Cover Crops – can be so much more!

Anne Verhallen, Soil Management Specialist – Horticulture, OMAF & MRA – Ridgetown

From ONvegetables in The Grower, August 2013

Cover crops are a must for vegetable soils; holding on to soil to prevent blowing in the spring, filtering and anchoring soils during heavy rainstorms and helping to stabilize field headlands and harvest driveways. But there is more, much more that cover crops can do in a vegetable production system.

Rotation benefits – achieving a good crop rotation can be challenging with high value vegetable land and limited land base. Select cover crops from plant families unrelated to your crops to ensure that you get the most rotation benefit. Getting a different root system and different crop residues in the system will encourage a wider variety of soil organisms and in particular, support beneficial organisms. Rotation research in field crops suggests a yield increase of approximately 10 per cent can be achieved with a well-planned and varied rotation.

Cover Crops Plant Family Root System/role
Oats, rye, wheat, pearl millet, annual ryegrass Grass Fibrous root system- Excellent soil structure builder
Clover, alfalfa, peas, vetch Legume Tap root system, some are more fibrous- Fixes nitrogen from the air
Oilseed Radish, rape, canola Brassica – related to other members of the cabbage family Tap root system- Fast ground cover and high nitrogen need
Buckwheat, phacelia Various Weak tap root and fibrous root systems

Suppress Weeds – Cover crops can help to suppress weeds at various times through the production year. An early spring cover crop prior to a later warm season crop can shade and delay weeds.  Similarly, planting a cover crop after an early harvested crop, like peas, can significantly reduce the weed biomass in the field and the potential seed production. Cover crops can suppress or reduce weeds in a variety of ways, most depend either upon allelopathy or basic shading/competition.

Sorghum top growth
Sorghum top growth

Cover crops in the sorghum family such as sorghum sudan have been studied extensively for their allelopathic control of weeds. Sorghum sudan root exudates contain an allelopathic compound called sorgoleone that has been shown to provide significant weed suppression with common weeds such as nightshade, pigweed and common ragweed (Czarnota et al 2001). Cereal rye is another cover crop with known allelopathic effects. Often the compounds that provide the weed suppression are very short lived and are only as consistent as the plant stand. With rye, the main allelopathic compound varies with rye variety and the maturity of the rye, declining as the rye matures (Regerg-Horton et all, 2005).

Cover crops like buckwheat and those from the Brassica family – things like oilseed and daikon radish, rape and canola, can cover and shade the soil quickly, reducing early weed seed germination. However, avoid these cover crops if you produce crops from the same family like broccoli and some Asian vegetables, in order to prevent disease. Warm season grasses like pearl millet and sorghum sudan also do an excellent job of growing rapidly and shading the soil under good conditions. Sorghum sudan managed with successive cuts has been shown to reduce the stand of Canada thistle through competition.

Reduce nematode populations – nematodes are a serious pest of many vegetable crops. However, it is important to test for and know the species of nematodes present in your fields. The species will help to determine cover crop choices. For example, the common nematode pest for most vegetable crops is the root lesion nematode. Almost any crop or cover crop is identified as an alternate host for that particular nematode. Read cover crop nematode claims with care – you need to know which nematode. For example Sunn hemp – a tropical legume is used in Florida as a nematode control cover crop for many vegetable crops. However, the target nematode is the root knot nematode.

Pearl millet top growth
Pearl millet top growth

Some cover crops like pearl millet and some varieties of sorghum sudan have been identified as non-hosts for nematodes, but this tends to be variety specific.

Another approach to controlling or suppressing nematodes is through the use of biofumigant cover crops like oriental mustard. Many of the brassica cover crops contain large amounts of glucosinilates – compounds that breakdown to materials very similar to chemical fumigation products. Brassicas are capable of growing large amounts of green material in a short time, if managed well. However, most of the brassica cover crops are also alternate hosts for root lesion nematode. Using mustard and other brassicas as biofumigants requires that the cover crop be chopped finely to release the compounds, worked consistently into the soil and the soil sealed through tillage and packing or irrigation.

Improve soil structure and drainage – cover crop root systems can help to break up mild tillage pans. For severe compaction a longer term rotational crop really should be grown to get root systems through the compacted zone.  Choose a fibrous root system like those found in spring cereals like oats, to glue soil particles together and build better soil structure.

Soil structure target Cover crop best suited
Better soil structure near the soil surface Choose a fibrous root system – the fine roots will help to bind soil particles together- Spring and winter cereals like oats, rye- Buckwheat or the warm season grasses like sorghum sudan
Mild tillage pans or compaction Choose a cover crop with a tap root such as cover crop radish or an aggressive fibrous root system such as sorghum sudan
Deeper compaction from rutting or harvest operations This is beyond using just a cover crop. Options include:- Rotate out of production for several years using sweet clover or alfalfa to break up the compacted layer and improve drainage- Or a combination of targeted deep tillage (done under appropriate soil conditions) combined with a cover crop program using both deep rooted and fibrous rooted cover crops such as cover crop radish and rye or oats and allowed to grow for 6 to 8 weeks minimum

Refer to the Midwest Cover Crop Council website for the Ontario Cover Crop Decision Tool for more information and to compare cover crops. http://mcccdev.anr.msu.edu/VertIndex.php

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