By Lee Stivers, Frances Tucker and Claudia Olivier
Integrating cover crops into vegetable production systems is a good idea. Cover crops add organic matter to our often carbon-starved vegetable soils, improve tilth in soils damaged by frequent tillage or heavy machinery; recycle plant nutrients; and in some cases suppress weeds and other pests.
Vegetable growers in New York have long used cover crops in their rotations, but are limited by the short growing season. Typical cover crops include wheat, rye, oats, barley, red clover and ryegrass. More recently, growers have also begun to use sudangrass (or sorghum-sudangrass hybrids) and buckwheat as summer cover crops, and hairy vetch as an overwintering leguminous cover crop.
Since 1993, we have evaluated a number of brassica species for their ability to grow rapidly, accumulate nitrogen, and suppress weeds and soil-borne diseases. These brassicas were planted in the late summer or early fall and were either left to winter-kill or were incorporated into the soil in early November before plants were damaged by the cold. This report summarizes the results of eight replicated on-farm trials.
Brassica Cover Crop Growth
Brassica cover crops grew rapidly during the cool fall period, generally reaching their peak biomass by approximately 75 days, depending on soil moisture. Most of the cover crops we evaluated accumulated between 1 and 2 T/acre aboveground biomass (dry weight) before being winterkilled (Table 1). However, in some years and locations, growth was less than 1 T/acre. Brassica juncea and B. napus plantings failed in three out of four trials; two due to extremely dry conditions of 1995, and one due to poor seedbed preparation and/or herbicide carryover.
Oilseed radish, white mustard, and yellow mustard typically were winterkilled by late December. Plants were still in a vegetative stage so no seeds were set. These cover crops left a dead residue mulch on the surface of the soil which proved very easy to incorporate with spring tillage. However, 50 to 75% of the biomass that accumulated through the fall was lost by the following April. This plant matter may have been fragmented and worked into the top surface of the soil, decomposed, or blown away by wind. No regrowth or volunteer plants were observed following tillage.
Forage kale, forage turnip, and “Sparta” canola were not completely winterkilled and were beginning to grow again by April in plots that were not fall-tilled. The forage turnips had already begun to form fleshy “roots” by late fall, contributing greatly to their biomass accumulation. Canola growth in the fall was better than that of forage kale. These cover crops may be very useful in situations where spring regrowth is desired. In these trials, no regrowth or volunteer plants were observed following tillage.
Brassica juncea and B. napus varieties were grown in plots that were disked in early November to incorporate the green plant material into the soil in order to test their abilities to suppress soil-borne diseases. Establishing good stands with these varieties proved more difficult than with the other brassicas tested. Sufficient soil moisture and a well prepared seedbed seem to be critical for successful establishment and growth of these species. In two trials, significant spring regrowth was observed in some varieties, even following fall incorporation at the vegetative stage. This suggests that, especially for the B. junceas, seeds may germinate over an extended period, which could result in serious weed problems.
Brassica cover crops are reported to be good scavengers of available soil Nitrogen. Having a cover crop taking up any potentially leachable N during the fall may be a way of recycling this important plant nutrient and minimizing environmental damage. Oilseed radish, white senf mustard, yellow mustard, kale and turnip were evaluated for their abilities to take up available N and release it for the following crops.
Brassica cover crops, which reached 1.5 to 2 T/acre aboveground biomass, generally took up around 100 lbs/acre of potentially leachable Nitrogen. Available N concentrations in the soil in cover cropped plots were usually lower than in bare controls, where N was likely leaching out of the soil profile. This provides further evidence that these cover crops were depleting leachable soil Nitrogen. However, as with the biomass residue, much of the N in the aboveground biomass could not be accounted for by the following spring. In April, available soil N tended to be higher in cover crop plots than in bare plots, so some of the N may have gone back into the soil through decomposition and water movement. Further research is needed for a complete understanding of N retention and losses associated with this practice.
Weed and Disease Suppression
Brassicas grown in the fall and left to be winterkilled were extremely successful in suppressing winter weeds, such as henbit and chickweed, especially when planted in late August. This was probably the result of both plant competition in the fall and light interception by the residue in spring. While “bare” plots were usually carpeted with these species by April, no measurable weed growth was found in several of the cover crop treatments.
When B. juncea and B. napus varieties were planted in late August and incorporated green in early November, they significantly decreased the severity of bean root rots in snap beans grown the following season. Brassica junceas, which contain high concentrations of the anti-fungal compound AITC, decreased bean root rot severity from 55 percent in controls to 33 percent. Brassica napus varieties, which don’t contain AITC, still reduced severity to 48 percent. While the effects of certain brassica species on some soil-borne pathogens look promising, these cover crops may potentially increase other pathogens, such as white mold or nematodes. Again, more research is needed to fully understand how these cover crops affect plant diseases.
Oilseed radish and yellow mustard are good choices for growers interested in trying brassica cover crops. These varieties are well developed, and seeds are available commercially for a reasonable cost. Yellow mustard is almost indistinguishable from white senf mustard, and seed is much more readily available. If spring regrowth is desired, forage turnip or “Spana” canola would be good choices. Brassica juncea varieties with high AITC concentrations are not yet available commercially. Breeding work on these varieties needs to be done to overcome the problems of seed dormancy and poor stand establishment.
About the authors: Stivers, Tucker and Olivier are, respectively: Cooperative Extension Educator and Aide, Cornell Cooperative Extension Lake Plains Vegetable Program, and Graduate Student, Cornell Univ. Dept. of Plant Pathology. Their article was originally published in the Proceedings of the 1998 N.Y. State Vegetable Conference.
|Table 1. Aboveground dry matter accumulation by nine brassica cover crops grown in a series of on-farm replicated trials in Genesee and Orleans Counties, N.Y., from 1993 to 1996. Seeding rates were 8 to 14 lbs/acre; seeds were broadcast by hand or hand-held spinner and raked in lightly.
Maximum Fall Growth
|– 1993 –
|– 1994 –
|– 1996 –
|Variety or type
Oilseed radish Raphanus sativus “Adaggio” 1.9 1.0 – – – – – –
Oilseed radish Raphanus sativus open pollinated – – 2.2 1.5 1.1 0.9 0.9 0.5
White senf mustard Brassica hirta ‘Martigena” 1.9 1.2 2.4 1.7 – – – –
Yellow mustard Brassica hirta open pollinated – – 2.3 1.6 1.4 1.0 0.3 0.6
Forage kale Brassica oleracea open pollinated – – 1.1 0.6 – – – –
Forage turnip Brassica rapa open pollinated – – 2.3 2.2 – – – –
Brown or Indian mustard Brasica juncea “Forge’ – – – – 1.2 1.1 1.0 0.9
Brown or Indian mustard Brassica juncea “Cutlass” – – – – – – 1.3 –
Canola Brassica rapus “Essex” – – – – – – 0.7 –
Canola Brassica rapus “Sparta” – – 2.2 1.5 – – 1.2 –