By Rory Cranston PAg
Regional Crops Specialist
With annual crops, fall weed control falls into two different categories: pre-harvest and post-harvest. Pre-harvest weed control is effective for controlling tall perennials and green weed growth that may interfere with harvest; however it is less effective at controlling problem weeds like Canada thistle, quack grass, dandelions and winter annuals.
Post-harvest weed control can be inconvenient, given the time constraints of harvest, but it is very effective at controlling these problem weeds. At this time in the season, many perennial plants and winter annuals are shutting down for the winter and are moving nutrients down into the roots. Applying a systemic herbicide now will move the chemical down into the root and provide great weed control
There are several factors to be considered in order to make post-harvest weed control as effective as possible.
Always read the product label. The label contains important product specific information like cropping restrictions, application rates and potential tanks mixes. For example, glyphosate rates are much lower when used in a pre-harvest situation, but can be increased for a post- harvest application.
To achieve the best possible weed control, the target weeds need to be actively growing.
Lower older leaves are not as effective at absorbing herbicide as new leaves. After harvest, wait until weeds have developed two or three new leaves before applying a herbicide. The new growth will optimize herbicide uptake.
It is important to manage straw and chaff at harvest. Weeds that are covered by chaff and straw will have a reduced contact area that can result in reduced control. Avoid windrows by chopping or baling straw. This will increase the leaf area to come in contact with the herbicide and can result in a more effective control.
Weather is an important consideration. The cold short days in fall can slow down herbicide activity in a plant. Frost is major concern. Producers should not apply a herbicide until two or three days after a light frost. A herbicide should not be applied after a severe killing frost, as it will be ineffective at controlling weeds.
For more information contact your local regional office or the Agriculture Knowledge Centre at 1-866-457-2377.