The weed issues that you need to address in the fall often depend on which weeds you have, and the growing conditions during summer, according to Dr. Bob Coleman, State Extension Specialist at the University of Kentucky. “Check and see what have, and plan for the most opportune time to control those particular weeds," said Coleman. "You need to go after annual weeds in the spring. Some of the perennial weeds can be controlled late in the year, like October-November, if your winter is still open.”
Know the life cycle of the plants you want to control in order to know when they grow and when they reproduce. If you plan to use herbicide on annual weeds, they should be sprayed when they are still young and actively growing, not when they are already mature. Perennial weeds, by contrast, can often be controlled by clipping to prevent them from going to seed, then sprayed when they regrow later in the year and are moving nutrients down into their roots for winter. Then you can get more herbicide into the root system. Fall is when those perennial plants are moving nutrients into the roots rather than bringing food up from the roots.
Timing is crucial when you want to go after a certain weed. You have to know when herbicides will have the most effect. “It depends on the seasonality of where you live," said Coleman. "Horse owners should be talking to their state extension people to know what the weeds and their cycles are in their areas and the best way to go after them. Some weeds, like thistles, can be handled by mowing. Timely mowing might reduce the growth and keep the unwanted plants from going to seed.” Mowing can keep most weeds from spreading and might eventually reduce their numbers.
“Short weeds like dandelion can’t be controlled by mowing because they are not tall enough," noted Coleman. "If certain weeds are a problem in your pastures, consult with your state extension service and ask what is available for control and which weeds you should attack in the fall. It will partly depend on growing conditions that particular year. If it’s hot and dry and nothing is growing in October, save your chemicals for a different time because the plants have to be actively growing for herbicide to kill them.
“Be judicious in use of chemicals, so you won’t be wasting your money or making a problem worse," said Coleman. "Read labels and use what is appropriate. If the label says to remove the horses for a certain length of time after you’ve sprayed a pasture, you need to do that."
Coleman said that some weeds are really hard to kill: "Spiny pigweed, for instance, is very hardy and it seems like the more you mow it, the worse it gets. Often the best strategy for fall weed control is fall fertilization—get the grass to grow better and stronger so it will be vigorous and out-compete the weeds. If you can minimize/eliminate areas of bare ground, there will be less chance for weeds to come in.
Sometimes grazing a pasture with other species (rotating the horses with sheep, goats or cattle) is a good way to control many types of weeds because ruminants will eat many plants that horses won’t eat, thus keeping them controlled so they don’t go to seed. “If you don’t have sheep or goats, you might know someone who does and who could bring their flock in for a few days to eat down the problem areas,” Coleman said.