During recent years, farmers have been encouraged to plant complex multi-species cover crop mixtures instead of cover crop monocultures. Some researchers and practitioners have suggested that complex cover crop mixtures are more productive or efficient than single species cover crops. In the northern Great Plains region, farmers often contend with limited precipitation. Therefore, cover crop response to soil moisture is important, because cover crops use water that might be needed to grow a main crop. Furthermore, with climate change, drought periods in these areas may lengthen or intensify, leading to a greater need to conserve soil water resources. Limited research has been conducted to understand how complex cover crop mixtures perform in relation to monocultures under different soil moisture levels. Therefore, the funded SARE graduate student project, entitled ‘Assessing multi-species cover crop responses to variable soil moisture and soil types,’ was aimed at comparing single-species with mixed-species cover crop performance under variable soil moisture regimes.
For this project, three monoculture cover crops (foxtail millet, cowpea, and sunflower) were compared to a three-species mixture (foxtail millet, cowpea, sunflower) and to a six-species mixture (foxtail millet, cowpea, sunflower, oat, hairy vetch, kale) under irrigated and dryland conditions at two locations in ND (Absaraka/Fargo and Mandan) during the 2016 and 2017 growing seasons. Stomatal conductance for each cover crop species was measured via a Decagon porometer to gauge water stress. Peak season above-ground biomass was assessed for each cover crop treatment, separated by species. Soil was analyzed at the end of the growing season to understand how the various cover crops and mixtures impacted soil nitrogen, phosphorous, and potassium. Limited data were also collected to understand weed-suppressive abilities of the various cover crop monocultures and mixtures. At the Absaraka site, we measured the weed suppressive ability of the cover crop residue left behind after flail mowing. Unfortunately, at the Absaraka site during 2016, precipitation was so heavy that the soil water content did not differ between the irrigated and dryland treatments, thus limiting our ability to assess responses to different water regimes. At the Fargo site during 2017, rainfall after planting the cover crops was so limited that emergence was quite poor and the plots were excessively weedy, which also negatively impacted our ability to assess cover crop performance during this year.
During 2016 at Absaraka, sunflower monocultures suppressed weeds in-season less well than 3 and 6 species mixtures. Foxtail monocultures and cowpea monocultures suppressed weeds as well as the 3 and 6 species mixes. Also, sunflower suffered more competition from weeds than foxtail millet. Sunflower density was low compared to cowpea and foxtail millet, which resulted in a more open and less competitive canopy. Cowpea monocultures and the cover crop mixtures suffered intermediate suppression by weeds. We were unable to assess these responses in 2017 because of extremely poor crop emergence (i.e., weeds were impacted by both crop competition as well as crop absence and these two impacts were confounded).
In terms of weed-suppressive action of the cover crop residue, results from 2016 indicated that although cover crop residue biomass did not differ among the cover crop treatments, both cover and weed-suppressive ability of residue containing foxtail millet were greater than residue lacking foxtail millet. Thus we concluded that, for weed suppression, cover crop species is more important than cover crop diversity. In particular, foxtail millet, whether in alone or grown in combination with other species, seemed to produce residue that was more weed-suppressive than residue from the other cover crop species. Thus, we suggest that this crop would be a good choice for a warm-season cover crop, if weed suppression is the main goal.
