Deciding on cover crop blends

Figuring out what to use in a cover crop blend can be an intimidating process. What species to use? What seeding rate? When should they be seeded? All would influence how the blend would look, work, and the success of it. There is some science involved, but making it work is as much art as science. Here are some tips.


Goals need to be in place before trying to put a blend together. To plan to use a cover crop is like saying a cereal crop is going to be grown. What market? When seeded? What seeding rate? Goals need to be in place for the main objective to overcome. Whether it is breaking up hard pan, fixing nitrogen, recovering saline areas, get supplemental grazing, or smothering weeds, the goals will dictate what species will be chosen.


The next step is to look at the rotation: What was there last year and what is going in next year? 


One of the reasons to use cover crops is to add diversity to the rotation. A rotation should include grass, broadleaf, and legume plants grown. Ideally, there should be combinations of each in both warm and cool season species. So if the last crop was barley and the next will be canola, the goal would be to add nitrogen. In that case, sweet clover, berseem clover, red clover, or crimson clover could be added to the barley, either while the barley is growing or immediately after harvest.


For full season cover crops, picking species that will not go to its reproductive stage before termination is important. Species like radish will bolt after 40 days after emergence so will need to be clipped or mowed to keep it in the vegetative stage. 


Alternatively, using species that are late maturing or biennials would be a lower maintenance option, like fodder beet, turnip, or forage collards. As the plant starts its reproductive stage, the plant will produce more lignin, which will be harder to work with the next year. The higher lignin content will produce more soil armour, litter protecting the soil from erosion and wider temperature fluctuations.


In a grazing scenario, picking species that have a low growing point and good regrowth would be priorities. Later maturing species would help keep the plants palatable throughout the summer. Matching the species of plants to the animals is important. Sheep and horses are nibblers when they graze where cattle are rippers. Some plants are better suited to one or the other. In either case, grasses are what produce tonnes, broadleaf plants will add protein, and legumes will be fixing nitrogen for the mix plus adding some feed quality to the mix.


Relay cropping is where a cash crop is seeded as normal, then a cover crop is seeded into the cash crop after it is established. The cash crop is harvested, and the relay cover crop then continues to grow. 


The challenge is to get species that will tolerate growing with the competition from the cash crop, then recover after harvest to continue growing. The other challenge is to have the relay crop not interfere with the harvest of the cash crop. 


Lentils would be an example of both. The solution is to seed the cover crop late so the lentil crop is close to maturity. This way the cover crop is in the seedling stage and will not be cut when harvesting the lentil crop. In a wheat crop, using species like Persian clover, phacelia, subterranean clover, and plantain, would give low growing species that should stay below the cutting height for the wheat crop.


Aerial seeding can be an effective method of getting the cover crop seeded into an established crop late in the growth cycle. The challenge in this method is to ensure good soil moisture at the soil surface. Because it is a cover crop, uniform germination is less important than for a cash crop, so a cover crop in different stages through the field is not an issue. The positive with aerial seeding is that a lot of acres can be covered in a short period of time, assuming the seeding rate is relatively low. Plus there are no tracks in the field from seeding.


Seeding rate is another important decision to be made. A stand-alone cover crop can range from a 1/3 to 150 per cent of a normal seeding rate. When seeding a relay cover crop, a lower seeding rate is normally used, where a smother cover crop will be seeded heavy to help out compete any weeds. 


Broadcasting seed will create a higher mortality rate, so there will be fewer plants established than a crop that is put into the soil. 


A mixture with large and small seeded species that are seeded too deep for the small seeded species will result in high mortality for the small seed species. Same goes for less competitive species or species that are seeded at the wrong time. This applies more for the warm season species that are seeded when nighttime temperatures are too low for vigorous growth and mixed with cool season species. 


A mix of sorghum Sudan mixed with oats that are seeded before June 1st on a normal year will end up being dominated by the oats. The same mix seeded the end of June, the blend will look like a sorghum Sudan stand. Some species, like sugar beets, are not overly aggressive and will be choked out of most blends, unless there is lower plant density allowing the sugar beet to have its own space.


Once the plan of the cover crop is made and goals are set, the next step is species selection. Planning the cover crop strategy and taking weather into account will get successful results. Diversity is the key on the path to soil health.