Underseeded Red Clover in Winter Wheat

Prepared by John Heard, MB Ag Soil Fertility Specialist

There is current interest in broadcast seeding red clover into winter wheat with spring nitrogen fertilizer.  The technical term for this is relay cropping.  The main reason is to provide biologically fixed nitrogen for the following crops.

Typical nitrogen (N) contributions are available from studies conducted by the University of Manitoba, where several legume crops were either spring sown into winter wheat and fall rye, or seeded following cereal harvest.  A following crop of oats was used to determine the nitrogen replacement value (Table 1).

Table 1.  Nitrogen replacement value in oats of various N fixing green manure crops in Manitoba (lb N/ac)

(Thiessen, Martens, et al. 2001)

Ontario fertilizer recommendations for corn use a 60-73 lb N/ac credit following red clover seeded into winter wheat (the higher number were killed by tillage, the lower were killed by herbicide alone — both operations done in the fall).

The lower N replacement value for red clover in Manitoba may be due to several factors:

• There is a shorter fall growth period than in Ontario.
• Nitrogen release from green manure plowdown crops better coincides with the N uptake pattern of long-season crops such as corn, potatoes, sunflowers and beans than for cool-season cereals, canola, etc.

• In the Manitoba study, the red clover was killed in the spring.  This further delays the release of N for following crops.  The annual alfalfa which winterkilled on its own provided more nitrogen for the following crop.

Nitrogen credits from green manure crops are generally related to the biomass produced.  A thumb-rule developed from University of Manitoba research is:

• Green manured legume crops produce 30 lb N per 1000 lb of dry matter forage, with 50% of this nitrogen available in year 1 and 15% in year 2.

Following are some of the production factors to consider in establishing red clover in winter wheat.

1. Type of red clover

   There is a difference between single and double cut red clover.  Double cut clover tends to be more aggressive starting, and will grow and flower the first year, producing knee-high vegetation after wheat harvest.  The main feature is greater top growth and N fixation or use for fall grazing, hay, etc.

   Single cut red clover needs vernalization to flower, stays vegetative the seeding year and concentrates on building a larger root in the fall, compared to the top growth which double cut clover produces.  This root development may be more beneficial for soil structure; adding tilth, drainage improvement, breaking up compaction, etc.

   In Ontario, where red clover is also grown to improve soil structure, both single and double cut clover are grown.

   Costs last year for pedigreed seed were about $2.60/lb for double cut (DC) and $2.00/lb for single cut (SC).  The common seed used in Ontario is about $1.25/lb.

   Dr. Martin Entz has conducted research on an annual type of clover called Berseem clover.  It winterkills on its own after producing much top growth.  Seed is available from Iowa growers.

2. Inoculation

   Ideally seed should be inoculated with rhizobium inoculant.  The species that is effective for clover (Rizobium trifolii) is different than that used for alfalfa (Rhizobium meliloti).  In practice, very little red clover is inoculated in Ontario.

3. Broadcast seeding

   Red clover seed is typically blended with granular nitrogen fertilizer prior to broadcasting.  The fertilizer and clover should be blended at the fertilizer plant.  Segregation isn't a big problem as long as the mix isn't handled too many times.  The fertilizer clover blend should be applied as soon as possible, but has not affected stands if stored dry for a couple of days.  Storage for any length of time would be expected to reduce inoculant efficacy more than clover germination.

   In former days when UAN solution was typically broadcast sprayed on winter wheat with floodjet nozzles, red clover could be mixed in the sprayer tank and applied.  However, it is now recommended that UAN be dribble band applied.  It also proved difficult to remove 100% of the seed from spray tanks which led to plugging of Tee-Jet nozzles.

   Some growers in Ontario that have had some difficulty in getting establishment in long-term zero till conditions are now running a double disk drill over the field in the spring to place the red clover seed into ½ inch of soil.  Both broadcast and drilled establishment methods have been successful at the Manitoba Zero Tillage Research Farm.

    

   Figure 1. Red clover broadcast seeded into winter wheat (some of the red clover seeds indicated by arrows).

