Sweet Clover Seed Production

The Plant

Generally, the cultivated forms of sweetclover are biennial. The two principal types are:

• White Sweetclover (Melilotus alba). The recommended Manitoba variety is Polara, a low coumarin type.
• Yellow Sweetclover (Melilotus officinalis). Recommended Manitoba varieties include Norgold, a low coumarin type, and Yukon, a "Madrid" type. Yellow sweetclover is earlier, finer-stemmed, usually less productive for forage and more dependable for seed than white sweetclover.

In the seedling year the plant consists of a single much-branched stem and tap root, and as the season progresses a crown is added. The top growth reaches maximum development in late summer after which root size increases throughout the fall period. Crown buds become large and conspicuous as the roots increase become large and conspicuous as the roots increase in size. Paralleling the increase in root weight is an increase in food reserves. The crown buds and the food reserves determine the amount of growth in the second year.

Storage of food reserves in the root and crown buds is dependent on photosynthetic activity in the leaves of the single stem. Thus, any reduction in the amount of top growth will interfere with food storage. Because new growth in the seedling year must come from stem (axillary) buds, sweetclover must be managed during the seedling year so that the single main stem is not clipped too low. In thick stands or under companion crops, many of the lower stem buds may not be viable. This leaves few buds capable of initiating regrowth.

In the second year, crown buds develop into rapidly growing stems. Food reserves in the roots are used by the developing stems and are not replaced. Thus second year growth consists mainly of top growth.

Site Selection

Sweetclover is an aggressive plant adapted to a wide range of soil and climatic conditions. It requires moisture for establishment but is moderately drought-tolerant and winter-hardy.

Sweetclover is most productive on fertile, well-drained clay and clay loam soils. However, it can be successfully grown on saline soils. Sweetclover should not be sown on acid soils or soils subject to flooding.

When producing pedigreed seed, isolate fields as required. Refer to the current Canadian Seed Growers' Association Circular 6, "Regulations and Procedures for Pedigreed Seed Crop Production" for complete information. 

Site Preparation

Sweetclover is susceptible to most herbicides used to control broad-leaved weeds. The competition from weed growth should be minimized by cultural practices before the crop is seeded.

Seeding

Seed should be scarified before planting, a process which removes the hull and scratches the hard coating on the seed. This is normally done by seed companies before sale. Prior to seeding, inoculate with Rhizobium melilotii to reduce the need for nitrogen fertilization of the stand. The normal seeding rate is approximately six lbs. per acre (seven kg per hectare) for close-drilled stands. A stand of one or two plants per square foot (19 to 20 plants per square metre) is considered to be optimal for seed production. The depth of seeding should be less than one inch (2.5 cm). Sweetclover should be seeded as early as possible in the spring to take advantage of favourable moisture conditions. The crop may be seeded with or without a sweetclover is seeded alone. If a companion crop is used it should be seeded first at one-half to two-thirds of the normal rate, followed by cross-seeding of the sweetclover.

For pedigreed seed production the crop may be planted in spaced rows to permit roguing of weeds and off-type plants.

Fertilizers

Sweetclover is a legume, and if seeds are properly inoculated nitrogen fixation should supply the nitrogen requirements of the plant. It may be necessary to apply phosphorus, potassium or sulfur. For specific recommendations fields should be soil tested.

Pollination and Seed Set

Honey bees are essential for pollination and seed set. One to two colonies per acre (three to five colonies per hectare) are considered adequate for good seed production. Greater concentrations of bees have been shown to increase seed set. Sweetclover is also attractive to other pollinating insects including leafcutter bees.

Harvesting

Sweetclover normally sets an abundance of seed. However, the somewhat indeterminate habit of growth and the loose attachment of the mature pods on the rachis (stem), result in heavy loss of ripe pods before and during harvest.

Highest yields of good quality seed are obtained by windrowing the crop when 50 to 60 percent of the pods have turned brown, black or white. Cutting should be done when the plants are tough damp from dew or rain. After a brief period of curing (several days to a week), the windrow is picked up and threshed. Use a slow cylinder speed and wide clearance of concaves to avoid shelled or broken seed.

The standing crop is not usually combined directly. Serious shattering accompanies the advanced stages of maturity necessary for such harvest.

Diseases

Sweetclover may be attacked by a number of diseases including damping-off, root rot and crown rot, stem rots and leaf diseases. The incidence of these diseases is usually light on forage stands and slightly heavier on seed stands. While these diseases damage the plant and negatively influence productivity, they are not normally considered a serious problem.

Insect Pests

The sweetclover weevil is this crop's main pest. The weevil chews the leaves of seedlings or second-year stands in the spring and, to a lesser extent, in late summer. Damage is likely to be most severe in years when growth is slow. Seedling stands may be completely destroyed, but because plants in second-year stands are growing more vigorously, damage is usually minimal. The weevil can be controlled through the use of insecticides, by tilling of second-year fields as soon as harvested, and by locating new stands as far as possible from established fields of sweetclover.

Damage to seedling stands may also occur from cutworms and grasshoppers. Cutworm damage is indicated when seedlings wilt and die because they have been cut off at or below ground level, or when the entire above ground portion of the plant is eaten. Grasshopper damage is severe when infestations are heavy. Both insects may be controlled by cultural methods and/or the use of insecticides (see Agriculture Canada Publication 1435).