Weed science requires good defense

Well, it never fails. We go out there with the perfect plans and plant the perfect pasture. In no time at all, undesirable plants find a way to grow with our crop.
Weed science. We used to say back in school that weed science held a lot of job security for us; weeds sprouting up in the wrong places are one of life’s certainties (along with death and taxes).
The longer our pasture soils remain somewhat undisturbed, the more biennial and perennial weeds we will see. Some weeds are simply annoying, some aggressively crowd out our forages, and some can be downright deadly to our livestock.
For this column, let’s discuss a couple of the principles of chemical weed control in pastures.
Identification. It sounds like common sense, but when a 2,4-DB application doesn’t seem to injure a Canada thistle all that much, we ought not be surprised, since Canada thistle isn’t even on the 2,4-DB label.
Picking the right chemicals and rates is pretty important. As with any herbicide, read the label all the way through.
Labels have lists of “labeled crops,” “weeds controlled,” and “weeds suppressed.” Make sure your pasture fills the bill in terms of both crops and weeds.
The labeled rates are a good guide to control, and reducing them by a large margin may reduce control by an equally large margin.
Do your weeds tend to reside in patches? Maybe a spot application of a cheaper, nonselective chemical (like glyphosate) would be preferable to spraying a large area with a more expensive selective chemical, saving time and money to boot.
Timing. Recently, I had a plant pathology professor relate some great wisdom to me: “If we don’t know much about the pest’s life, how can we expect to control it?”
During the pasturing season, I get at least three calls a week about chemical control failures of Canada thistle, and my first question to them (after “What and how much did you use?”) is “When did you spray it?”
If the answer is July or August, that’s part of the problem. You’ll be trying to kill a thistle when it’s not nearly as susceptible as it is right before and during budding (and that goes for most broadleaf perennial weeds).
Larger perennials and biennials are most vulnerable in the fall. For timing applications to perennial grass weeds, they are most susceptible to herbicide action from boot to seedhead growth stages.
Pick the right time and method, and our control dollars stretch further and are used more effectively.
Restrictions. Do take note of grazing restrictions some chemicals might have at specific application rates; that information is contained in the label. If you use two chemicals tank-mixed, use the longer of the two restrictions.
Good luck and happy grazing.
(The author is an Ohio State University Extension agriculture and natural resources educator in Wayne County. Questions or comments can be sent in care of Farm and Dairy, P.O. Box 38, Salem OH 44460.)