During 2016 at Absaraka, cowpea stomatal conductance (a measure of water stress) did not differ among monoculture, 3 species mix, and 6 species mix. Since water was plentiful at this site during 2016, this result suggests that the performance of cowpea with regard to withstanding water stress was not affected by growing in species mixtures. Conversely, at Mandan during 2016, cowpea stomatal conductance when grown in mixture was drastically reduced compared to growing in monoculture. Since the Mandan site was somewhat water-limited during 2016 (compared to Absaraka), this result suggests that the other species in the mixtures might have competed with cowpea for water. Alternatively, cowpea stomatal conductance for cowpeas growing in mixture may also have been reduced because cowpea is shorter than sunflower and foxtail millet. Although we tried to always measure sunlit leaves, if a species was completely shaded, then no sunlit leaves would be available. This would lead to reduced stomatal conductance, as shaded leaves are not as active photosynthetically as sunlit leaves. However, because this effect was not seen in Absaraka, differences in soil water availability may have played a role. Either way, the performance of the cowpea may have been reduced when growing in mixture with sunflower and foxtail millet under limited soil water. Conversely, stomatal conductance of sunflower and foxtail millet did not differ between monocultures and mixtures. Our results indicate that the ability of individual cover crop species to withstand water stress may or may not change when that species is grown in mixtures with other species. This in turn may affect the ecosystem services accrued from each species. For instance, if a farmer wanted to grow the 3-species mix for weed suppression and nitrogen fixation in a water-limited situation, the cowpea might under-perform, and the amount of N added could be limited.
At Fargo during 2017, irrigation did not affect cowpea stomatal conductance when cowpeas were grown in monoculture. But, perplexingly, for cowpeas grown in a 3-species mixture, stomatal conductance was greater for ambient rainfed plants than irrigated plants. Conversely, for cowpeas grown in a 6-species mixture, stomatal conductance was greater for irrigated compared to ambient rainfed plants, as one would expect. At Fargo during 2017, neither foxtail millet nor sunflower stomatal conductance was not affected by soil moisture treatments or cover crop mixes. These results suggest, again, that for some species, plant performance may change when a species is grown in a mixture with other species compared to growing alone in monoculture. Cowpea was more impacted by growing in mixtures than foxtail millet or sunflower. This could be because of the shorter stature of cowpea, or perhaps less extensive root architecture of this species compared to the other species.
Soils planted to cowpea and sunflower monocultures contained more soil N than soil planted to the three-species mixture (123 and 128 lbs N per acre, respectively, vs. 91 lbs per acre. But the six species mixture was associated equal soil N compared to cowpea and sunflower monocultures. This is probably because the six species mixture contained 2 legumes (cowpea and hairy vetch), whereas the three species mixture only contained one legume, cowpea. Soil phosphorous and potassium were not affected by the cover crop treatments.
The reader may consider the outcomes of our study in light of two ecosystem services: weed suppression and nitrogen provision. Cowpea and sunflower provided the greatest benefit in terms of soil N, but provided the least weed suppression, especially in terms of weed suppression via residue mulch. Foxtail millet provided excellent weed suppression in-season and also produced a highly weed-suppressive residue when terminated via flail mowing. Therefore, one might think that combining these crops would provide a good solution, combining attributes of all three species. However, although the three-species mixture at Absaraka during 2016 provided excellent weed suppression, it did not contribute as much soil N as cowpea alone or sunflower alone. Furthermore, this result occurred under ample soil moisture. Data from Mandan in 2016, a much drier site, suggested that cowpea performance in terms of stomatal conductance was reduced when cowpea was grown in mixture (i.e., the cowpea had reduced ability to compete for limited water compared to the other species in the mix). These results suggest that predicting ecosystem service provision by cover crop mixtures is complex (performance when grown in monoculture doesn’t necessarily predict performance when grown in mixture) and that soil moisture is a factor that should be considered.
The results of this study will be relevant to farmers who are being encouraged to plant multi-species cover crops, but who are concerned because the benefits of planting these types of cover crops under variable moisture conditions have not been thoroughly documented. Although our results are preliminary, we provide some evidence that much more research should be done to determine the benefits of growing cover crop mixtures, ideally under various soil types with various types of stresses, including water stress. The reason for growing mixtures is usually to provide more than one ecosystem service. However, mixtures can be more expensive to plant and more difficult to establish and manage. Therefore, more research is needed to determine if the benefits of growing cover crop mixtures outweighs the costs and risks, especially under resource-limited conditions.
