Harvest is well on its way in most areas of the province, which means there may be time left to do some fall work in the field. Adverse weather conditions add additional obstacles to get through when it comes to making management decisions be the most efficient and profitable as the season closes. We all know that herbicide efficacy is largely based on temperature and the growing conditions that the plant is living in. With that being said, there is always a sweet spot where the optimal performance will occur, but there are also some warning signs we need to look out for in these later months of the growing season.
As night time temperatures continue to drop, it is critical that we are keeping our best management practices at the top of mind when it comes to fall burn off. If the nighttime lows are dropping below 0°C, make sure to consider these key points before an herbicide application: Duration of the frost, the severity of the frost, the weather leading up to the frost, and the target weed species.
Duration, Severity, And Weather Leading Up To The Frost
When temperatures drop to -2 to -3 there is usually minimal plant damage. At this point, you could spray later that day as long as the temperature reaches 8°C and stays there for 2-4 hours after application. As temperatures dip below -5°C, you will start to see greater damage to plants. You should hold off spraying for a couple of days so that you can assess the damage that was caused by the frost and make a better decision on whether or not an herbicide application is necessary. If there is enough healthy tissue left to take up the herbicide (roughly more then 60%), and the temperature is going to get up to 10°C, you could consider spraying. If there is a heavy frost the night after you spray, you could also see reduced efficacy with the application.
Spraying when weeds are actively growing is the key to getting good control with a herbicide. With cooler temperatures drastically slowing down the metabolism of the plants, they will experience less efficient take-up of the chemical. This is especially important when applying a systemic herbicide like glyphosate.
Target Weed Species
When you are deciding whether or not you want to do a fall herbicide application, it may be useful to take a look at what weeds you are aiming to kill. Fall is a great time to hit perennials, biennials, and winter annuals. These species can undergo a more severe frost event than germinating spring annuals. If an annual weed species looks like it will set seed before freeze up, it may be worthwhile to spray.
Research shows that Kochia regrowing after a mid-August harvest will likely not set seed before a killing frost event. This is important to note when it comes to herbicide resistance. If you are planning on using glyphosate to kill the annual weeds, specifically Kochia, you might just be adding to the selection pressure and increasing your risk of developing resistance.
Fall is a great time to do some groundwork and get your fields ready for spring. Always keep the best management practices in mind to help you get ahead of the game!