The Southwest Booster
A variety of factors to consider during selection for Cover Cropping mixes
Cover crops, or poly crops, have been very popular in agricultural news lately. There are many claims regarding the improvement of soil health and nutrient cycling coming from seeding diverse mixtures. While these mixtures have potential environmental benefits, such as reducing fertilizer, it can be difficult to know what plants should be included in a blend. Though there is some research supporting these practices, one of the many questions remaining is around the optimal number of species. Pre-blended mixes can include anywhere from two species to an upwards of 18, but we really do not know where the point of diminishing returns lies. After how many species do the additional species stop adding notable value?
When designing your cover crop blend, there are a few questions you can ask yourself that may assist you in getting the right mix for your operation.
The goal of the crop
If the main intent is to grow feed, you may consider including more cereal species that have high biomass rather than novel flowering species designed to attract pollinators. If you are trying to manage compacted soil, you may include more brassica and legume species that have deep tap roots that can break-up compacted soil rather than species with fibrous root systems that may struggle. Brassica species also have been known to have biofumigant properties, meaning these plants have the some fungicidal and nematodal properties—though not all brassica species are suited to this task. Having differing root systems also will influence infiltration and nutrients in the soil profile.
The moisture of your region
Plants are broken into three categories, based on their method of photosynthesis: C3, C4 and CAM
Roughly 95 per cent of all plants are C3 plants and are considered cool and damp loving. When completing photosynthesis, these plants open their stomata (pores) cells which can allow water to leave their cells. Many of the crops we grow on the prairies, such as wheat, barley and canola, fall into this category. On the other hand, C4 plants complete photosynthesis with their stomata closed, saving the release of excess moisture but sacrificing extra energy and resources of the plant. The C4 plants we typically see are corn, sourghum-sudangrass and millet, as well as some of our competitive weeds on the prairies such as pigweed and leafy spurge. CAM plants are adapted to dry environments (such as cacti and pineapples) and we typically would not seed them into a cover crop mix.
Consider the functional groups of the species included
There are several ways to delineate these groups, but for now we will break it down into warm or cold season and grass or broadleaf groups.
Warm season grasses, such as teff grass and corn, will thrive in high heat. Cool season grasses, such as rye grass and barley, will do better in cooler conditions but have a higher protein content. Warm season broadleaf plants, such as cowpeas, have lower protein but grow best between 32 and 35 C. Cool season broadleaf plants, such as radishes, turnips and alfalfa, have higher protein but also can have the beneficial properties of brassicas and deep rooting legumes that were discussed earlier.
Ultimately, when deciding on what species to include in your cover crop, it comes down to specific needs of your operation. You do not need to include a species from every functional group, nor do you have to stick to only one functional group. By increasing the number of species, you may buffer risk by having species that grow at varying environmental conditions, although be mindful that this may also increase cost. The perfect cover crop mix will balance economics while still meeting nutritional requirements and all environmental restrictions and likely look very different on your operation than it will on your neighbours.