A post-harvest herbicide can help manage perennial weeds such as Canada thistle, quackgrass and dandelion.
“At this time of the year perennial weeds are preparing for winter by putting energy reserves into their root systems,” said Mark Cutts Alberta Agriculture and Rural Development crop specialist. “A fall application of a glyphosate product results in the herbicide being moved down into the root system and generally provides very good control of these weeds.”
In annual cropping systems, options for perennial weed control in the fall include a pre-harvest or post-harvest glyphosate application. Applying glyphosate to a standing crop is referred to as pre-harvest and has become a very common management practice. In certain situations such as when retaining seeds for planting, where a pre-harvest glyphosate application is not feasible, a post-harvest might be.
When considering a post-harvest application, producers should consider several factors to ensure success.
“One important component of post-harvest weed control is ensuring there is sufficient leaf area present on the target weeds,” said Cutts, adding that weeds growing into the crop canopy, will be removed during harvest so a re-growth period is necessary to ensure enough leaf material is present to kill the plant.
Canada thistle requires three to four new leaves that are several inches long while quackgrass should be six to eight inches in height with at least three or four new leaves.
“For dandelions, the removal of plant material during harvest typically benefits control,” said Cutts. “The removal of the crop canopy allows for improved access to dandelions and better herbicide coverage will occur.”
Producers should also consider the impact of frost on the growth of the target weeds.
“If the majority of the perennial weeds consist of leaf tissue that is more than 60 per cent green, a glyphosate product can be applied,” said Cutts. “Typically dandelions are the most tolerant to frost, quackgrass has moderate frost tolerance and Canada thistle is considered the most sensitive. Knowing the target weeds and their frost tolerance will help you get good control.”
Harvest management can also impact the success of a post-harvest glyphosate application.
