Diquat vs. Glyphosate - Pre-Harvest Desiccation

Written By: Eric Surine

As we start to move into harvest, we get ready for the last step in many of our crop plans before we get the crop in the bin - fall desiccation. When it comes to crop desiccation, we have two types of herbicides that we can use, contact herbicides and systemic herbicides.

Choosing which product to apply as a desiccant can rely on a few factors:

1. Time
2. Weed pressure
3. End use of the crop

Contact
The active ingredient diquat (also known as Reglone Ion, Craven, Desica) is commonly used in pulses to allow for a quicker dry-down of your crop. A quick dry-down process allows for a more efficient harvest. Many growers will find that they are able to combine later into the night and start earlier in the morning with a crop that is completely dried down. Diquat works by breaking down the cell walls of the plant. It will only break down the cells that it comes into contact with so adequate water volume is recommended. It will typically take between 4-10 days between application and harvest. 

The timing of application is also critical to the success of the product. Diquat is photoreactive, which means that it becomes reactive when it is exposed to ultraviolet rays or visible light. The best time to apply diquat is early morning or late into the evening, when the sun is least active. There needs to be enough time for the product to cover the surface of the plants before being exposed to direct sunlight for the product to work. Diquat will start to work once it is activated by sunlight.

Since diquat is a contact herbicide, it will not translocate through the plant and there is zero risk of the product translocating into the seed. Many grain buyers will offer a premium for pulses that are desiccated with diquat, and some will not accept pulses that have been desiccated with glyphosate. It is important that growers communicate with their grain buyers before desiccating to ensure that the product they are choosing to use will not jeopardize their grain sales. Diquat will not affect seed germination, this is important to consider if the grower is planning on saving seed for next season.

Systemic

Systemic herbicides are typically recommended for crops like canola, where a dense canopy can make the dry-down process more difficult. A systemic herbicide will translocate through the entire plant, allowing for the dry-down of thicker and greener canopies. Glyphosate (Roundup Transorb, Stonewall, RT 540) is commonly used as a systemic desiccant. While commonly used to desiccate canola, it can also be used to desiccate pulses and cereals.

Our crops are growing slower in the fall than they do earlier in the season, and since the herbicide needs to move through the plant for it to work, it can take longer for the plants to die. In the fall, it usually takes between 10-14 days for glyphosate to show activity.

Pre-harvest applications of glyphosate is another tool used to help manage weed pressure. Glyphosate will clean up weeds that were missed earlier in the season and will help growers gain control of hard-to-manage weeds. Glyphosate does have the potential to affect the germination of a seed. This is very important to keep in mind if growers are wanting to save seed for the next season.

Staging
 

	Diquat	Glyphosate
Canola	Apply once 90% or more of the individual seeds have turned colour. Combine within 14 days of application.	Apply at least 50-60% seed colour change in the least moisture areas of the field. Bottom seeds are completely black/brown. Top seeds are green but remain firm when rolled.
Green Peas	Bottom pods are dry and seeds are detached from pods. Middle pods are light green to yellow, seeds split when squeezed. Upper pods are green but starting to turn.	Majority of pods are  brown. 75-80% of the plant has turned colour. Peas are tart and firm. Peas in the bottom pods are detached and rattle when shaken.
Yellow Peas	Crop is yellow overall. Lower and middle pods are dry, seeds detached from pods. Upper pods are leathery, peas split when squeezed.	Majority of pods are  brown. 75-80% of the plant has turned colour. Peas are tart and firm. Peas in the bottom pods are detached and rattle when shaken.
Lentils	Crop is 75-80% yellow overall. Stems are turning yellow. Bottom pods are dry but not split, seeds will rattle within the pods. Middle pods are light green to yellow, seeds are soft but not juicy. Upper pods may still be green and a bit fleshy.	Lowest pods are brown and rattle when shaken, lowest pods make up the bottom 15% of the plant. Seeds from the top thirds of the plant are fully formed and firm.
Cereals	Apply at hard dough stage, seed is less than 30% moisture and physiologically mature. Will be able to leave a thumbnail impression in the seed.

Surfactants and Additives

Except for Reglone Ion, diquat requires the addition of a non-ionic surfactant. Glyphosate does not require the addition of any other product, but there are several additives that can be used to increase the efficacy of our pre-harvest herbicides.

Hi-Activate:

• Can be used wherever a non-ionic surfactant is required on label
• Reduces surface tension and increases spreading/dispersion of spray droplets
• Improves coverage and plant uptake
• Critical for herbicide performance when the addition of a non-ionic surfactant is required

Crimson:

• Water conditioner with added de-foaming agent
• Prevents herbicide antagonism (positively charged ions like calcium, iron and magnesium can tie up negatively charged ions like glyphosate)
• Pre-dissolved formulation to prevent sedimentation and plugged nozzles
• Increases herbicide efficacy
• Can be used on all crops and any tank-mix

InterLock:

• Reduces spray drift and increases penetration into the canopy
• Improves spray droplet coverage
• Increases herbicide efficacy
• Registered for use with all herbicides

Masterlock:

• Improved product deposition and penetration into the canopy
• Reduces spray drift
• Optimizes the adhesion of spray droplets on the plant surface
• Enhances sticking and spreading, allowing for better coverage of the plant
• Increases herbicide efficacy

*Pending registration with diquat