,
,
 

Label use of pre-harvest herbicides

 
  From the October 15, 2018 issue of Agri-News
Subscribe to our free E-Newsletter, "Agri-News" (formerly RTW This Week)Agri-News
This Week
     Agri-News HomeAgri-News Home
 
 
 
 The use of pre-harvest herbicides is very common. Harry Brook, crop specialist at the Alberta Ag-Info Centre, explains why conditions have to be right for these applications to be effective.

“Why and when are you applying pre-harvest herbicides,” asks Brook. “Is it to control late flushing, perennial weeds? Is it to quicken the dry down of your crop and even out maturity? A lot of fields I have seen this fall seemed to have been sprayed while conditions were less than ideal.”

Herbicides only work on plants and weeds that are actively growing. Explains Brook, “That means temperatures should be in the double digits, and sunny conditions help. How can a plant actually take up a chemical and translocate it to where it will activate if it is dormant? Many fields in Central Alberta were sprayed when temperatures were cool and there was no sun. It is very doubtful it will have any effect on the weeds or the crops. These were the very conditions we have received since late August.”

The glyphosate label contains the warning that “wet, cool or cloudy weather conditions between the time of application and anticipated harvest may slow down the activity of the product, thereby delaying crop dry down and harvest date.”

The primary registration for pre-harvest glyphosate is for fall weed control of perennial weeds. “The secondary result can be a quicker dry down and evening out crop maturity, but it is very weather and temperature dependent. If conditions aren’t good, you may have wasted your time and money, and the crop will not respond,” says Brook.

Brook says to think of herbicide application from the plant perspective. “Anything that might impair the plant photosynthesizing and respiring will also reduce the effectiveness of an herbicide. Not only will cold temperatures reduce herbicide activity, so will drought, hail, and frost. After any of these events, it takes time for a plant to regain photosynthetic activity. Spraying a crop under these poor conditions is not effective and is mostly a waste of money. Still, a lot of fields were still sprayed this fall when they shouldn’t have been. Swathing under these conditions would provide a quicker crop dry down and protects the crop from lodging risk due to snow fall.”

All chemical reactions work faster the warmer it is. “Most plants shut down at 5 C, and crops like corn and soybean are especially sensitive to cool temperatures,” explains Book. “It takes an extended warming trend to get them back to normal photosynthetic activity. Most of the crops and weeds that grow in Alberta are temperate adapted species. They grow best in the temperature range of 15 to 25 C.”

Brook adds that another reason to be certain of using the right tool for the right conditions is the threat of herbicide resistance. “Overusing one herbicide or tool will shorten the life and range of effectiveness of that herbicide. Overuse of glyphosate in other parts of the world has resulted in massive weed problems, and we see the tip of the iceberg here in Alberta with kochia. Proper stewardship of our herbicide tool will keep it available to producers into the future.”

“Farming on the prairies deals with unpredictable and extreme changes in weather,” says Brook. “Whether you use a pre-harvest herbicide spray to control weeds or to speed up crop maturity and drydown, you have to apply the herbicide tool when it can be successful. Not all tools are useful all the time. This fall is a case where swathing may be the most effective tool to even out crop maturity and speed harvest.”

For more information about effective use of pre-harvest herbicides, contact the Alberta Ag-Info Centre at 310-FARM (3276).

Contact:
Alberta Ag-Info Centre
310-FARM (3276)

 
 
 
 
view Agri-News RSS FeedAgri-News RSS Feed      Share via AddThis.com

,
view Agri-News RSS FeedAgri-News RSS Feed      Share via AddThis.com

For more information about the content of this document, contact Harry Brook.
This document is maintained by Christine Chomiak.
This information published to the web on October 1, 2018.