Desiccation or preharvest weed control?

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	Desiccation or preharvest weed control? Growers cannot have both in one herbicide.
Written 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.

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
	Reglone	Glyphosate
Wheat, barley, oat	No	Yes
Canola	Yes	Yes
Flax, solin	Yes	Yes
Lentil	Yes	Yes
Pea	Yes	Yes
Chickpea	Yes	Yes
Lupin	Yes	Yes
Fababean	No	Yes
Soybean	Yes	Yes
Dry bean	Yes	Yes
Forage	Yes	Yes
Sunflowers	Yes	No
Potatoes	Yes	No


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 Thiessen.

Reglone desiccant	Glyphosate preharvest
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 performance	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 Hall.

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.

Growers cannot have both in one herbicide.