Every hay grower fights it: a field that started clean slowly filling with foxtail, crabgrass, thistle or buttercup until the horse customers wrinkle their noses and the feed value drops. Weeds steal moisture, nitrogen and sunlight from the grass or alfalfa you actually want, and a weedy bale is worth less the day it comes off the field. This guide covers weed control in hay fields and pastures the way it actually plays out — the cultural moves that do most of the work, which herbicides fit which situation, the label restrictions that trip people up, and what to do with weedy hay you have already baled. Grower experience is quoted verbatim from the hay forums, with the agronomy checked against university extension sources.
This is written for hay and forage producers running grass, alfalfa or mixed stands for round bales, small squares or grazing — not for lawn care. If you make or sell hay, this is your playbook.
Start here: the cheapest weed control is a thick stand
Before you price a jug of chemical, look at your management. Weeds are opportunists — they colonize the bare ground, thin spots and short stubble that a vigorous stand never leaves open. The growers who fight weeds least are usually the ones who feed, cut and rest their stands the best.
Three cultural levers do most of the work:
- Fertility and pH. Fertilize to soil test and keep lime up. A well-fed grass or alfalfa stand simply out-competes most weeds. Thin, hungry, acidic fields are where foxtail and buttercup take over.
- Cutting interval. Cutting on a tight schedule removes weed tops before they set seed and keeps the crop canopy closed. As one grower put it, "alfalfa can be fertilized well and cut on a less than 35 day interval to keep most weeds from becoming problems."
- Stand density and rest. Don't overgraze pastures to the dirt, and give hay ground a chance to thicken. Bare ground is an open invitation.
"Alfalfa can be fertilized well and cut on a less than 35day interval to keep most weeds from becoming problems."
— Jimmy Bartlett, on HayTalk
Soil pH is worth a special mention because it comes up constantly with foxtail. Foxtail thrives on acidic, low-fertility ground, so growers often reach for lime first — "Foxtail only thrives in acidic soils. Limestone is what you need," as one put it. But lime is a slow, partial fix, not a silver bullet, and another grower pushed back immediately: "My soil ph is just fine and I lime regularly. I still have bouts of foxtail." Treat pH correction as one leg of the stool, alongside fertility, cutting timing and — when needed — herbicide.
Know your enemy: the weeds hay growers fight most
The right control depends entirely on what you are fighting and whether it is a grass or a broadleaf. That distinction matters more than anything else, because a broadleaf herbicide won't touch foxtail, and a grassy-weed product won't touch thistle. Here are the usual suspects on the hay forums.
| Weed | Type | Where and when it shows up | Best control angle |
|---|---|---|---|
| Yellow & knotroot foxtail | Grass (annual / perennial) | Hot, late-summer regrowth; thin, acidic stands | Pre-emergent pendimethalin; tighten cutting; lime and feed the stand |
| Crabgrass | Grass (summer annual) | Warm season, short stubble, open ground | Pre-emergent, or quinclorac on small plants; crop competition |
| Quackgrass | Grass (perennial, rhizomes) | Cool-season, spreads underground | Very hard selectively; spot glyphosate during renovation |
| Canada & musk thistle | Broadleaf (perennial / biennial) | Pastures, field edges, overgrazed spots | Aminopyralid or 2,4-D at the rosette; mow before it seeds |
| Common milkweed | Broadleaf (perennial) | Alfalfa and grass hay, patchy | Hard to kill; spot-treat; it is toxic, so keep it out of horse hay |
| Buttercup | Broadleaf (winter annual / perennial) | Overgrazed, wet, acidic spring pasture | 2,4-D on early rosettes; fix drainage and fertility |
| Marestail, pigweed, waterhemp | Broadleaf (annual) | Disturbed ground, new seedings | Pre-emergent plus timely broadleaf; watch for glyphosate resistance |
Identify before you spray. A field that looks "weedy" from the road is often mostly one or two species, and knowing whether they are grasses or broadleaves tells you 80% of what herbicide will work.
Herbicides for grass hay and pasture
Here is the hard truth that surprises new hay growers: there are very few selective herbicides that kill a grassy weed inside a grass crop. Foxtail in orchardgrass or timothy is chemically similar to the crop, so your options are limited and timing-sensitive. A grower fighting foxtail in timothy heard it straight from his extension agent: "there were no selective herbicides for foxtail in cool season grasses." That is close to the reality, so grass-on-grass weed control leans hard on pre-emergents and cultural control.
Pre-emergent: stop the weed before it sprouts
Pendimethalin (Prowl H2O) is the workhorse. It is applied to an established stand before the weed seed germinates — typically early spring for foxtail and crabgrass — and it forms a barrier in the soil surface. Two practical points growers stress: it needs moisture to activate, and timing is everything.