4. Seeding rates

   Commonly recommended seeding rates are 6-8 lb/ac if seeded early (before winter wheat greens up) or 10 lb/ac once wheat has started growth and is more competitive. General guidelines from Ontario are for 9 lb/ac (10 kg/ha).

   Remember one lb/ac supplies 6 seeds/square foot and 13-14 seedling plants per square foot is more than sufficient.

5. Weed control

   It is important to control volunteer canola and winter annual broadleaf weeds in many winter wheat fields. Registered herbicides for cereals underseeded to red clover include MCPA amine, MCPA/MCPB (Tropotox Plus) and 2,4D-B for broadleaf weeds and Achieve, Avadex and Avenge for grasses (Guide to Crop Protection). Some other broadleaf herbicides are commonly used but are not labeled as such. One should remember that the objective is not to produce high yielding forage in June — but solely to establish some runt plants that can grow vigorously once wheat is harvested.

   Of course pre-harvest glyphosate applied prior to winter wheat harvest would kill the red clover before it provides much benefit.

6. Fall termination of the red clover

   In most cases it is preferable to terminate the clover in the fall. Problems with spring termination include depletion of soil moisture, cooler soils, a delay in nitrogen release and the challenge in providing total kill with herbicides.

   For killing clover, nothing beats the moldboard plow but glyphosate plus disk or heavy cultivator tillage will work. If you are going to burn it down for no-till, it has to be done in the fall, before there is a killing frost. Addition of phenoxy herbicides to glyphosate will aid in control.

   Fall timing of control is a compromise between getting good control and letting the plant grow as long as possible to fix nitrogen.

   This issue of fall termination is not an issue when growing a legume that will winterkill such as annual alfalfa or Berseem clover.

Other red clover facts:

1. Red clover does not affect winter wheat yields or protein — since clover is frost seeded into an established cereal stand that has initiated regrowth and is rapidly establishing leaf area.
2. Red clover generally does not affect spring cereal yields, despite being at less of a competitive disadvantage. Ontario and Michigan studies found no effect on barley and oat yields, respectively.

   Figures 2 and 3. Red clover growth prior to cereal harvest after spring broadcast seeding into winter wheat (left) or seeded with spring cereals (right).

3. Red clover can be seeded too early. Two to three week old seedlings are less tolerant of frost than younger seedlings. One week old seedlings may tolerate -4°C. This sensitivity to frost may account for some of the uneven establishment some years.
4. Uneven spreading of cereal straw may lead to smothering of clover and variable stands.
5. Red clover establishment is reduced as N rates applied to the winter wheat increases — due to greater competitiveness of the wheat.
6. Very little clover biomass is produced in the growing cereal crop — the bulk of the growth occurs following cereal harvest. Biomass produced after harvest is affected by red clover density, precipitation, temperature, length of growing season between cereal harvest and killing frost and soil texture. Biomass is less on coarse soils.
7. Fall growth is not influenced by residual N from the cereal or manure applications.
8. Release of N from red clover is higher when mechanically tilled vs. no-tillage.
9. N release from clover proceeds rapidly for 4 weeks after burial and then proceeds slowly.
10. N release from red clover better coincides with the N uptake pattern of late season crops like corn and potatoes than for cool-season crops. N release occurs during June and July.
11. The efficiency of N uptake from red clover and other legumes is only about half that as from fertilizer N. However, cover crops contribute twice the amount of N as does inorganic N to the organic N pool in the soil.
12. Red clover reduces leaching losses of N, by taking up residual N and through water use in the fall.
13. The non-N rotational benefit of red clover is about 5% - chiefly through soil structural improvement (increased wet aggregate stability and increased organic matter levels).
14. Some pest problems may occur with red clover. Where corn is zero tilled in humid environments, slug feeding is more prevalent. Use of red clover in the cereal crop curtails some of the herbicide options, including preharvest glyphosate. Hence quackgrass and perennial weeds must be controlled elsewhere in the rotation.

More information on red clover as a plowdown is available at:

Red Clover - Cover Crop (OMAFRA)

For further information, contact your MB Ag Office.