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Mustard: more than just a condiment

Using mustard to control weeds, nematodes and other pests may be an alternative to chemicals.

November 20, 2007  By Rosalie I. Tennison

Substantial research is being done on a plant that most of us think of as the
forebear of what we put on hot dogs. Nevertheless, a team of Washington researchers
has learned much about using mustard in numerous applications and believes that
further research will enable fine-tuning of the ideas to give growers some organic
options for pest control.

In fact, mustard may be an alternative control for many problems pestering
growers. The research being conducted in Washington State shows that using some
varieties of mustard as green manure will suppress weed seeds and some soilborne
pests. When used as green manure, the decomposition of the plant material has
shown efficacy on reducing germination of some weed species. In related research,
some mustards are more effective than others when used as a 'bio-fumigant'.
However, in other research the use of mustard to reduce nematodes, particularly
in potatoes, has proven less effective than other plant variety options.

"We see some weed suppression when mustard is planted as a fall cover
crop, cut when still green and disced into the soil," reports Dr. Rick
Boydston, a weed scientist with the USDA's Agricultural Research Service. "We
also see weed reduction with other fall-planted cover crops if the weeds that
germinate with the cover crop are not allowed to produce new seed in the fall."
As the leader of a group of researchers from several departments, Boydston says,
by working together, they have learned a lot about how mustard works and how
it might be used. But, there is still much to understand.


"We see that small seeded weeds are the most sensitive to this process,"
adds Boydston. "Sometimes we just see a delay in emergence." What
the scientists do know is that part of the bio-fumigant effect is attributable
to isothiocyanates, the chemical by-products that result from the decomposing
plant material. These by-products have proven toxic to adjacent organisms. Sometimes,
according to Boydston, farmers do not get the mustard incorporated in the fall
and have to do the operation in spring. If the mustard has not been winter-killed
and is still green, they have to wait two or three weeks before spring planting
because the mustard could have an adverse affect on the crop.

The researchers see the use of bio-fumigants as possible alternatives in crops
that offer high returns because the cost to plant a crop that will be plowed
into the ground has to be balanced by the value that crop can provide. Boydston
says potato growers have seen the highest value in using mustards because of
the nematode and weed seed suppression, and the added benefit of the nitrogen
that may be trapped in the soil courtesy of the green manure.

However, another researcher, who specializes in nematode research in all crops,
says mustards have proven less effective for nematode suppression than some
other plants she has examined. Dr. Ekaterini Riga says, while mustard did offer
some nematode control in potatoes, it was not enough, when used alone, to meet
the high standards of nematode tolerance in Washington. A former researcher
for Agriculture and Agri-Food Canada in Ontario, Riga says using mustard for
green manure and nematode suppression may work well in Ontario because the industry
has different standards for allowable nematodes in the soil. The drawback for
using mustard as green manure anywhere in Canada, she believes, is the shorter
growing season that would not allow a farmer to harvest one crop, plant mustard,
allow it to mature and then plow it down, all before winter sets in. "I
have tried a new cold-tolerant mustard variety that may do really well in Canada
for this purpose," she says. "It could be incorporated as late as

Andy McGuire, an agricultural representative with USDA in Washington, says
he has been evaluating mustard varieties to find those that are most effective.
"We have seen some inconsistency in the weed control," he says. "But,
we did note some verticillium wilt control in potatoes when using the mustard.
Unfortunately, the mustard cover crop is not cheap to grow so it has to be used
on a high return crop. Nevertheless, some potato growers have been able to use
mustard as a replacement for other fumigants and that was a cost saving."

Much more to learn
Boydston says there is still a great deal to learn about using mustard and,
because it is expensive, the team is considering other crop species that might
offer the same pest suppression at less cost. "We are looking at sudangrass
and wheat vetch in addition to mustard as a cover crop," he says. Because
mustard is expensive, he adds, the team is also examining it as a cash 'biofuel'
crop rather than a cover crop. They do not know yet if it will have the same
suppressive effects when grown as a crop rather than being used as green manure.

Then there are the combinations of varieties that McGuire has been considering.
He learned that a mixture of Oriental and white mustard varieties were the most
effective. "Many of the varieties we are growing were developed in Italy,"
he says. "They are varieties that were developed specifically for biomass

Riga, while not giving up on mustard entirely, found that arugula offers similar
nematode suppression as mustard but, because it is a cash crop, it could pay
for itself. "It has some economic value," she says. "Growers
could harvest a crop of arugula then plow down the second growth. In areas where
the growing season is shorter, arugula may be a better option than mustard to
get the same result."

Boydston agrees that not all mustards overwinter well and, if they cannot be
plowed into the soil in the fall, growers would not likely get the same weed
or nematode suppression. In addition, he sees some issues with cross-pollination,
particularly with canola, if mustard is used as a cover crop for weed suppression.
"You don't want to create more problems with cross-pollination, so growers
need to be careful how to work mustard into a crop rotation." In addition,
some mustards may create weed problems in rotation crops.

Another area of interest to the researchers is the use of mustard seed meal
for weed suppression, but the cost to haul and spread the meal is prohibitive.
Boydston says seed meal might make sense for high value crops or greenhouses,
but not for lower value field crops.

Despite working with mustards for weed, nematode and pest control for about
four years, the researchers agree there is still more to learn. While the principle
that mustards offer value as bio-fumigants is sound, the economic value, most
effective varieties and best methods for use still need to be determined. In
addition, Boydston's and Riga's research show mustard may only be the initial
approach, other plant varieties may prove to have the same or better effectiveness,
but more economic value. For example, Riga is also considering a variety of
radish for nematode suppression.

If anything, the last four years of research with mustard have raised as many
questions as it has answered, but the members of the team believe they have
made progress. What needs to be done now is to formulate definitive recommendations
for using mustard as a weed and nematode suppressant and bio-fumigant, if that
is possible. -30-



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