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Early Season Pasture Thistle Control

By Jerry Tesmer
Extension Educator, Fillmore and Houston Counties

When the grass started greening up, it also meant other less welcome plants will soon be appearing. Normally, early May is excellent for early season pasture weed control, but if this weather trend continues, consider moving the timing up. Plants that are easily controlled when small and tender become more difficult to control as they mature. Also, early control of pasture weeds allows more grass to grow and the pasture will support additional grazing.

There are a many broadleaf weeds to be concerned with, but Bull Thistle, Musk Thistle, and Canada Thistle seem to be most common. The good news is the process of controlling thistles often controls other broadleaf weeds.

Bull Thistle and Musk Thistle are biennials, which mean they take two years to complete their life cycle. They form a rosette (a flat group of leaves at ground level) and store food in their roots the first year and flower (produce seed) the second year. Control measures, chemical or mechanical, are most effective when applied during the first year's growth. If treatment is delayed until the second year, early season application of herbicide before bloom is important. In most cases you will have both years present in your pasture.

If you have only a minor problem with scattered plants, mechanical control can be effective. The rosettes are too generally too low to be mowed effectively, so digging the first year plants is your most dependable method. The second year growth can be mowed, but multiple trips will be needed to successfully prevent the thistles from producing flowers. Once you have flowers, you have seed. As a perennial, Canada Thistle can be a tougher weed to deal with. It not only produces seeds, it also spreads by underground rhizomes.

If you chose to use herbicide control, a number of choices are available. I counted fourteen options in the Grazing Restriction Table (page 41) in the U of M Extension Publication Plants Commonly Found in Established Minnesota Horse Pastures. Check it out at click on Agriculture, than Horses. Horse pastures have the same weeds as cow, sheep, and goat pastures.

Anytime you use herbicides reading the label is a must. The label will list any precautions and grazing limitations for milk and meat animals. However, many labels do not list horses. Extension Educator Krishona Martinson suggests horses should be excluded for seven to ten days after spraying.

This is another good argument for splitting pasture into multiple paddocks, not only will you increase grazing productivity, you have an opportunity to control weeds in each paddock When the animals are rotated out of a current pasture into a new one use that opportunity to dig, mow, or spray your thistles.

If you are trying to maintain a legume in your pastures, be aware that any of the broadleaf herbicides will eliminate both alfalfa and clovers. Mechanical control or spot spraying will be your only alternatives.