In Canada and the United States, cover crops are heavily promoted as an essential component of soil health in grain/oilseed/pulse production systems. Ideally, cover crops are grown before or after the main cash cropping season so that a growing season is not devoted solely to cover crops. Under the right conditions, cover crops can help to improve water infiltration and retention, reduce soil erosion, and provide livestock fodder.
However, cover crops are not ideal for all climates outside of the main cash crop season. In areas of Canada and the United States north of the 43rd parallel and west of the 100th meridian, cover crops usually will not work outside of the main summer cropping season due to short growing seasons and/or relatively dry climates. While irrigation is important in some areas (i.e. Central High Plains, South-Central Saskatchewan) dryland acreage of grain/oilseed/pulse cash crops is significantly greater than irrigated acreage across this region. Considering the expense of irrigation infrastructure and the limits of existing water resources, dryland production of grains/oilseeds/pulses north of the 43rd parallel and west of the 100th meridian will remain an important method of commercial production well into the future.
While various forms of wheat-fallow rotation became dominant across the region between the 1920s and 1970s, the financial and environmental viability of wheat fallow-only systems started to be questioned as a result relatively low wheat prices along with severe soil erosion and poor soil function resulting from long periods of soil being left bare. A number of practical innovations have been adopted in these colder and/or drier climates that have allowed for the adoption of continuous cropping systems (with some important exceptions in extremely dry climates of interior Washington State or the Southern High Plains). At present, continuous cropping dryland systems use a number of innovations which allow for soil health practices to be used minus cover crops. They include:
1) Maximizing post-harvest crop residue: This can be achieved using stripper headers (small grains), pan headers (sunflowers), or specially configured conventional corn headers. Another benefit of not processing residue through combines is reduced machine wear and tear and lower fuel consumption.
2) No-till drills or row crop planters configured to handle large amounts of the previous year’s cash crop residue: Newer no-till drills or planters use a significant amount of downforce along with discs to cut through residue, establish good seed/soil contact at optimal planting depths, and minimize hairpinning. While no-till equipment has been widely available since the 1970s, a host of innovations have significantly improved no-till seeding and planting equipment since 2010.
3) Broad and flexible cash crop rotations: Ideally, 5-6 cash crops are grown in rotation and rotations are set on an annual basis. Cash crops are chosen based on interrupting pest and disease lifecycles, diverse herbicide programs, post-harvest residue maximization, and market demands. Typically, small grains play a key role in these broad rotations because they provide significant post-harvest residue relative to oilseed and pulse crops.
4) Synthetic N fertilizer optimization: Synthetic N rates can often be reduced thanks to improved soil function, broader cash crop rotations, variable rate fertilizer equipment, and monitoring N requirements throughout the growing season.
5) Other synthetic input optimization: Broad cash crop rotations can result in reduced requirements for fungicides, insecticides, and herbicides.
The practices that I describe have become standard approaches for soil health practitioners in drier and/or colder areas of North America. During the past decade a number of other innovations have been added to these standard practices, which include:
1) Mixed grain intercropping: Raising two or more cash crops at the same time and separating after harvest can provide specific agronomic benefits like reduced lodging and reduced fungal pressure.
2) Variable rate seeding: Like variable rate fertilizer, variable rate seeding allows for seed cost savings and in some cases, higher yields.
3) Ruminant livestock integration: The grazing of livestock on early-stage cash crops, cash crop residue, or forage crops are extremely old practices that have been used worldwide. However, there has been a recent resurgence of interest in grazing livestock on dryland cash crop fields after a significant decline in the 1970s.
While cover crops are an important soil health tool in the right contexts, they can be difficult to make work in dryland cash crop systems in drier and/or colder climates in Canada and the United States during the main growing season. Cover crops can certainly be grown during the main summer growing season to improve soil function, but farm operations must be willing to give up a cash crop for one year and ideally a cover crop raised during the growing season would be grazed by ruminant livestock.