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Controlling round-leaved mallow

Some acceptable control possible.

November 20, 2007  By Bruce Barker

Round-leaved mallow is not very common on the prairies, but it can cause localized
yield losses where it is present. So if you have round-leaved mallow problems,
research at the University of Saskatchewan might help.

"One of the reasons we looked at round-leaved mallow was that plant breeders
were having trouble with it in their breeder plots. Round-leaved mallow likes
open canopies, and those plots were ideal, especially the single line plantings,"
says University of Saskatchewan research assistant Ken Sapsford, who conducted
the trials in conjunction with Rick Holm of the Crop Development Centre, with
financial support from Saskatchewan Agriculture, Food and Rural Revitalization.

Round-leaved mallow is an annual broadleaf weed introduced from Europe. It
thrives in open areas and is a prolific seed producer. When present in a cultivated
crop, it can provide strong competition. Many broadleaf weed herbicides have
little activity on round-leaved mallow.


In the first year of the two year University of Saskatchewan trial, Sapsford
looked at nine different herbicides, some with and some without round-leaved
mallow on the label, at two application timings. In the first year of trials,
Dyvel DS, Estaprop, Frontline and Trophy gave little or no control when applied
at the one to three leaf stage, and suppression (about 70 to 80 percent) at
the four to six leaf stage when rated 35 days after application. Based on the
first year of data, Sapsford selected the best products and in the second year
he had five herbicides applied at two application times.

Figure 1. Visual control of round-leaved mallow 28 to 35 days after
application. Source: Ken Sapsford, University of Saskatchewan.

In that second year, the registered herbicides used were Attain (control) and
Refine Extra (suppression). Prestige, Frontline 2,4-D, and Spectrum were also
used in the second year of trials, although round-leaved mallow is not on their
labels. Refine Extra was applied in a tank-mix with 2,4-D. All herbicides were
applied at the one to three leaf stage and the three to six leaf stage. The
trials were on wheat, and visual weed control ratings were conducted seven and
28 to 35 days after application. Seed yield of wheat and round-leaved mallow
was measured. Herbicides were applied at recommended label rates.

Choosing the best
Sapsford says that based on the two years of data, Frontline 2,4-D, Spectrum
and Refine Extra+2,4-D tank-mix applied at the one to three leaf stage, and
Attain applied at the three to six leaf stage, provided commercially acceptable
control (better than 80 percent). Attain at the one to three leaf stage had
inconsistent control. Prestige, a pre-mix of Attain and Lontrel, provided marginally
acceptable control.

"In the first year, Attain applied at the early application was not as
good as the late application. We think the reason was a second flush of weeds
after the herbicide application. Since Attain does not have any residual, its
control suffered," explains Sapsford.

Frontline 2,4-D, Refine Extra and Spectrum have short-term residual and provided
control of the second flushes. Sapsford says that these products also gave excellent
control at the three to six leaf stage, which provides greater flexibility in
application timing – essentially providing an application window from the
one to six leaf stage of round-leaved mallow.

Figure 2. Wheat yield. Source: Ken Sapsford, University of Saskatchewan.

Looking at yield in the first year, all treatments increased wheat yield compared
to the check except for Attain at the one to three leaf stage. In year two,
Attain at the one to three leaf stage had significantly lower wheat yield than
the check.

While Frontline 2,4-D and Spectrum are not registered for control, and Refine
Extra is rated as suppression, Sapsford does not expect to see round-leaved
mallow on the labels anytime soon. The weed is such a minor problem that it
is not worth the effort for herbicide companies to get it on the label. He says
that his two years of research does not have enough data points to apply for
a Minor Use Permit.

"What we wanted to do was see if there were herbicides available that
would control round-leaved mallow when spraying for other broadleaf weeds,"
explains Sapsford. "While these products may not be registered for round-leaved
mallow control, what we have seen from our research is that they can provide
commercially acceptable control." -30-


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