This project aimed to assess multi-species cover crop responses to variable soil moisture and soil type by comparing single-species with mixed-species cover crop performance under variable soil moisture regimes. The results will be relevant to farmers who are being encouraged to plant multi-species cover crops, because the benefits of planting these types of cover crops under altered precipitation have not been thoroughly documented. The results of this study will benefit both organic and conventional farmers who are interested in using multi-species cover crops but are concerned about water use efficiency and productivity. Although we evaluated a small subset of possible cover crop species and multi-species combinations, we plan to use the results as preliminary data to apply for a larger grant to extend the results to many more species. Through outreach efforts at one field day, about 10 farmers learned about how multi-species cover crops may perform under different levels of soil moisture typically encountered in the northern Great Plains. This information will help farmers who are hesitant about using cover crops decide if and how multi-species cover crops might be incorporated into their production systems. We expected that approximately five farmers will decide to change their cover cropping strategies as a result of being exposed to our research results. But we were unable to assess whether or not any farmer practices were changed. In addition, the proposed project resulted in approximately 1000 scientists/researchers and 500 farmers/practitioners/dealers learning about multi-species cover crop water use efficiency at two conferences (American Society of Agronomy Annual Conference and MOSES Organic Farming Conference).
Educational & Outreach Activities
Results were presented at the 2016 NDSU Horticultural Research Field Day. Approximately twenty growers came to the field day and were able to observe the cover crops at their height of growth.
Results were presented at the following scientific conferences:
Franco JG, Beamer KP, Gramig GG. 2017. Cover crop-weed dynamics in two contrasting management systems in the northern Great Plains. Western Society of Weed Science Annual Meeting. March 13-16. Coeur d’Alene, ID.
Results will also be presented at the 2018 ASA-CSSA-SSSA Annual Conference in Baltimore.
One objective of our research was to quantify the weed-suppressive ability of monoculture vs. multi-species cover crops, both in terms of in-season weed suppression by the crop canopy and next-season suppression by cover crop residue. The canopy cover provided by foxtail millet (91%), the three species mixture (foxtail millet, cowpea, and sunflower; 89%), and the six species mixture (foxtail millet, sunflower, cowpea, kale vetch, oat; 93%) was greater than the cover provided by sunflower alone (49%) and cowpea alone (51%). Interestingly, none of the cover crops or mixtures differed with respect to residue mass, so the differences in cover were not due to differences in biomass. Foxtail millet, a three species mixture , and a six species mixture all suppressed weeds very well (75, 64, and 78%, respectively), whereas cowpea alone and sunflower alone provided poor weed suppression (26 and 15%, respectively). Since the suppressive ability of foxtail millet alone did not differ from the mixtures in terms of weed suppression, we conclude that foxtail millet was the superior crop in terms of leaving a weed suppressive mat. Observationally, this species formed a thick cohesive mat of residue that remained intact over winter. Also, during the growing season, virtually NO WEEDS were present in any plots containing foxtail millet. Therefore, growing foxtail millet as a warm season cover crop might be useful for dealing with weedy fields in situations where herbicides cannot be used.
Another objective of this project was to assess how soil water content affects the performance and productivity of various cover crops species when grown in monoculture vs. mixtures. We measured stomatal conductance of the cover crops as a way to gauge water stress. Cowpea stomatal conductance (a measure of water stress) sometimes differed according to whether this species was grown in mixture or monoculture, especially under water-limited conditions. When soil water was reduced, cowpea often seemed to suffer more water stress when growing in mixtures as opposed to growing alone in monoculture. Conversely, foxtail millet and sunflower stomatal conductance were relatively unaffected by soil water status or cover crop mixtures.
We also measured differences in end-season soil nutrients that might be attributed to the cover crops. We saw that sunflower and cowpea monocultures were associated with the greatest amounts of soil nitrogen (NO3), but soil phosphorus and potassium did not appear to be affected by the cover crops.
