covered controls (Figure 2). The influence of location on the distribution of B. scoparia was due to the lack of establishment of the seeded B. scoparia at the Evans Research Farm site. As seen in Figure 2, B. scoparia was present across the site at Cache Junction. The use of hairy vetch did seem to help reduce B. scoparia presence though no cover crop at any rate was distinguishable from the non-cover cropped control plots (Figure 2).


Determination of Cover Crop Competitiveness in Semi-Arid Regions

Initial observations of the performance of cover crops as inter-seeded cover for competitive suppression in small grain crops showed a few differences. For June in terms of average percent cover of the cash crop within the location percent cover was similar across all treatments (Figure 3). The differences seen in terms of grain crop and weeds average percent cover were all observed based on the location of planting. Weeds abundance observed in June in terms of average percent cover seemed to be consistent across the location (Figure 3). This would imply that the inter-seeded cover crop did not have any influence or allow for the competitive suppression of weeds. Inter-seeded cover crops were observed to establish better when inter-seeded at the same time of the grain crop seeding. Both Kaysville and Field 17 locations were inter-seeded at the time of planting and displayed greater average percent cover of the inter-seeded cover (Figure 3). Though overall percent cover of the inter-seeded cover crop was low, again this is likely due to the limited water availability due to the drought conditions in 2021.

Overall, for June plant life establishment at Field 17 was low (Figure 3). This is likely due to the delay and variability in water application as there were limiting water resources due to drought.

Figure 3: The average percent cover of the cash crop, cover crop, and weeds seen for each cover crop treatment for Evans Research Farm, Kaysville Research Farm, and Field 17 Research Field observed during June and July.

The average percent cover observed in July was similar for those locations which were inter-seeded at the same time as grain crop planting. Overall, across both locations (Kaysville and Field 17) both the grain crop and weeds average percent cover observed were similar (Figure 3). The inter-seeded cover did establish differently between the sights, though the overall establishment of the inter-seeded cover was low. At both sites, medium red clover has some establishment. At Kaysville, cicer milkvetch was seen to have some establishment though this was not observed at Field 17. At Field 17 both frosty berseem clover and prairie clover were also seen to establish, though this was not observed at Kaysville (Figure 3). These observations would support that inter-seeding cover cropping was not successful at competitively suppressing weeds in this system. However, with the low establishment of cover through this study due to drought conditions further investigation would be recommended.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

1 Published press articles, newsletters

Participation Summary:

Education and outreach methods and analyses:

The findings from this study will be presented at the upcoming Spring 2022 WSWS conference. Both a poster presentation and a talk will be given discussing the use of these tactics at this conference. All findings will also be submitted as a journal publication to communicate our findings on the efficacy of these methods for the Western US. Field days, workshops, on-farm demonstrations, and trainings in the summer of 2022 will be dependent on producer interest and university policy restrictions. We will be disseminating our findings to interested producers in our area through a newsletter. We aim to provide field demonstrations and any in-person or online training to any producers interested in the findings of this study. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.