Preharvest Glyphosate Applications

Sep 01, 2021

Written By: Candice Harris, Agronomy Lead (North)

Preharvest herbicide application is the practice of applying herbicide, either glyphosate or a desiccant type product, to a physiologically mature crop in order to accelerate the dry down process. This article focuses on the use of glyphosate as a preharvest tool. 

Physiological maturity is achieved when the crop reaches 30% moisture or less. At this stage grain fill is done and the grain itself will be almost ready to combine. The purpose of the glyphosate application is really to eliminate any green matter that is in the canopy. The green matter can be annual or perennial weeds, regrowth, or it can be the green leaves and stems from the crop itself.

Preharvest applications of glyphosate are a common management practice for cereal crops including wheat, barley, and oats. Glyphosate can also be used for some pulses and oil seed crops including peas, faba beans, flax, and canola. 

Situations where preharvest glyphosate is a necessary practice include:
  • Cool and wet falls in which the plants are prone to staying greener longer.
  • Fields that have seen fungicide applications. Fungicide will help keep the plants healthier, as a result they can stay greener longer into the growing season.
  • Fields that have rolling topography. Hills and depressions mature at different rates.
  • Years where we have undergrowth or regrowth of the crop.
  • Fields that have green weeds present.
Applications of preharvest glyphosate will dry down the crop faster than if we were to let it dry down on its own. Accelerated dry down provides key benefits to our farm management strategies.

Key benefits of preharvest glyphosate applications:
  • Increased harvest efficiency. Preharvest applications will even out the crop and allow for even maturity of the field. It will increase the speed of dry down of the plant matter, so the maturation to combining interval will be decreased. The drier plant matter will go through the combine easier, potential drying costs are decreased, and it will help with forward planning and organization of harvest activities. 
  • Increased grain quality. When we have fields that contain a lot of green matter in the canopy, our options really are to swath or to apply a preharvest product. This is because we need to give that green matter time to dry down before it is combinable. Preharvest glyphosate will allow the crop to remain standing until it has reached a combinable stage. Swathing can leave crops susceptible to adverse fall weather conditions. If the crop goes through several wet/dry cycles while in the swath, we run the risk of grade loss. Mycotoxins and moulds can form in the grain, bleaching may occur, and we may run the risk of additional losses, specifically for wheat if sprouting occurs. Allowing the crop to stand can protect it from quality loss from these scenarios. 
  • Annual and perennial weed control. We may have weeds present in our fields at harvest time for a variety of reasons such as low spots that did not get seeded or that drowned out, seeder misses, sprayer misses, or simply problematic areas. Presence of green weeds in a mature crop will extend the harvest interval, decrease combine efficiency, and potentially cause an increase in dockage due to the presence of weed seeds in the grain. A well-timed glyphosate application will provide the best short and long term control of annual and perennial weeds, while maintaining harvest productivity. 
Keys to success:

There are a few practices that we must be mindful of in order to see the best results from our applications.
  1. Apply at the proper stage. We need to ensure that the crop is at that critical moisture stage. Early applications will cause shrunken and shriveled kernels that will appear similar to frost damaged grain. Applying too early will also cause potential issues with glyphosate residue in the grain. If you have questions or concerns on that critial timing, please feel free to reach out to your local G-Mac's AgTeam Agronomist or ACE so that they can provide you with crop specific information. 
  2. Use proper water volume. Remember that we are trying to target the green matter in our canopy. That means we need to get our product down onto the leaves and weeds that are deep in the canopy. 10-15 gallons of water is ideal. Also, if you have hard water, ensure that you are using an ammonium sulfate based water conditioner such as Crimson NG, as this will help you get maximum efficacy out of your applications. If you are unsure of your water quality, collect a sample in a clean jar and have your local G-Mac's AgTeam retail send it in for a water analysis. 
  3. Be mindful of environmental conditions. The environment plays a huge role in the way our herbicides work and fall applications are no different. Be sure to apply when you have several consecutive days of warm and sunny conditions. These conditions will give you the best and most rapid results from your applications. 
Preharvest glyphosate is a tool that when used in a well-timed application will create value by creating efficiencies in both combining as well as overall harvest management. Preharvest applications increase opportunity to achieve high grain quality and to provide good weed control. 

For any questions on the information provided here or pertaining to specific crops on your farm, please reach out to your local G-Mac's AgTeam Agronomist or ACE to help you navigate through your questions and concerns.


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