Taken together, these results suggest that the performance of a cover crop species may change when that species is grown in a mixture with other species as compared to growing alone in a monoculture. This may especially be the case when some resource, such as soil water, is limited. At this time, our ability to predict or estimate benefits (such as soil N produced or weeds suppressed) for even single-species cover crops is limited. Estimating or predicting the benefits provided by multi-species cover crops is infinitely more complicated than predicting benefits of single species cover crops. Not only must we be able to predict performance under different soil types, with different resources and different weather, but we must consider intricate interactions between each species in the multi-species mixture. Every mixture will be different.
Our results are based upon only a small set of measurements for a small subset of cover crop species. Nevertheless, these results illustrate the difficulties one may expect when trying to predict outcomes of growing multi-species cover crops. Planting and managing multi-species cover crops can be both expensive and management-intensive. Estimating costs vs. benefits is critical to ensuring economic, environmental, and social aspects of sustainability associated with incorporating cover crops into agricultural production systems. Before recommending use of multi-species cover crops, additional research should be conducted to further explore costs. vs. benefits, and to develop frameworks or models for predicting the performance of cover crop mixtures under various environmental conditions.
We confronted many logistical obstacles during the course of this research. Coping with limitations imposed by access to labor and equipment, as well as challenges posed by the vagaries of weather, gave our research team new appreciation for the hurdles farmers encounter every season. Many farmers, as well as agricultural researchers, manage their field operations in consideration of current year objectives only. However, when one chooses to consider long-term sustainability of production, management may become more complex. For instance, instead of considering only the weed management outcomes of weed management tactics, one may want to consider soil quality outcomes, or pollinator health outcomes. Including these additional considerations makes management more complex, and also more subject to failure when factors beyond the farmer’s control (like weather) intercede. Recognition of these challenges further cemented our belief that research can help farmers achieve better sustainability, because researchers can assume more risk when testing new approaches. In particular, we realized the many challenges associated with incorporating cover crops into agricultural production systems. Some obstacles, such as those presented by less than ideal weather, cannot be altered. However, access to better ability to estimate the concrete benefits of planting cover crops, especially complex cover crop mixtures, would help farmers better gauge if the cost:benefit ratio would be favorable.
We presented results from this study at the 2016 NDSU Horticultural Research Field Day. At this field day, numerous participants were very impressed with the cover and vigor of the foxtail millet stands. Many were unfamiliar with this crop, and so the opportunity was valuable in that several growers were exposed to a new cover crop.
As a result of the outreach associated with this project, we met and started interacting with Ross and Amber Lockhart, of Heart and Soil Farm. We developed a new project about living mulches with the help of Amber and Ross, which was funded by the ND Specialty Crop Block Grant Program.
We still need more research aimed at assessing the costs and benefits of multi-species cover crop mixtures compared to monoculture cover crops. These mixtures are widely touted and recommended, but the benefits are not substantiated by peer-reviewed science. The benefits needed depend on the goals of the farmer. If the farmer has one goal, such as weed suppression, chances are great that a single species cover crop will be best for that purpose. For instance, our research shows no benefit in terms of weed suppression from growing a mixture of cover crop species because foxtail millet alone was the superior weed suppressor. On the other hand, if one is attempting to achieve multiple goals, a mixture of species may make sense. For instance, if one wanted to suppress weeds AND supply soil N, then one might choose the 3-species mix we tested (sunflower + cowpea + foxtail millet). However, we showed that under conditions of plentiful soil moisture, the 3-species mixture suppressed weeds very well but did not enhance soil N as much as cowpea or sunflower grown alone. Furthermore, we also showed that, at the much drier Mandan site, cowpea stomatal conductance (a measure of water stress) decreased markedly when cowpeas were grown in the three-species mixture compared to growing alone. So especially under conditions of limited moisture, a mixture may not perform as one might expect based on the performance of cover crops in monoculture. Our results are preliminary, but do point to the need to conduct more research about multi-species cover crops before widely recommending their use to farmers.