"I plan on spraying max rate of Prowl H20 late March or April before the foxtail germinates. Also, prowl requires incorporation into the soil so spray just before a nice rain if possible."
— Missouri hay hauler, on HayTalk
That lines up with university guidance. University of Georgia Extension notes that Prowl H2O for forage-grass weed control should go on before weeds emerge and needs roughly 0.25 to 1 inch of rain within about 7 days to move it into the soil where germinating seeds are (UGA Extension). A split application — part in early spring and part after first cutting — often gives better season-long control than a single pass. The big catch: you cannot overseed grass into a treated field for many months, because the same chemistry that stops weed seeds will stop your new grass seed too. If a field is thin and you plan to reseed, you have to choose between fighting foxtail and thickening the stand that year.
Post-emergent: catch grassy weeds while they are tiny
Quinclorac (Facet L) can knock back crabgrass and foxtail after they emerge, but only when the weeds are small. Growers and weed scientists agree the window is narrow.
"The UK weed prof. says Facet-L will kill foxtail post-emergent. BUT!..you must hit it at 3" or less. If it is over 6" tall..'don't waste your time and money'."
— Edd in KY, on HayTalk
The lesson: scout early and spray small. Post-emergent grass herbicides lose efficacy fast as weeds mature, and a foxtail plant that is already heading is past saving for the season — mowing to stop seed set is your only move at that point. Product labeling for grass hay varies, so confirm your crop is on the label before you buy.
Herbicides in alfalfa and mixed stands
Alfalfa gives you more chemical options than grass hay because you can use products that kill grasses without hurting the legume. Growers commonly run pendimethalin (Prowl H2O) for pre-emergent foxtail and crabgrass control in established alfalfa, and imidazolinone products such as imazethapyr (Pursuit) or imazamox (Raptor) for a mix of small grass and broadleaf weeds. On Roundup Ready alfalfa, glyphosate becomes a clean, broad-spectrum option that a conventional stand could never tolerate.
One grower laid out the classic season-long plan for foxtail and crabgrass in an alfalfa/orchardgrass mix:
"Based upon your suggestion of Prowl early and then after first cut... I research Quinclorac and it seems it is especially useful against crabgrass... It seems that both are friendly to Alfalfa and OG."
— calico190xt, on HayTalk
Mixed grass-legume stands are the tricky case. Anything that cleans up broadleaf weeds will also hurt the clover or alfalfa you are trying to keep, and anything that kills grassy weeds risks the grass component. In a mixed stand, cultural control and spot-spraying usually beat a blanket application. Here is a quick reference for the products that come up most often:
| Product (active ingredient) | Targets | Where it fits | Watch-outs |
|---|---|---|---|
| Prowl H2O (pendimethalin) | Foxtail, crabgrass — pre-emergent only | Established grass hay & alfalfa | Needs rain to activate; no overseeding for months after |
| Facet L (quinclorac) | Crabgrass, foxtail — post-emergent | Labeled grass situations | Only works on weeds under ~3 inches; confirm crop is on label |
| 2,4-D / dicamba | Broadleaf weeds | Grass pasture & hay | Injures alfalfa/clover; watch drift onto neighbors |
| GrazonNext / DuraCor (aminopyralid blends) | Thistle, tough broadleaves | Grass pasture & hay | Aminopyralid carryover — see restrictions below |
| Pursuit / Raptor (imazethapyr / imazamox) | Small grass & broadleaf weeds | Alfalfa | Rotation restrictions; weeds must be small |
| Glyphosate | Broad-spectrum | Roundup Ready alfalfa; spot / renovation | Kills conventional stands; resistant weeds exist |
This table is a starting point, not a recommendation. Rates, approved crops and restrictions differ by state and product — your local extension weed guide and the product label are the final word.
Grazing and haying restrictions you must respect
The label is a legal document, and forage herbicides carry restrictions that can burn you if you ignore them. Two categories matter most:
- Grazing and harvest intervals. Many products let you graze soon after the spray dries but require a waiting period before you cut hay, and some add extra restrictions for lactating dairy animals. Check the interval for your class of livestock.
- Aminopyralid carryover. This is the big one. Aminopyralid (in GrazonNext, DuraCor and similar) is fantastic on thistle but persists through the animal — it stays active in hay, in manure, and in compost made from either. Treated hay and manure generally cannot be moved off the farm or used on gardens, because the residue will kill tomatoes, beans and other broadleaf crops. If you sell hay or manure, read the aminopyralid restrictions carefully before you spray (University of Maryland Extension).
