Seed & Chemical
Desiccation or preharvest weed control?
By Bruce Barker
When a year like 2009 rolls around, with uneven and late crop maturity, the topic of speeding up harvest also rolls around. With a late harvest looming, and uneven seed maturity causing problems, many producers go looking for a way to speed harvest. Unfortunately, there is only so much that can be done, depending on the crop, crop stage, and seed maturity. “You can either desiccate your crop, or you can have good perennial weed control, but you really can’t have both in the same product,” says Dr. Linda Hall, weed scientist at the University of Alberta.
When a year like 2009 rolls around, with uneven and late crop maturity,
the topic of speeding up harvest also rolls around. With a late harvest
looming, and uneven seed maturity causing problems, many producers go
looking for a way to speed harvest. Unfortunately, there is only so
much that can be done, depending on the crop, crop stage, and seed
maturity. “You can either desiccate your crop, or you can have good
perennial weed control, but you really can’t have both in the same
product,” says Dr. Linda Hall, weed scientist at the University of
Hall is referring to the two common choices for preharvest, in-crop
application. Reglone is registered as a desiccant on some pulse and
oilseed crops, while glyphosate is registered for preharvest perennial
weed control on cereals, oilseeds and pulses.
|Registered crops for preharvest applications|
|Wheat, barley, oat||No||Yes|
Reglone speeds crop drydown, while glyphosate provides perennial weed control.
|eglone nine days after application provides fast desiccation for earlier harvests.
|(Photos courtesy of Syngenta Canada)|
Ed Thiessen of Syngenta Canada says that herbicide application before
harvest can often bring increased profitability by providing several
benefits, depending on the herbicide used, including maximizing yield
and quality, facilitating direct combining of standing crops, managing
in-crop weed escapes for easier harvesting and reduced dockage,
controlling perennial weeds in subsequent crops, and controlling
harvest timing. “There are typically three scenarios where a producer
will consider a herbicide application before harvest. Those include as
an aid to harvest a good crop, to improve harvest of an uneven crop,
and for weed control,” explains Thiessen.
Syngenta sells both Reglone and the glyphosate Touchdown, so Thiessen
can also offer a good perspective on both of the products. “The number
one question we get from growers leading up to harvest is whether they
should apply Reglone or glyphosate on their crop and what benefits they
will see with each,” he says.
Unfortunately, myths persist surrounding the use of glyphosate as an
aid to crop drydown, Reglone for providing perennial weed control, and
both as an aid to speeding maturity of green seed. “You have to be
careful to ask the right question, or you may get the wrong answer.
Make sure you know the strengths and weakness of each product,” says
|Contact non-selective||Systemic non-selective|
|Fast acting (days)||Slow acting (weeks)|
|Desiccates crops and weeds||Kills crops and weeds|
|Strong activity on dicot crops||Strong activity on monocot crops|
|Affects green tissue of all plants||Affects meristems of all plants|
|Best on senescing plants||Best on actively growing plants|
|Burns top growth of annual and perennial weeds||Kills perennials, inconsistent on annuals|
|Apply when majority of seed is mature||Apply at less than 30 percent seed moisture|
|Does not affect seed germination||Adversely affects seed germination|
|Temperature, day length and light intensity affect
|Frost, drought and temperature affect performance|
|More expensive||Less expensive|
|Source: Thiessen, Syngenta Canada.|
Neither will speed seed maturity
Hall says that Reglone is an excellent crop desiccant for broadleaf
crops, while glyphosate provides excellent control of perennial weeds
and can be used to even out maturity of cereal crops. However, she says
that neither can be used to speed up seed maturity. “If you have a
green seed, you have a green seed. There are only two things that
affect seed maturity: the genetics and the environment. You won’t
enhance maturity by applying either of these two products,” explains
With Reglone, too early an application will desiccate the leaves and
leave the green seed in the pod. With glyphosate, if seeds are still
maturing, the active ingredient will translocate to the seed and can
inhibit future seed germination, but will not speed seed maturity.
Thiessen says that some growers experienced a loss of seed quality in
2009 because of application to an immature crop, especially on pulses.
