Cover Crop Potential
By Greg Stewart, OMAFRA Corn Specialist

If cover crops were politicians, you would have to admire their campaign managers, or perhaps more appropriately their 'spin doctors'.

When the issue of the day was fuel shortage, cover crops ranging from winter rye to hybrid poplars were going to save the
Applying manure to actively growing cover crops or establishing cover crops after manure incorporation may help reduce nitrogen losses.
day with enhanced biomass production. When soil erosion and residue cover became the hot topics, cover crops were going to be there stabilizing soil particles and adding residue whenever the main crop came up short. It was an easy pitch to promote the cost savings of a legume cover crop in the face of high nitrogen fertilizer prices. And now in the face of increased pressure regarding nutrient management, cover crops are out there front and centre.

Fall Nitrogen Capture

An efficient cover crop, within the context of nutrient management, is one which will: 1) tie up or sequester available nitrogen in the fall, 2) secure this nitrogen from leaching over the course of the fall/winter/spring when crop demand is low, and 3) release it to the corn crop in time for its phase of rapid growth. This target of available nitrogen in the fall may be a result of leftover nitrogen that the crop did not use or come from late summer or fall applications of manure. The ability of a cover crop to capture this nitrogen in the fall hinges mostly on its ability to grow rapidly after the main crop has been harvested. This has been the traditional advantage held by an underseeded cover crop like red clover compared to those cover crops that need to be seeded in August. To assess the ability of various cover crops to sequester nitrogen following winter wheat, trials were conducted in the early 1990s by University of Guelph researchers T. Vyn, J. Faber, K.Janovicek and E. Beauchamp. Their findings, outlined in Table 1, indicate that all four of the cover crops examined (red clover was spring underseeded while fall rye, oilseed radish, and oats were drilled following wheat harvest) were able to draw down soil nitrate levels compared to the plots that had no cover crop. Interestingly, even red clover, with its ability to fix nitrogen from the air, was as effective as the other cover crops in reducing soil nitrate concentrations by the end of the growing season.

Table 1 Influence of cover crops on soil nitrate concentrations in the surface 60 cm (24 inches) when sampled in early November of the establishment year. Average of four sampling trials: Ayr, 1992 and 1993; Kirkton, 1992 and 1994. (Vyn et al., Agronomy Journal 92: #915-924)

Cover crop

Soil Nitrate (PPM)

Red Clover


Fall Rye


Oilseed radish




No Cover


Other research has found that actively growing red clover will serve as a sink for manure nitrogen that is surface-applied. Nitrogen credits have been found to multiply in these systems; that is, the nitrogen credit for the manure applied to the cover crop is higher than the combined credits of the red clover and manure when acting separately. One of the ongoing difficulties with the use of red clover as a cover crop is the variability in the stand across the field. One approach to dealing with this is to direct-drill other cover crops (i.e., oilseed radish, peas, oats, etc.) after wheat harvest into just those areas of the field where the red clover stand is poor.

Table 2 The influence of cover crops on soil nitrate concentrations taken in the pre-sidedress time frame (PSNT) and on corn yield when no fertilizer nitrogen was applied. Average Kirkton, 1993 and 1995. (Vyn et al. Agronomy Journal 92: #915-924)

Previous Year
Cover Crop

Pre Sidedress
Nitrogen Test (PPM)

Corn Yield (bu/ac)
(no fertilizer nitrogen applied)

Red Clover



Fall Rye



Oilseed Radish






No Cover



Establishing a vigorous cover crop where manure is to be applied of course implies that the manure (and cover crop) will not be incorporated with tillage until later in the growing season. This may be difficult to manage in the face of greenhouse gas issues, odours or other concerns which may dictate immediate incorporation of manure. In these situations, the manure application combined with the act of tillage will often foster higher levels of soil nitrate. This nitrate could be made less available to leaching or other losses if a cover crop was established immediately after the tillage operation. Of the cover crops tested which could be fall-seeded after a manure application and incorporation, oilseed radish appears to have greater potential to tie up soil nitrate than oats, fall rye or ryegrass.
Fall-seeded oilseed radish can grow vigorously given good conditions prior to season-ending frosts. Uneven stands continue to plague red clover as a cover crop. Filling in poor areas by drilling other cover crops after wheat harvest is an option.

Nitrogen Availability to the Corn Crop

The problem with cover crops is that after all the talk, you may end up with less money in your wallet than when you started. For cover crops to be both environmentally and economically sound, the nitrogen that is sequestered in the fall needs to be efficiently made available to the corn crop, ideally around mid June. Red clover has been shown in trials to release its nitrogen (via mineralization) in a pattern that is nearly perfect for corn growth. Oilseed radish also has generally good characteristics in terms of making its nitrogen available. However, some findings have indicated that the nitrogen in oilseed radish may be released into the soil earlier than red clover, before any significant uptake by the corn crop, and thus become more susceptible to losses from leaching or denitrification. Tony Vyn and his colleagues also pointed out that the nitrogen release from cover crops such as fall rye and oats tended to be slower than desired, and in some cases reduced corn yields below those where no cover crop had been grown (see Table 2).

Fortunately, we have some direction on ways to fit cover crops into corn production systems, but the need for integrated approaches to tillage, manure management and cropping sequences continues to grow.

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