In many parts of Manitoba spring cereals were seeded early this year, and harvest may occur directly after winter wheat harvest. An early harvest means that there may still be two months of warm weather between harvest and freeze-up — warmth that could be used to grow a late-season cover crop.
Cover crops are known to protect soil from erosion, reduce nitrogen leaching, build soil organic matter, and suppress weeds. Growing a cover crop also adds diversity to the system and promotes biological activity in the soil, as many beneficial micro-organisms rely on living plant roots. If the cover crop is a legume it can contribute a considerable amount of nitrogen (N) to the cropping system.
Legume cover crops can be grown in Manitoba by double cropping, which is planting the cover crop after harvest of the main crop, or by relay cropping, which involves planting the cover crop into an established first crop. In most areas, it is too late to plant a relay crop this year, but it is a great time to think about double cropping a legume after harvest of your winter or spring cereals.
There are many options for late-season legume cover crops, but in years with sufficient moisture, forage pea performs well when planted after harvest of winter wheat or fall rye.
Chickling vetch and black lentil are even more drought tolerant than pea and can also be planted as double crops, though their biomass production tends to be lower than pea.
Cover crop mixtures may also be used, with legumes making up a large proportion of the mix. When choosing a cover crop seed availability needs to be taken into consideration and seed for certain cover crop species may be more readily available in Saskatchewan.
The growth potential of a late-season cover crop depends on planting date and late-summer and fall weather, including both temperature and moisture.
While we can’t predict this year’s weather, historical weather data tells us that at many locations in southern Manitoba, there is often adequate warmth and moisture to produce a cover crop after an early cereal harvest.
The table below shows the average growing degree days (GDD) and precipitation accumulated for various locations in Manitoba after winter wheat harvest, as well as the minimum GDD and precipitation accumulated in this period in three out of four years (75 per cent probability).
In one out of four years, the number of GDD and amount of precipitation accumulated would be expected to be lower than the values given in this table.
A cover crop can be grown successfully with as little as 400 GDD. In research at several sites and over several years in southern Manitoba, forage pea cover crops seeded in the third week of August produced 450-900 lbs./acre of dry biomass with 400-500 GDD. Peas planted in early to mid-August generally produced 800-2,100 lbs./acre of dry biomass with 590-725 GDD.
Available moisture after cereal harvest also affects cover crop establishment and growth. Precipitation at this time of year is extremely variable and can be sporadic. It is also important to remember that soil moisture may have been depleted to a depth of several feet by the cereal crop.
Despite these challenges, the pea cover crop established successfully at three different sites over four years in the study described above. Even in years with only 70-90 mm of rain during August and September, pea biomass production generally ranged from 450-1,200 lbs./acre.
The size of the N benefit depends on the growth of the legume cover crop. If legumes are inoculated and actively fixing nitrogen, you can expect to add 25 lbs./acre of nitrogen to your soil for every 1,000 lbs./acre of dry legume biomass produced.
Whether this nitrogen benefit increases the yield of the following crop depends on a number of factors, including the organic matter (OM) and fertility levels in the soil. In soils that are depleted of OM and nutrients, much of this nitrogen may go into the long-term “savings account” in the soil.
In such cases, it may take several years of growing legume cover crops or using other soil improvement practices before the benefits become quantifiable. Rather than a loss, this can be seen as an investment in soil health and nitrogen reserves that will be repaid gradually in future years.
Where soil OM and nutrient levels are adequate, the fertilizer replacement value of a legume cover crop can be up to 50 lbs./acre of nitrogen. In a Manitoba study, a 900 lbs./acre legume cover crop increased yield of unfertilized oats by 30-35 bu./acre.
Growing a cover crop may or may not require you to change fall field management and spring seedbed preparation, depending on your current practices and other factors such as weeds.
Annual cover crops such as peas winterkill readily and don’t necessarily require any direct termination. If weed pressure is low and seeding into high-residue conditions is possible, cover crops may be allowed to grow right until freeze-up. In cases where control of perennial or other weeds is needed, a fall herbicide application or tillage operation may be used after the main period of cover crop growth.
Late-fall or spring tillage may also be used as necessary to manage residue and create a good seedbed for the next crop. Tillage may also speed up the release of nutrients from cover crop residue.
For farmers with access to livestock, grazing the cover crop provides some economic return and can also enhance nutrient cycling. In an intensive grazing system with high rates of forage utilization, livestock can convert a 1,000 lbs./ac. cover crop into 60-70 lbs./acre of live-weight gain while transforming the nutrients in the cover crop into forms that are readily available to the next crop.
In recent research at Carman, a pea-soybean-oat cover crop was seeded on August 20 after fall rye harvest and produced 425 lbs./acre of biomass before being grazed on Oct. 2. The following year, unfertilized wheat yielded 25 bu./acre after the grazed cover crop and only 13 bu./acre where no cover crop was grown after fall rye harvest.
Although there are far more benefits to growing a cover crop than the value of the nitrogen or feed produced, it is also important to consider the economics of a late-season cover crop.
Based on MAFRD’s 2015 crop production costs, the seed, inoculant, and fuel costs associated with planting a late-season cover crop of peas is $49.75/acre. In a year with average heat and rainfall a cover crop planted in early to mid-August could produce 1,000-2,000 lbs./acre of dry biomass, which would result in a nitrogen contribution of 25-50 lbs./acre, and a fertilizer replacement value of $12-$24/acre. The benefits of reduced soil erosion, nitrogen leaching, and increases in soil OM are difficult to quantify, but should not be discounted.
If moisture and heat are not limiting in your area this August, consider trying a cover crop. For more information on how to include cover crops on your farm, visit the Natural Systems Agriculture website.