Assessing Multi-Species Cover Crop Responses to Variable Soil Moisture and Soil Types

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.