Cover Crops: Frost Termination

Frost termination can be a viable (and cost-free) method of cover crop termination provided it does not interfere with cash crop production during the primary growing season and sufficient soil moisture is available following harvest. Ideally, a cover crop planted for frost termination in the Fall would be able to produce significant biomass growth and it would be killed by below freezing temperatures prior to seed set. Due to these requirements, frost termination of cover crops works best in continental (inland) climates between 35 and 50° N. However, in locales with the potential for mild winters, a backup plan for termination should be in place in the event of the cover crop surviving the winter.

Frost-terminated cover crops would be viable following an over-wintered small grain harvested in late June or early July, or after a pulse crop harvested between August and mid-September. If a cash crop is harvested in mid-summer, warm season annuals such as millet, sorghum, buckwheat, and sunflowers will produce a significant amount of biomass ahead of the cooler temperatures and shorter days leading up to the first frost. If a cash crop is harvested in August or September, cool season species like mustard, daikon radish, oats, and white clover would be more suitable.

Overwintered or spring planted cover crops should produce a significant amount of biomass prior to termination. In cooler climates or years with cooler springs, there may be very little cover crop growth until mid-May and this growth delay can have significant impacts on cash crop planting dates. This is not an issue with frost terminated cover crops because they produce biomass during the late summer and early Fall and in turn, this dead cover crop biomass can be direct seeded into the following spring during optimal planting windows. Therefore, frost-terminated cover crops might be a viable option in drier and colder commodity producing areas of Canada and the U.S.

Some dryland/livestock mixed farm operations on the U.S. central High Plains have had good success with planting warm-season forage mixes in late Spring, grazing at various points during the summer, relying on frost to terminate the warm-season forage mix in the Fall, and then direct seeding a cash crop the subsequent spring. The only disadvantage to this use of frost-terminated cover crops is that one season of cash cropping is missed. However, the residue and added fertility from ruminants can contribute to higher yields and lower synthetic fertilizer requirements in cash crops the following growing season.

The major issue with the frost termination method is that it violates Principle 4 of soil health practices (Continual Live Plant/Root). This means that no biomass or root exudates will be generated between cover crop termination in the Fall and cash crop planting the following Spring. However, the diversity added to the cropping systems, potential for grazing, heavy residue, and the flexibility in cash crop planting dates in the following Spring will override any disadvantage to not having live continual live plant/roots in place during Winter and early Spring.

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