Legumes, including alfalfa, red clover, and birdsfoot trefoil, fix nitrogen from the air and make it available to the pasture grasses. They also improve pasture nutritional quality and palatability. Legumes can easily be incorporated into a pasture stand by frost seeding. The seed is simply broadcast over the surface of the ground in early spring. Frost seeding works by allowing the freeze-thaw action of the ground during March and April to work the seed into the soil.
Timing is critical.
In most years, the seed should go out in February or early March to ensure that it will be worked in by the time the ground is warm enough for germination. The key to successfully introducing new species into a pasture stand is minimizing competition from established plants through removal of as much existing live vegetation and thatch as possible and maximizing seed to soil contact. We are almost always more successful introducing legumes into an existing sod than we are grasses. In the grass dominated plant community of most pastures, legume seedlings have the advantage of filling an open niche, while grass seedlings are competing directly with established grass plants for nutrients, water, and space. One exception is frost seeding annual ryegrass into existing pastures.