Proper identification of weeds key to control

UNL Extension

Late summer always seems like a time when weeds can become quite noticeable in pastures. While they may be an eye sore, control methods now are 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 an open space, providing the sunlight, nutrients, and water to grow. Besides being too large to kill at this point, most annuals have already set seed. Spraying at this point will do nothing 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. The only sure-fire tool available is to collect it by hand, a very labor intensive option. Plants that have yet to set seed may be mowed off to improve appearance and stop some seed production. Another option is to mark the spot as an area to keep an eye on next year. A spring herbicide application when plants are smaller is better for annual control and killing 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, over winter and initiate growth in the spring. Species like henbit, downy brome, and mustard are best controlled later this fall in the rosette stage. If a preemergent option is available for the species you want to control, make sure to look at the length of residual and time your application to provide protection through the entire fall growing period.

Pulling the trigger too soon could leave a window of opportunity for growth before the first hard freeze.

Again, marking these locations to come back in early spring and check for any breakthrough plants will provide more complete control.

Perennial weeds like western ragweed, ironweed, and verbena are still noticeable but require different control. Once again, spraying weeds now does little good. Weeds are too large to kill and herbicides may only slightly reduce some seed production. Just like annuals, spring herbicide applications are best when plants are small and young tissues more easily absorb the chemical.

However, if additional control is needed, waiting closer to a killing frost is best. This provides the double whammy of stressing the plant heading into winter and allows more product to be translocated down to the shoots and roots as nutrients are pulled down for winter storage.

For all weeds, dry conditions can throw a wrench in plans as well. Plants that have gone dormant due to drought are not absorbing and translocating herbicide like normal, so little control will come from an application. It’s 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. If you need a hand, UNL produces two handy plant ID books available in PDF 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 most important, is focusing on the 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, most reoccurring pasture weeds are a symptom of poor grazing management.

These plants are opportunistic invaders, taking advantage of an open piece of 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 reduce weed populations economically over time.

Pasture weeds may look unsightly now, but hold off on spraying. Improve grazing management and time herbicides for the best window of control so herbicides won’t be needed as often 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) 2546821 or email: ben.beckman@unl.edu