Top Crop Manager

Features Agronomy Weeds
Uptake and timing: getting the best value from inputs

When choosing herbicides, it is important to consider how site of uptake influences weed control.

November 20, 2007
By Top Crop Manager


Herbicides are registered on the basis of delivering commercially acceptable
control, rated at 80 percent control of target weeds. Most farmers, though,
would not be happy with just 80 percent control. Rather, they seek out the herbicide
that delivers the best control under all possible environmental and application
situations. In finding herbicides that work the best, understanding how herbicides
are taken in by the weed, the site of uptake sheds some light on how well weeds
are controlled.

"Certainly, the way a herbicide is taken up by the weed is important,"
says weed scientist, Eric Johnson with Agriculture and Agri-Food Canada (AAFC)
at Scott, Saskatchewan. "With graminicides (grassy weed herbicides), a
lot of the weed control comes primarily from contact with the meristems (growing
points), especially the Group 1 herbicides."

Johnson says that some graminicides do not translocate readily within the plant
and that is where differences in performance may emerge. He explains that these
herbicides must hit the meristem of the weed to get the best control. "Some
herbicides have more limited translocation than others. That's where application
parameters become more important in control," says Johnson. "If we
are going to see a problem with Group 1 herbicides, we start to see it with
low water volume and coarser droplets."


Several Group 2 herbicides have both foliar and root uptake. Everest and Assert
are taken in by the leaves and also have soil activity which is taken up by
the roots. Brian Schilling, technical service representative with Arysta LifeScience
Canada at Saskatoon, says "Everest doesn't seem to be affected by environment
and application parameters as much as Group 1 products. He says that flexibility
comes from the dual sites of uptake and pays off in years when an early application
of Everest is made to very young, rapidly growing weeds and growing conditions
favour multiple weed flushes.

"If a herbicide has several sites of uptake, it may be able to perform
under a wider range of application or environmental conditions," says Schilling.
"We've seen where several flushes of wild oats and green foxtail can be
controlled by the soil activity, and that can increase your application flexibility."

Greater application flexibility with multiple sites
of uptake

In research trials at AAFC Saskatoon, research scientist Tom Wolf, along with
Eric Johnson at the Scott Research Farm investigated how droplet size and water
volume affected grassy weed herbicide performance. Three water volumes of four,
eight and 12 gallons per acre were applied through medium, coarse and very coarse
droplet nozzles. The researchers were trying to find out where weed control
drops off when increasing droplet sizes (to reduce drift) and while lowering
water volumes (to increase application efficiencies).

The researchers found that products with dual soil and leaf uptake were better
able to perform under a wider range of application variables. In the trials,
Group 2 products had consistent and high wild oats control at all water volumes
and droplet sizes tested. "With the Group 2 chemistries, we found they
worked well under every application parameter at full label rates. The weather
conditions favoured Everest because of a light rain we received shortly after
application," Wolf says. "Because of the soil activity, it seemed
to be quite resilient under varying application methods."

In the first year of trials, the rain moved the Group 2 herbicides into the
root zone. Coarse sprays and lower water volumes did not affect Group 2 weed
control at full label rates. "Is it the result of soil uptake? We didn't
assess that and a lot of other factors could be involved," says Wolf. "We
do know, though, that Group 2 herbicides produce a different response to application
parameters, if soil type and crop rotation allow a producer to use them."

Wolf notes that without rainfall soon after application, the Group 2 results
were not so pronounced in the second year of trials.

Several Group 1 herbicides were also included in the research. Both Assure
and Horizon provided very good non-residual wild oats control at recommended
label rates and control did not tend to vary with water volume for medium and
coarse sprays. For Assure, application in a very coarse spray resulted in reduced
oats control at all three water volumes, with the greatest loss at the lowest
water volume.

Horizon saw decreased control when moving to very coarse sprays at the lower
water volumes. At the highest water volume and very coarse droplets, weed control
was still acceptable but tended to be more variable than it was with the finer

One possible reason for the reduced control with Group 1 herbicides at very
coarse droplets and lower water volumes is that targetting the plants becomes
more difficult. With the vertical orientation of the grassy weed, getting larger
droplets to stick is harder to achieve.

Schilling says that the dual sites of uptake with Everest can help increase
weed control and allow better performance under tougher application parameters.
However, he says farmers should still focus on good weed coverage. Poor spray
patterns and poor coverage may still result in poor weed control. "We are
seeing a lot of high clearance sprayers using coarser sprays and lower water
volumes with good results," he says. "But you need to be careful.
We need to better understand how coarse droplets affect soil activity and root
uptake." -30-