Management of Oilseed Radish and Yellow Mustard Green Manure Crops

Introduction

Green manure crops of oilseed radish (Rhaphanus sativus) and white mustard (Sinapis alba) provide several benefits to any crop production system. One of the main reported benefits of these green manure crops is reduced populations of nematode, weed and disease pests. Pesticide inputs to potato and sugarbeet crops can often be reduced because of the lower pest pressure when these crops follow oilseed radish or white mustard. Compared to standard rotations, higher quality sugarbeet and potato crops are generally grown on soils where oilseed radish or white mustard residues have been incorporated. The residues from these green manure crops add humus that improves soil tilth, water holding capacity, and nutrient availability. Fall-planted green manure crops can reduce soil erosion and tie up residual nitrogen from the previous crop that might otherwise be leached below the root zone during the winter. Nutrients taken up by the green manure crop are released into the soil after incorporation of the residue, where they become available for uptake by the following crop.

There are several potential disadvantages of radish and mustard green manure crops. One of the biggest constraints to adoption of green manure cropping systems is the additional costs for seed, water, fertilizer and tillage. Green manure crops also require additional labor and management, often at times during the season when these inputs are needed for planting or harvest of marketable crops. Cover crops reduce flexibility in choosing the crops that precede or follow in the same cropping season. In regions with a long frost-free period, spring planted green manure crops should be followed by short-season crops such as sweet corn, onion seed or dry beans. Fall plantings should be preceded by early maturing crops such as peas, barley or winter wheat. In regions with a short growing season, it may not be feasible to grow another crop in the same season as the green manure.

Cultivar Selection

The choice of an appropriate cultivar will depend on the primary reasons for including the cover crop in the rotation, adaptability of the cultivar to the local climatic conditions, and types of crops grown in the rotation. One of the main reasons to grow white mustard or oilseed radish is to suppress weeds, diseases and nematodes. Unfortunately, there is little information on the impact of cultivar selection on the level of suppression of these pests. Therefore, the best choice in most situations will be a cultivar that grows well in your region. However, there is good information on using oilseed radish or white mustard to suppress sugarbeet cyst nematodes. The nematode suppression results from the green manure crops ability to "fool" the nematodes into hatching when a good host plant is not present. When a host plant - such as sugarbeets, certain weeds, or certain kinds of radishes and mustards - grow close to sugarbeet cyst nematode eggs, a hatching factor exudates from these plants, stimulating the eggs to hatch. Properly timed destruction of these plants will result in the nematodes dying from starvation before they have time to reproduce.

European scientists developed a trap crop system based upon development of sugarbeet cyst nematode-resistant varieties of oil radish and white mustard that produce this hatching factor. These special varieties produce enough hatching factors during a ten-week growth period to initiate hatching of almost all the cyst nematode eggs. The newly hatched worms quickly invade the roots, but in these specially developed varieties, the nematodes do not develop into egg-laying females, and reproduction does not occur. Few viable eggs remain in the soil to infect the next crop of sugarbeets.

Studies at the Parma Research and Extension Center indicate that certain cultivars provide better suppression of cyst nematodes than do other cultivars (Table 1). In general, radish crops provide slightly better suppression than do mustard crops.

¹ Resistant (R) or moderately resistant (MR) to sugarbeet cyst nematode.

The cultivars listed in Table 1 can be obtained from the following seed suppliers:

• Beta Seed
  741 Spyglass Way
  Eagle, ID 83616
• Hilleshog Mono-Hy
  NW District Sales Manager
  25660 Stephen Ln.
  Parma, ID 83660

Planting Date

A major concern in growing radish and mustard green manure crops is the relatively long growth period required to produce an acceptable amount of plant biomass. It takes 8 to 10 weeks of at least 60 degrees F soil temperature to get satisfactory plant growth and cyst nematode suppression. Frost is also a concern because it will kill the plants if the temperature drops below 28 degrees F for mustards, or 25 degrees F for oil radishes. Early plantings generally produce more plant biomass, provided frost damage is avoided (Table 2).

For fall planting, the last week of July to the second week of August is the optimum planting date in southwest Idaho. For spring planting, the first two weeks in March are recommended in southwest Idaho. Planting dates in other regions should be adjusted according to local soil and air temperature conditions, and danger of a killing frost.