“Reglone can accelerate the crop and weed drydown, which can assist in
getting the crop off faster, but it must be applied at the correct seed
stage for the best seed quality,” he explains.
Reglone is the ‘hare’ of preharvest applications
Reglone is a fast, contact herbicide. It enters the cell and interferes
with high-energy electron transfer causing production of toxic hydroxy
radicals that disrupt cell membranes. This causes the cells to
desiccate rapidly within hours of application. Because desiccation
happens so rapidly, the herbicide does not move away from the site of
contact where the droplet hit. “It does not translocate, it doesn’t
move away from where it hits the plant, and it doesn’t move to the
seed. So you can effectively use it on a crop that you are going to use
for seed when applied at the correct crop stage,” says Hall.
She also explains that Reglone does not kill the crop, but desiccates
it. Typically, in about two to three days, there should be very good
desiccation. However, some regrowth is possible.
Proper application timing for Reglone occurs when the seeds are
virtually mature. In this case, desiccation can be a very beneficial
harvest management strategy.
Hall cautions that Reglone on canola is difficult to manage because
canola is easy to shatter. “If you desiccate canola, you can really
increase shattering. A late application can increase shatter, too early
and you can increase green seed. You really have to be desiccating just
one or two days before harvest, so timing is very critical,” says Hall.
If applied properly, Hall adds that Reglone provides excellent fast
desiccation of broadleaf crops and excellent annual weed desiccation,
but it provides no perennial weed control. It cannot be used on
cereals, because of residue issues.
Glyphosate is the tortoise
Glyphosate works more slowly, which is a good thing for perennial weed
control. Glyphosate enters the plant and is concentrated in the cells,
including the phloem, where it is translocated along with sugars to the
actively growing parts of the plant. There it slowly kills the plant by
diverting carbon flow. This slow mode of action is ideal for perennial
weed control because it moves to the roots and rhizomes.
Thiessen explains that glyphosate is not a desiccant and that the
distinction is more than just semantics. He says that because it is
slow working, it is ideal for perennial weed control, but does not
provide the benefits of a desiccant.
The timing application for glyphosate is when seed moisture is less
than 30 percent. The product label provides further guidelines on which
visual symptoms can be used to estimate this crop stage. Glyphosate
cannot be applied to any crop grown for seed. “It is well established
that glyphosate can inhibit seed germination,” says Hall.
Glyphosate will slowly even maturity of cereal crops and is the only
product that can be used preharvest on cereals. However, as previously
mentioned, it will not speed maturity of green seed. As a result, the
primary objective of glyphosate preharvest is perennial weed control.
“Both products bring strengths, but they are not replaceable.
Unfortunately, you cannot have both in a single product and they cannot
be effectively mixed,” says Hall.
Ensure proper application
Thiessen says that applying a herbicide prior to harvest takes
additional care. The crop/weed canopy is often three to four feet tall,
which impacts on application parameters like boom height. Reglone also
requires thorough coverage of the crop, so increased water volumes and
attention to droplet size and canopy coverage are critical.
Reglone application is critical because it is a contact herbicide. The
only way to make the product work is to get very thorough coverage of
the crop leaves. Increase the water volume to get better coverage. What
it touches is what it desiccates.
Go slow with sprayer speed as well. Hall explains that if travelling
too fast, one side of the crop will be desiccated while the other side
will not be dried down. High boom height and a smaller droplet size
will help with coverage. Droplet size is a compromise between coverage
and drift. A non-ionic surfactant is absolutely critical, as is clean
water, because dirty water will deactivate the active ingredient.
Thiessen says that the two products can be applied sequentially to
achieve the best of both worlds. It can be costly so the return on
investment needs to be calculated; applying glyphosate first, followed
by Reglone three to five days later, can provide both perennial weed
control and improved crop desiccation.
While both products have their strengths, unfortunately they cannot
provide growers with what many wanted in 2009: a way to speed seed
maturity so that the harvest could proceed sooner without any green and
immature seed. For that, there is only Mother Nature to rely on.
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