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Hit dandelions in the fall

September applications work best, but spring applications may contribute to long-term control.


November 26, 2007
By Bruce Barker

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Recently, dandelions have moved up in the rankings in weed surveys across the
prairies. Some research correlates the move to reduced tillage as a reason that
dandelion has increased in abundance, while other research finds no correlation.
Regardless of the reason, dandelion control requires the correct timing and
multiple hits of herbicides.

 6a
Late September (left) treatments provided better control than late
October (right).

As a perennial weed with a deep taproot, dandelion control must focus on killing
the growing point and root. Very small segments along the length of the taproot
can form new shoot and root tissue, making control difficult, even with tillage.
And because dandelion starts growth early in the spring, in-crop weed control
can be difficult.

Recently, several research projects have refined dandelion control recommendations.
Ken Sapsford, a research assistant at the University of Saskatchewan, has analyzed
the results of three studies that he was involved in which covered 96 treatments.
The studies compared post-harvest versus spring control, post-harvest September
and October control, and a final study looking at spring timing.

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"All three trials complemented each other, and they came out strongly
supporting September dandelion control," says Sapsford.

During the first trials, Sapsford says the October post-harvest applications
provided disappointing results, so they decided to include a September application
as well. Treatments were done in the last week of September and the last week
of October. Herbicides evaluated were glyphosate at the one litre per acre rate,
two high rates of 2,4-D (equivalent to 12 and 16 ounces of 2,4-D per acre, far
above what is normally used in-crop), Express, Express Pack, Spectrum and Amitrol
(pre-seed).

Weed control ratings were done in early May, mid June, early July and August.
These results showed that late September treatments provide better control than
late October treatments. September applications of Express, Express Pack and
Spectrum gave greater than 80 percent control through to mid August the following
year.

Glyphosate applied in September at one litre per acre controlled dandelions
up to July, while the October glyphosate controlled dandelion until the early
June rating. With glyphosate at one litre per acre, spring pre-seed control
did not provide consistent control in all years. The key to spring pre-seed
control was higher rates applied to early growth stages of dandelion.

Amitrol applied in the spring gave good control, but the plants grew back later
on in the season. "We got better control with September treatments than
October," explains Sapsford. "Fall was always better than spring and
September was always better than October."

Sapsford says that the growing pattern of dandelion offers an explanation on
why September treatments are better. He says that established dandelion plants
have a big surge of growth in the early spring and flower in early May. Then
they go dormant for much of the growing season. As the crop canopy fills in,
dandelion plants become harder to hit with herbicides. Once harvest is completed,
dandelions resume active growth, translocating energy reserves to the root for
overwintering. Piggybacking herbicide on the translocation helps to move herbicides
into the root for better control.

Figure 1. Fall vs. spring application, dandelion, glyphosate at 1.0L/ac
and 1.5L/ac. Source: Ken Sapsford.
 6c

For post-harvest treatment to be successful, dandelions should be actively
growing in the fall. Dandelion is a tough plant, so it can tolerate some fall
frost, but Sapsford could not quantify how much frost was too much, since other
factors such as moisture conditions and length of frosts also impact the tolerance.
Post-harvest applications will be more successful once the crop residue has
settled and the harvest dust has blown off the dandelion leaves. "Good
coverage with herbicides is still important," he says.

Spring glyphosate helps fall control
Sapsford says that while a 0.5L/ac pre-seed burnoff rate is not very effective
on spring dandelion control, it helps with fall dandelion control. In his research,
he always applied a 0.5L/ac pre-seed glyphosate burndown to the plots prior
to seeding in the spring. He feels the burndown helps with fall control, even
if he is not getting commercially acceptable control in the pre-seed application.

"In our first trials, we saw some improvements in the October treatments
if a spring burndown had been done, so I think it will help with fall dandelion
control," explains Sapsford.

Going into the spring of 2006, many fields missed post-harvest herbicide applications
due to the late harvest of 2005. Compounding the problem, the wet fall was also
conducive to dandelion growth. The best advice in this scenario is to try to
control dandelions early in the spring for maximum yield benefit to the crop.

Glyphosate is registered for pre-seeding control in the spring, at one litre
per acre for dandelions up to six inches in diameter, and two litres per acre
for larger dandelions. Sapsford had some success in controlling dandelions with
higher rates of glyphosate, but says that farmers should not expect long-term
control. Rather, the dandelions will likely re-grow later in the season.

In his spring pre-seed trials, supported by Dow AgroSciences, Sapsford found
that PrePass provided better control than one litre per acre of glyphosate while
a tank-mix of 0.5 litre per acre of glyphosate and Express was equal to one
litre per acre of glyphosate applied alone. "Spring applications can give
you some yield protection, but they aren't as good as fall control," says
Sapsford.

Figure 2. Relationship between dandelion ground cover and canola yield
in zero-tillage fields.
 6d

Expectations for in-crop control should be similarly tempered. In cereal crops,
some herbicides are available for control/suppression, but they will only provide
top growth control and the dandelions will regrow. In Roundup Ready canola,
glyphosate is registered for suppression, while Liberty provides top growth
control in Liberty Link canola. There are not any registered herbicides for
in-crop control in pulses.

While in-crop herbicide application can help protect yield, the dandelion problem
will still be there in the fall. Even with fall application, Sapsford says that
dandelions eventually come back. With his best fall control – Spectrum,
Express Pack, or Express applied in September providing 98 percent control as
rated in May and June the following spring – by the end of the August,
control was down to 80 percent.

"Long-term control will not be achieved with a one-time treatment,"
says Sapsford. "We have to look at a systems approach over a few years."

Sapsford started a multi-site systems trial in the fall of 2005 to further
refine dandelion control strategies. It is looking at four crops in rotation
and the interaction of early fall, early spring, a combination of herbicide
intensities, and Roundup Ready treatments.

"With pulses in the rotation, we don't have in-crop control options. We're
looking at what type of herbicide intensity we need to control dandelions before
pulses, along with the economics of each system," explains Sapsford.

Dandelions can cut yields
Dandelions
can cut no-till canola yield by 50 percent.

Research conducted at the University of Manitoba by student Nathan Froese and
Professor Rene Van Acker of the Department of Plant Science provides some of
the first western Canadian data on how dandelions impact yield. The research
looked at in-crop density, ground cover, leaf diameter and how that impacted
canola yield.

Froese, who has since graduated, found that in conventional tillage fields,
no relationship was found between any measure of dandelion infestation and canola
yield. That is not to say that dandelions did not cause yield reductions. Rather,
the yield losses due to dandelion infestation could not be predicted based on
the number of plants per square foot.

However, in zero-till fields, there was a strong correlation between the percent
of ground that dandelion covered in canola crops and the loss of yield. At two
sites, if dandelion covered roughly 50 percent of the ground at the in-crop
sampling stage, then canola yield was decreased by 50 percent.

The research showed that there was a linear relationship between canola yield
and dandelion ground cover. Essentially, the more dandelions, the less yield
in no-till fields.

The Bottom Line
In a short five years, direct seeded acreage has more than doubled
in our area. While this brings many benefits, there are also drawbacks, such
as new diversified weed species coming to the forefront. Weeds such as dandelions
and foxtail barley are very competitive and can reduce yields dramatically if
left unchecked. Unfortunately, tillage seems to be the quickest cure and not
many want to take a step backwards.

We have found that a pre-seed application of Prepass has worked, although it
comes with an added cost. Fall applications have been complicated by an extended
harvest. Kenton Possberg, Humboldt, Saskatchewan.

 


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