Field Preparation

Because early planting is critical for the effectiveness of green manure crops, it is important to quickly incorporate previous crop residues and perform tillage operations to loosen the soil. When following cereal crops, remove straw immediately after harvest, or chop it as short as possible and work it into the ground. Loosen soil deep enough to allow dense root penetration and optimum aeration for hatching of cyst nematode eggs. These tillage operations can also be used to incorporate fertilizer. If time permits, irrigate to germinate volunteer cereal and weed seeds. Finally, prepare a good, firm seedbed to encourage rapid germination and establishment of the green manure crop.

Planting

Planting can be accomplished with a grain drill, or the seed can be mixed with fertilizer and applied with a fan spreader truck. A light harrowing would be necessary to cover the seed after spreading. Seeds should be covered to a depth of 1/2 to 3/4 inches. A seeding rate of 25 lbs per acre is recommended to insure a dense stand that will reduce weed competition. A dense stand is also required to stimulate adequate hatching of cyst nematode eggs.

Fertilization

Approximately 50 lbs N per acre is required to establish a good crop of radish or mustard. An additional 15 lbs N per acre should be applied for each ton of straw incorporated following a cereal crop to aid decomposition (up to 50 lbs N/acre). The nitrogen added to the green manure crop should be taken into account when determining nutrient requirements for the following crop.

Irrigation

Adequate soil moisture and aeration are important for both seed germination and nematode egg hatching. Following a cereal crop, the soil is usually very dry and irrigation is required immediately after the green manure crop is planted. Under gravity irrigation systems, furrows should be put into the field shortly after planting. For sprinkler irrigation systems, water can be applied immediately after planting to promote rapid germination. After the plants emerge, an additional two to three irrigations may be necessary to provide adequate plant growth and to stimulate nematode egg hatching.

Weed Control

Weeds act as alternate hosts for numerous diseases and nematodes, and thereby reduce the effectiveness of green manure crops. Weeds also provide competition and reduce growth of the green manure crop. Weed control in green manure crops can be accomplished by incorporating several integrated management practices. Pre-irrigation will help germinate volunteer grains and broadleaf weeds so that they can be controlled by tillage before planting. Establishment of a dense stand of radish or mustard will also reduce weed populations. There are several herbicides registered for broadleaf and grassy weed control in radish and mustard crops. However, plant back restrictions and carryover effects on the following crop should be carefully considered before application of any herbicide to a green manure crop. Registered herbicides for preplant or pre-emergence control of broadleaf weeds include Dacthal, Roundup, and Treflan. Poast is registered for post emergence control of grasses. Consult the label for specific use rates and restrictions.

Incorporation

The effect of timing and method of incorporation of green manure residues on pest suppression is not well understood. Therefore, current incorporation recommendations are mainly based on cultural factors. In most situations it is advisable to incorporate the radish or mustard residues while the tissue is green, and before seed formation. This will help prevent problems with volunteers in subsequent crops. In regions where Cole crop seed is produced, the prevention of seed formation is essential, and may be required by local regulations. The residues should be chopped as finely as possible to facilitate incorporation and breakdown in the soil. After chopping, the residues should be incorporated by disking or plowing. In regions where soil erosion is a concern, the residues may be left on the soil surface during vulnerable periods.

Economics

Growers that plant green manure crops in Washington and Idaho report total variable costs of $23 to $58 per acre for establishment of the crop. Seed costs usually make up the largest portion of the total cost of establishing the crop.

Table 3 gives approximate costs of production for sugarbeets with and without a green manure crop.

This example assumes that under the standard rotation, Temik would be required for control of sugarbeet cyst nematodes. Under the green manure system, Temik would not be required, but there are additional expenses for seed, tillage, fertilizer, irrigation and Poast (for control of volunteer grain in the green manure crop). The additional expenses incurred in producing these green manure crops should be weighed against the benefits of reduced pesticide use and improved crop yield and quality. However, it should be pointed out that many of these benefits are difficult to measure, and are very dependent on the management system.

Table 4 gives examples of the type of yield increases that occur in sugarbeets when following various white mustard and oilseed radish cultivars.