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United States Department of Agriculture

Agricultural Research Service

Title: Take Cover from the Elements-Brassica Cover Crops Can Control Weeds and Reduce the Use of Crop Protectants in Vegetable Rotations

Author
item Boydston, Rick

Submitted to: American Vegetable Grower
Publication Type: Popular Publication
Publication Acceptance Date: February 1, 2004
Publication Date: March 1, 2004
Citation: BOYDSTON, R.A. TAKE COVER FROM THE ELEMENTS-BRASSICA COVER CROPS CAN CONTROL WEEDS AND REDUCE THE USE OF CROP PROTECTANTS IN VEGETABLE ROTATIONS. AMERICAN VEGETABLE GROWER. MARCH. 2004. p. 18-19.

Interpretive Summary: The use of fall-planted Brassica cover crops has increased recently as a result of multiple benefits to growers such as nematode, disease, and weed suppression, soil conservation, nutrient cycling, and increasing soil quality (soil water infiltration and soil aggregate stability). Popular Brassica cover crops include white mustard, oriental mustard, and oilseed radish. Brassicas are normally seeded at a rate of 6 to 10 lbs seed per acre. Fall-planted brassicas are able to produce cover quickly and smother weeds in comparison to many other cover crops. Brassica tissues contain glucosinolates, which upon decomposition in the soil, are converted to numerous compounds. Some of these compounds have broad biocidal activity affecting nematodes, insects, fungi, and weeds. The suppression of soil-borne pests and pathogens by release of biocidal compounds has been termed biofumigation. In general, green fresh residue incorporated into the soil will release greater amounts of suppressive compounds than dried, mature residues. Decaying root tissue may also play an important role in the biocidal activity of Brassica green manures. Mainly small seeded annual weeds such as, pigweed, shepherdspurse, green foxtail, kochia, hairy nightshade, puncturevine, longspine sandbur, and barnyardgrass are inhibited by incorporating Brassica residues in soil. Fall-planted Brassica cover crops fit well into vegetable cropping systems following early harvested crops. White mustard and brown mustard have become popular fall-planted cover crops in the potato producing regions of the Columbia Basin of eastern Washington. Planted in mid to late August, white mustard emerges quickly and produces a large amount of biomass before succumbing to freezing temperatures. As a component of integrated weed management, utilizing Brassica cover crops in vegetable rotations could improve weed control and reduce reliance on herbicides.

Technical Abstract: Brassica cover crops offer numerous benefits to growers such as nematode, disease, and weed suppression, soil conservation, nutrient cycling, and increasing soil quality (soil water infiltration and soil aggregate stability). White mustard (Brassica hirta, syn. Sinapis alba), brown mustard (Brassica juncea), oilseed radish (Raphanus sativus), and rapeseed (Brassica napus), have been utilized as cover crops with varying success. Rapid emerging, vigorous growing cover crops shade the soil, reducing light intensity and creating a cooler environment. In addition, members of the Brassicaceae have frequently been cited as potentially allelopathic. Low levels of isothiocyanates (ITCs) released from root tissues may play a role in weed suppression during Brassica cover crop establishment. Early season weed suppression in the main crop is often noted when following a Brassica cover crop. Brassica tissues contain glucosinolates, which upon decomposition in the soil, are converted to numerous compounds including isothiocyanates (ITCs). Over 100 distinct glucosinolates have been identified from brassicas and other plant families. Glucosinolate types and concentrations vary within and between Brassica species and in different plant tissues. The suppression of soil-borne pests and pathogens by release of biocidal compounds, such as ITCs from glucosinolate-containing tissues, has been termed biofumigation. ITCs inhibit seed germination and seedling growth to varying degrees and the ability to inhibit germination may be related to volatility. Many small seeded annual weeds such as, pigweed, shepherdspurse, green foxtail, kochia, hairy nightshade, puncturevine, longspine sandbur, and barnyardgrass are inhibited by incorporating Brassica residues in soil. Glucosinolate levels in brassicas can also be increased by mechanical wounding or attack by pathogens. Manipulation of some of these factors could result in greater efficacy of Brassica cover crops on weeds and other pests. Biomass production is influenced by species, cultivar, planting date, planting rate, fertility, winter conditions, and growing region. In most cases early season weed suppression obtained with Brassica cover crops must be supplemented with herbicides or cultivation to avoid crop losses from weed competition. As a component of integrated weed management, utilizing Brassica cover crops in vegetable rotations could improve weed control and reduce reliance on herbicides.

Last Modified: 12/21/2014
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