How brassica species can aid Pakistani agriculture?

By FAREEHA ZAFAR/FAROOQ AHMAD KHAN

An in depth look has shown that most brassica species release chemical compounds that may be toxic to soil borne pathogens and pests, such as nematodes, fungi and some weeds. However, they are attracting renewed interest primarily because of their pest management characteristics.

Brassica and mustard cover crops are known for their rapid fall growth, great biomass production, and nutrient scavenging ability.

Most brassica species release chemical compounds that may be toxic to soil borne pathogens and pests, such as nematodes, fungi and some weeds. Cover crops offer many benefits for farmers seeking to reduce their reliance on external inputs. These include maintaining and improving soil quality, preventing erosion and, in some cases, allopathic weed control. Because of their potential contribution to pest management, there is increased interest in growing brassicas, both as cover crops and as seed crops harvested for oil production.

The mustards usually have higher concentrations of these chemicals. Brassicas are increasingly used as winter or rotational cover crops in vegetable and specialty crop production, such as potatoes and tree fruits. There is also growing interest in their use in row crop production, primarily for nutrient capture, nematode trapping, and bio toxic or bio fumigation activity. Some brassicas have a large taproot that can break through plow pans better than the fibrous roots of cereal cover crops or the mustards. Those brassicas that winter-kill decompose very quickly and leave a seedbed that is mellow and easy to plant in.

With a number of different species to consider, you will likely find one or more that can fit your farming system. Don’t expect brassicas to eliminate your pest problems, however. They are a good tool and an excellent rotation crop, but pest management results are inconsistent. More research is needed to further clarify the variables affecting the release and toxicity of the chemical compounds involved.

How can brassicas help?

Brassicas can provide greater than 80 per cent soil coverage when used as a winter cover crop. Depending on location, planting date and soil fertility, they produce up to 8,000 lb. biomass/A. Because of their fast fall growth, brassicas are well-suited to capture soil nitrogen (N) remaining after crop harvest. The amount of nitrogen captured is mainly related to biomass accumulation and the amount of N available in the soil profile. To maximize biomass production and nutrient scavenging in the fall, brassicas must be planted earlier than winter cereal cover crops in most regions, making them more difficult to fit into grain production rotations.

Pest management

Pest suppression is believed to be the result of glucosinolate degradation into biologically active sulfur containing compounds called thiocyanates. To maximize pest suppression, incorporation should occur during vulnerable life-stages of the pest. It varies depending on species, planting date, growth stage when killed, climate, and tillage system. Be sure to consult local expertise for best results.

The use of brassicas for pest management is in its infancy. Results are inconsistent from year to year and in different geographic regions. Different species and varieties contain different amounts of bioactive chemicals. Be sure to consult local expertise and begin with small test plots on your farm.

Disease management

Study of brassica green manures in potato cropping systems compared winter rape (Brassica napus) and white mustard (Sinapis alba) to no green manure, with and without herbicides and fungicides. The winter rape system had a greater proportion of Rhizoctonia-free tubers (64 per cent) than the white mustard (27 per cent) and no green manure (28 per cent) treatments in the non-fumigated plots. There was less Verticillium wilt incidence with winter rape incorporation (7 per cent) than with white mustard (21 per cent) or no green manure incorporation (22 per cent) in non-fumigated plots.

Nematode management

Rapeseed, arugula, and mustard were studied as alternatives to fumigation. The brassica cover crops are usually planted in late summer (August) or early fall and incorporated in spring before planting mustard.

Several brassicas are hosts for plant parasitic nematodes and can be used as trap crops followed by an application of a synthetic nematicide. Washington State University nematologist Ekaterini Riga has been planting arugula in the end of August and incorporating it in the end of October.

Nematicides are applied two weeks after incorporation, at either a reduced rate using Telone or the full rate of Mocap and Temik. Two years of field trials have shown that arugula in combination with synthetic nematicides reduced M. chitwoodi to economic thresholds.

Longer crop rotations that include mustards and non-host crops are also effective for nematode management. For example, a three-year rotation of potatoes>corn>wheat provides nearly complete control of the northern root-knot nematode (Meloidogyne hapla) compared to methyl bromide and other broad-spectrum nematicides.

Weed management

Like most green manures, brassica cover crops suppress weeds in the fall with their rapid growth and canopy closure. In spring, brassica residues can inhibit small seeded annual weeds such as pigweed, shepherds purse, green foxtail, kochia, hairy nightshade, puncturevine, longspine sandbur, and barnyard grass (Munoz and Graves), although pigweed was not inhibited by yellow mustard. The density of 16 weed and crop species was reduced 23 per cent to 34 per cent following incorporation of brassica green manures, and weed establishment was delayed by two days, compared to a fallow treatment. However, other short-season green manure crops including oat, crimson clover, and buckwheat similarly affected establishment.

Early season weed suppression obtained with brassica cover crops must be supplemented with cultivation to avoid crop yield losses from weed competition later in the season. As a component of integrated weed management, using brassica cover crops in vegetable rotations could improve weed control.

Soil structure management

Most mustard have a fibrous root system, and rooting effects are similar to small grain cover crops in that they do not root so deeply but develop a large root mass more confined to the soil surface profile. Some brassicas  like forage radish, rapeseed, turnip produce large taproots that can penetrate up to 6 feet to alleviate soil compaction this so-called “bio drilling” is most effective when the plants are growing at a time of year when the soil is moist and easier to penetrate. Their deep rooting also allows these crops to scavenge nutrients from deep in the soil profile. So, as the large tap roots decompose, they leave channels open to the surface that increase water infiltration and improve the subsequent growth and soil penetration of crop roots.