Weeds are unwanted plants on the farm. They pose a serious threat to the plants and can lead to poor yield. Although weeds can provide some useful functions such as covering the ground, decreasing erosion or scavenging nutrients, those weeds will eventually shed seeds and may contribute to problems in the growing crops.
Cover crops serve important functions such as enriching soil organic matter, cycling nutrients, and protecting soil from water and wind erosion, but aside from this, they can help prevent the buildup of weed populations while simultaneously providing benefits such as reductions in nitrogen leaching or increases in soil organic matter.
Cover crops help control weeds in two ways. First, they compete with weeds for available water, light, and nutrients. Second, many have allelopathic effects, producing biochemicals that influence weed emergence, growth or persistence.
Sometimes, cover crops are planted primarily to control weeds. Planted with this purpose in mind. They work best when dealing with perennial weeds but can also help starve out annuals when used over consecutive seasons. And they can be as effective (or more) on some of the worst perennial weeds.
They can be highly weed suppressive while still achieving other agronomic benefits through careful selection of mixes and the inclusion of aggressive species like certain grasses.
A vigorous, fast-growing cover crop competes strongly with weeds for space, light, nutrients, and moisture, and can thereby reduce weed growth by 80–100% for the duration of the cover crop’s life cycle.
In some cases, cover crop mixtures may be better than individual cover crops. For example, oats may be used as a nurse crop for hairy vetch planted in early fall. The oats grow more quickly in the fall, providing partial soil coverage and nutrient-trapping benefits before they are winter-killed, which prevents competition with the hairy vetch.
Choose the Right Cover Crop for the Climate and Season
Identify the weed species you want to control. It’s important to get a cover crop well established before the target weed begins to emerge.
Choose cover crops based on your objectives. If weed suppression is an objective, select an aggressive species that will cover the ground quickly. If you desire a cover crop that will protect the soil through the fall and winter and suppress winter annual weeds, plant a winter cereal in late summer or early fall.
Sow warm-season, tender cover crops like soybean, buckwheat, and millet at least six to eight weeks before the rainy season to ensure rapid growth and good biomass production.
They may establish slowly and become weedy if sown immediately after the spring frost date when the soil is still cool, but they will grow rapidly and overwhelm most weeds if sown later when the soil and weather are really warm.
Be Sure to Use High-Quality Seed
Even a day’s delay in cover crop establishment that results from weaker seed can give the weeds a chance to get a foothold. Patchy stands from poor quality seeds can defeat the purpose of cover cropping altogether.
Use current-year seed from a reputable source for cool-season cereal grains, buckwheat, and soybeans. High-quality pea, bell bean, cowpea, millet, and sorghum-sudangrass seed that is properly stored (kept cool and dry) can be used for two or three years; vetches and clovers can last up to five years. When in doubt, get new seeds.
Seeding Rate, Row Spacing, and Planting Arrangement
The seeding rate and arrangement of the cover crop can influence weed suppression. Planting at higher-than-normal seeding rates and in narrow rows can influence the amount of soil cover, particularly in the first several weeks after seeding. Thick, dense cover crop stands can help reduce the establishment of weeds.
Weed seedlings double their size in fewer days than large-seeded crops and will begin closing the gap—unless and until they are shaded out by cover crop canopy closure.
Individual cover crop seedlings should be fairly uniformly distributed across the field, and close enough together so that they will intercept most of the incident light and their roots will occupy most of the soil volume within a few weeks after planting.
It is important to provide adequate soil fertility to cover crops to ensure they are competitive and successful. This is particularly true for small grains like cereal rye and wheat, which require adequate nitrogen. Lime may be necessary to maintain or raise the soil pH for legumes like hairy vetch and red clover.
If the soil is dry and it is at all feasible to water a newly-planted cover crop, go ahead and do it. One sprinkler irrigation may be all it takes to get a drought-tolerant summer cover crop like cowpea, sunn hemp, millet, or sorghum-sudangrass established.
Most cool-season cover crops are less drought-hardy and depend on rains to thrive. Plantings of oat, field pea, and/or bell bean may be particularly difficult to establish in dry seasons; try barley, radish, or mustards instead.
Feed the Cover Crop, Not the Weeds
One important caution about fertilizing cover crops: fresh manure and other materials that rapidly release a lot of soluble nutrients can compromise the cover crop’s ability to outcompete weeds.
In soils with fairly low levels of soluble N (especially nitrate), N-fixing legume cover crops have a big advantage over most weeds and they can effectively shade them out.
Slow-release sources such as high-quality compost are more likely to support vigorous cover crops without speeding the growth of nutrient-responsive weeds. If a “hot” material like fresh manure or chicken litter is the only organic fertility source available for restoring depleted soil, follow its application immediately with a nonlegume.
In order to gain the maximum benefits from a cover crop, grow it to maturity—full height, full heading with pollen shed in grains and grasses, and full bloom in legumes and other broadleaf crops.
Allowing a cover crop to grow as long as possible before controlling it reduces weed populations through competition for light, nutrients, and moisture. In no-till, letting the cover crop achieve maximum dry matter production (often at flowering or beyond) will increase weed suppression.
Although cover crops restore nutrients in the soil, prevent erosion, and choke out weeds. There comes a point where you have to kill the cover crop.
All plants want to reproduce, and if the cover crop is allowed to seed it can build up a seed bank in the soil that could haunt you for years to come.
Therefore, you must prevent the cover crop from self-seeding by terminating it before seed or pod formation, unless a second generation of the cover crop through self-seeding is desired.
Cover crops can be terminated mechanically or with herbicides. Each method has advantages and disadvantages.
Mowing can be effective, but the mulch may degrade quite rapidly because it has been chopped.
Ploughing can be an effective physical control for cover crops, but the benefit of a weed-suppressive surface mulch in the subsequent cash crop is lost.
By understanding your cover crops you’ll be better equipped to select a herbicide—contact or translocating. Contact herbicides require thorough coverage since they only affect the part of the plant they touch.
Whereas translocating herbicides don’t require as much coverage since they move and attack the growing point of the plant. In both cases, it’s important to apply adequate rates.
Tillage can be a good alternative when herbicides aren’t an option. Since some farmers prefer not to use herbicides or Mother Nature doesn’t provide the right conditions for application, tillage is an alternative.
Tillage can destroy the cover and integrate the biomass into the soil while preparing the seedbed.