Cover Crop Categories: Brassicas

As cover crops grow in popularity, there are many questions to consider when first getting started.  Cover crops can provide many benefits including improved soil health and reduced soil and nutrient loss.  Some species are better suited for Iowa’s weather and growing season than others.  To help make species selection decisions a little easier, we will highlight several different categories of cover crops in individual blog posts in the coming weeks.

Cover crops can be categorized into three main categories: grasses, brassicas, and legumes.  This post will focus on brassica cover crops.  Brassica cover crops include mustards, kale, oilseed radish, rapeseed, turnip, and winter canola.

Many farmers use brassicas because of their large taproots which can ease soil compaction near the upper soil layers and break through plow pans better than the roots of cereal cover crops.  Some brassica species produce taproots that can penetrate up to six feet into the soil profile.  This deep rooting ability can address issues related to soil compaction, water drainage, and excessive nitrate leaching.

Oilseed radishes with long taproots

Brassicas are also useful for controlling erosion, suppressing winter weeds, scavenging nutrients, and for producing a large amount of biomass in a short period of time which adds organic matter to the soil.  If your field has excess nutrients after crop harvest, brassicas can take up between 30-50 pounds of N per acre.

Using brassica cover crops in the fall means that you don’t have to worry about terminating your cover crop in the spring with herbicide or tillage.  Brassicas are not winter-hardy and will typically die after sustained freezing temperatures.  If planting brassica cover crops in the fall, try to plant them at least 35 to 42 days before the average 28 degree frost date in your area.  This normally requires aerial or overseeding into the standing row crop in late August or early September.

Winterkilled Yellow Mustard (left) and winterkilled Oilseed Radish (right)

Brassica cover crops can be seeded in a variety of ways, including drilling, broadcast seeding, overseeding, and aerial seeding.  Brassicas have a smaller seed size which should be taken into consideration when choosing a method of planting.  Brassicas perform particularly well with aerial seeding due to the small size of the seed, their spherical shape, and their ability to germinate in cool weather.  Generally, brassica cover crops have a low seeding rate of between 5 to 10 pounds per acre, depending on the type of brassica.

In Iowa, brassicas are often mixed with other brassicas, cereal rye, or oats.  By pairing a brassica and an overwintering species, such as cereal rye, you are able to achieve the benefits of the brassica’s tap root system while also providing growing cover in the spring.  Be sure to talk with your cover crop seed provider to compare the prices of available brassica species.  Given the relatively short growing season in Iowa, many of the brassicas such as rapeseed and brown mustard will develop the same size tap root but at a lower seed cost.  For more information on cover crop mixtures, check out the cover crop species trail conducted by Practical Farmers of Iowa.

Finally, when thinking about when to plant your brassica cover crop, be aware that brassicas are sensitive to herbicides.  Pay attention to the labeling on your herbicide that you used earlier in the growing season to ensure a successful establishment of your brassica cover crop.  For more information about herbicide use and cover crop establishment, see the Iowa State University Extension publication, “Herbicide use may restrict grazing options for cover crops.”

Stay tuned for more blog posts about the other cover crop categories – grasses and legumes – in the coming weeks!

–Julie Whitson