Specialty Products

Oilseed Radish

Oilseed radish  has been grown on a few farms in Ontario as a plowdown - cover crop. Experience to date suggests that it could be a useful soil improving crop for late-summer seeding.

The number one plowdown crop in Ontario is red clover seeded in the spring in either winter wheat or spring cereals. This crop does a good job of improving soil structure and fixing nitrogen from the air through nodules on the roots. Seed cost is reasonable and seed is readily available. Establishing red clover costs very little; red clover does not harm the yield of the companion crop or the following crop. Thus there are good reasons for the continued popularity of red clover as a plowdown.

However, there are situations where for one reason or another, red clover does not get seeded in the spring or fails to establish well. In 1988, the severe drought killed seedling red clover in some wheat fields. There is interest in a cover crop that can be seeded after cereal harvest and still provide significant benefits to the soil by winter. Oilseed Radish appears to be a crop useful in this situation. Oilseed Radish has been used for this purpose for several years in Europe. In Ontario, several farmers, particularly organic growers, have been using Oilseed Radish as a cover plowdown crop for at least four or five years.

Plant Description

Oilseed Radish (Raphanus sativus, var. oleiformis) is a plant developed only for plowdown and has no other use. When planted in the spring, it will grow to a height of six feet by maturity. Flowers are predominately white with a purple tinge. Root shape is somewhat variable with a central tap root shaped like a carrot or parsnip or overgrown radish. Fine roots grow out from the central tap root. Oilseed Radish belongs to the Brassica family and thus does not have nodules and cannot fix nitrogen from the air. The crop will tolerate fall frosts but does not survive the winter in Ontario.

Culture as a Cover Crop

Oilseed Radish is usually planted after cereal harvest between late July and early September. The crop is planted later in areas with more heat units available.

Usually growers adjust the seeding date so that the crop does not have time to set seed before winter. In the 2700 H.U. zone, a seeding date of August 15 will usually mean no  seed set. If the crop is approaching seed set before heavy frosts, growers will usually plow it under or chop it down in the fall. One can normally expect about two feet of top growth six to eight weeks after seeding. This growth provides excellent soil protection if left in place over winter. However, the residue "shrinks" enough over winter that crops such as soybean can be no-tilled into the residue the following spring.

Seeding rates used range between 10 and 20 lbs. / acre. When the seed is broadcast and worked in with harrowing or light cultivation the higher seeding rates are probably needed. Oilseed Radish seed is not much larger than canola seed; it is easy to bury seed too deep for emergence. Where the seed is drilled in, 10 to 12 lbs. / acre should be adequate. Regardless of seeding method, firming the soil after seeding is a good practice with summer seeding especially if the soil is somewhat dry. Oilseed Radish is a heavy nitrogen user. The crop responds well to residual nitrogen left from the previous crop or from manure. In soils that are very low in nitrogen, Oilseed Radish growth will be much slower.

Volunteer wheat does not seem to be too much of a problem as it stays short in the fall and the Oilseed Radish "leaves it behind". However, volunteer oats or barley are much more competitive and can reduce the Oilseed Radish stand by 50%. Some growers will work a spring grain field once after harvest to induce grain sprouting and then rework and seed Oilseed Radish about two weeks later.

Cautions as a Plowdown Crop

If planted early or fall growing conditions are very good, Oilseed Radish can produce viable seed from a summer seeding. This seed can remain viable in the soil for several years. Herbicides which control mustards and canola should give good control of volunteer Oilseed Radish.

Cautions if Grown for Seed

If one plants Oilseed Radish in the spring to produce seed, seed shatter before harvest is not a problem but there will certainly be some seed loss during combining. One will have to be prepared to control volunteer plants in future years.

One should also be aware that Oilseed Radish is quite susceptible to white mold attacks during the July flowering period, particularly on wetter years. Thus it should not be grown for seed if the susceptible crops such as white beans, canola and sunflowers are also being grown on the farm. (There are no reports of white mold problems when grown as a fall cover crop, probably because of low temperatures if the crop flowers in October).

One would expect that flea beetles could cause significant damage to spring seedings. However, there have been no reports of damage to August seeding from this insect.

Why consider Oilseed Radish?

Oilseed Radish has the ability to grow very quickly from an August seeding; tolerates the lighter frosts of September-October, but dies over winter so that it is not a problem to kill the following spring. The combination of a deep tap root, fibrous side roots and strong top growth, provides a lot of fresh organic matter and should improve soil structure. The crop absorbs and holds in the plant nutrients from the soil or from a recent application of manure. This can be particularly important with nitrogen, a nutrient that otherwise usually leaches out of the soil over winter.


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