Late summer always seems like a time when weeds can become quite noticeable in pastures. While they may be an eyesore, control methods now can be limited. Proper ID can help with making the right management call.
Annual weeds like horseweed, sunflowers, snow-on-the-mountain, and buffalo bur can be plentiful, often showing up in areas of pastures that have relatively thin grass stands, in areas where animals congregate, or if some overgrazing has occurred. These weeds are taking advantage of open space, providing the sunlight, nutrients, and water they need to grow.
Besides being too large to easily kill at this point, many annuals have already set seed. Spraying now may do little to prevent that seed from maturing and starting a new plant next year.
To control the population of annual weeds next year, preventing seed spread is key. At this point, pulling or collecting plants by hand is one of the surest options, although it is obviously labor intensive. Plants that have yet to set seed may be mowed off to improve appearance and stop some seed production. We can also mark these areas as spots to keep an eye on next year. A spring herbicide application when plants are smaller is generally a much better option for summer annual control and gets plants well ahead of seed set.
Another group of annuals we may run into later on are winter annuals. These plants sprout in the fall, overwinter, and initiate growth again in the spring. Species like henbit, downy brome, and mustards are often easier to control later this fall while plants are still small and actively growing. If a pre-emergent or residual option is available for the species you want to control, make sure to look at the length of residual and time your application appropriately. Again, marking these locations to come back and check later this fall or early next spring can provide more complete control.
Perennial weeds like western ragweed, ironweed, vervain, Canada thistle, and leafy spurge require a different approach. Spraying large, mature weeds right now may not provide great control, but the fall window can still be very useful.
As we get later into September and into October, many perennial weeds begin moving nutrients down into their roots for winter storage. Herbicides applied while plants are still actively growing can move with those nutrients into the root system. This can improve control and provide that double whammy of stressing the plant heading into winter while also getting more product down into the roots and shoots.
That doesn’t mean every perennial weed will disappear after one treatment. Tough species like Canada thistle and leafy spurge often require repeated control over several years. But a properly timed fall application can be an important part of that plan.
Dry conditions can throw a wrench in our plans as well. Plants that have gone dormant due to drought are not absorbing and translocating herbicide like normal, so little control may come from an application. It is better to hold off until moisture renews growth, either this fall or next spring.
When it comes to herbicides, getting a proper ID is critical for making the correct management decision. Different weeds respond differently depending on their life cycle, growth stage, and the herbicide being used. Always read and follow the label, including any grazing or haying restrictions.
If you need a hand with identification, UNL produces two handy plant ID books available in PDF form free online: “Common Grasses of Nebraska” and “Common Forbs and Shrubs of Nebraska.” As always, your local Extension office is also willing to lend a hand.
The last point to consider for pasture weeds, but arguably the most important, is focusing on grazing management for long-term weed control. This includes using the proper stocking rate and developing a good rotational grazing plan. While this doesn’t work for all weeds, especially some of our more nasty perennial species like leafy spurge and Canada thistle, many recurring pasture weeds are a symptom of poor grazing management.
These plants are opportunistic invaders, taking advantage of open ground and weakened competition from their neighbors. As such, an important objective is to increase the health, vigor, and density of your grass. Healthy, competitive grass stands are essential to reducing weed populations economically over time.
Pasture weeds may look unsightly now, but don’t let appearance alone determine when to spray. Improve grazing management, properly identify the weed, and time herbicide applications for the best window of control. Doing so can improve control now and help reduce how often herbicides are needed in the future.
Ben Beckman is a beef systems Extension Educator serving northeast Nebraska. He is based out of the Cedar County Extension office in Hartington. You can reach him by phone: (402) 254-6821 or email: ben.beckman@unl.edu.
