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Fall Herbicide Application

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From the cab of your combine, it is the perfect time to start thinking about next year’s crop. Fall herbicide applications can be a reliable way to help keep fields clean of those winter annual and perennial weeds that will be tough to control the following spring. 

When determining to do a fall herbicide application, consider the history of the field and plan for the following growing season. 

Scouting

When you’re out harvesting this fall, take a good look at the field for small, germinated weeds. Asses the species, health, and size of the weeds present, making sure they are actively growing and have enough leaf surface to actively take the product in. Questions you may want to consider: What were some of the troublesome weeds throughout the season? Are any of those weed species perennial or winter annuals? 

Winter Annuals

Fall germinating winter annuals are controlled effectively in the fall because the plants are small and susceptible to the herbicide. These weeds will continue to grow through the fall frosts and will resume growth in the early spring. Some examples of tough to control winter annuals include Narrow Leafed Hawksbeard, Prickly Lettuce, and Night Flowering Catchfly. 

 Perennials

In the fall, perennials are sending reserves down to the roots to overwinter and will carry the herbicide to the roots. For optimal control, it is important to ensure there is enough regrowth after being cut at harvest for the herbicide to hit its target. Some common examples of tough to control perennial weeds are Dandelions, Canada Thistle, Horsetail (commonly found around sloughs), and Foxtail Barley. 

Spray Conditions

Optimal weather conditions are preferred for an effective fall herbicide application. Ideally warm fall days, temperatures greater than +10 are when the plants are actively growing. Frost and continuous low temperatures will send the weeds into shut down. Making it best to wait a few days after cold temperatures, assess the health of the weeds, and apply an application only after the weeds start growing again. 

Tank Mixing

Tank mixing is another essential consideration when it comes to fall burn-off. Tank mixing allows for herbicide rotation, especially in instances where there is a presence of resistant weed biotypes that limit herbicide group options in-crop. Although a heavy rate of glyphosate may be a straightforward and cost-effective solution, it is important to incorporate a tank mix partner to get multiple modes of action, ensure effective control and mitigate the chance of resistance. Keep in mind any re-cropping restrictions. Heat LQ, Express and 2,4-D are common tank-mix partners to consider. 

A fall weed control application can reduce the spring workload, get your crop off to a weed-free start, and allow you to get into the field earlier. Contact your local Synergy AG agronomist if you need assistance identifying weed species or creating an optimal fall burn-off plan. 

Fall Cleanup

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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!