For a broader species-by-species rundown of what is labeled where, University extension weed guides such as Virginia Tech's Control of Common Grassy Weeds in Pastures and Hayfields are the authoritative reference and are updated regularly.
What to do with weedy hay you already baled
Sometimes the weeds win a cutting and you are staring at bales you can't sell as premium horse hay. Don't dump them — weedy hay still has value if it is dry and safe. Forum growers reliably move it three ways:
- Sell it as goat, sheep or "cheap cow" hay. As long as the grass is edible and the bale is safe, someone will feed it. Growers routinely price rough grass hay at a few dollars a bale, load-your-own.
- Grind it into a ration. "Any beef feedlots around? Run that junk hay through a tub grinder, add it to a TMR and it will get eaten right up," one grower advised — a mixer wagon hides a lot of weeds.
- Sell it as bedding or erosion mulch. Dry, stemmy, weedy hay beds livestock well, and grading and erosion-control contractors buy it to blow on bare ground.
"Any beef feedlots around? Run that junk hay through a tub grinder, add it to a TMR and it will get eaten right up."
— Gearclash, on HayTalk
One safety note that always comes up in these threads: if the weedy hay went up a little tough or green, watch the internal temperature. Wet bales can heat, mold and in bad cases catch fire, so monitor a stack of marginal hay before you store it tight. (See our guide on hay bale heating and fire prevention.)
A simple weed-control calendar
Weed control is a season-long rhythm, not a one-time spray. Here is a practical checklist you can run every year:
- Fall: Soil test. Line up lime and fertilizer. Scout for winter-annual broadleaves (buttercup rosettes) — a fall or dormant-season broadleaf spray can be very effective while the crop is dormant.
- Late winter / early spring: Apply lime and fertilizer. Put pre-emergent (pendimethalin) on before foxtail and crabgrass germinate, timed just ahead of rain.
- Spring green-up: Scout weekly. Spot-spray small grassy weeds while they are under 3 inches; hit broadleaf rosettes at the right stage.
- After first cutting: Consider the second half of a split pre-emergent. Keep the cutting interval tight so the canopy stays closed.
- Late summer: Watch for a foxtail flush in hot, dry regrowth. Mow to stop seed set on anything you can't spray.
- Any time: Fix the bare spots. Overseed thin areas (outside the pre-emergent window) so weeds have nowhere to move in.
Frequently asked questions
How do I get rid of foxtail in a hay field?
Stop it before it sprouts. Apply a pre-emergent like pendimethalin (Prowl H2O) in early spring before the foxtail germinates, timed just ahead of rain so it activates. Post-emergent quinclorac can catch foxtail only while it is under about 3 inches. Long term, lime and feed the stand and tighten your cutting interval so the crop crowds it out.
What herbicide is safe on grass hay?
Broadleaf products such as 2,4-D and aminopyralid blends are safe on established grass hay and clean up thistle, buttercup and similar weeds. Killing a grassy weed inside grass hay is much harder — pre-emergent pendimethalin is the main tool. Always confirm your specific grass is on the product label first.
Will lime by itself get rid of weeds?
Not by itself. Correcting low pH with lime makes your grass more competitive and discourages acid-loving weeds like foxtail, but growers who lime regularly still get weed flushes. Treat lime as one part of a program that also includes fertility, cutting timing and, when needed, herbicide.
Can I spray a brand-new seeding for weeds?
Carefully. Most pre-emergents and many post-emergents can injure seedlings, and pre-emergents block your new grass or alfalfa seed from establishing. On new seedings, lean on mowing above the seedlings and use only herbicides specifically labeled for new stands at the right growth stage.
Is weedy hay safe to feed?
Usually yes, if the hay is dry and free of toxic weeds. Cattle, goats and sheep will eat most grassy, weedy hay, and grinding it into a mixed ration hides the stems. Avoid feeding hay heavy with toxic plants such as milkweed, and never feed moldy or overheated bales.
The bottom line
Good weed control in hay fields and pastures is 80% management and 20% chemistry. Keep the stand thick, fed and cut on time, identify whether you are fighting grasses or broadleaves, pick the herbicide that matches, and respect the grazing and haying restrictions on the label. Do that and you protect both your yield and the price your hay commands. And when you have grown a clean, high-value crop, finish the job right: bale it with quality bale net wrap that keeps a tight, weather-resistant package so the hay you worked all season to keep clean doesn't spoil in storage.
Grow it clean, keep it clean. Explore XES bale net wrap for a tighter, better-protected bale — or start with a trial roll and see the difference on your next cutting.
Related reading: Fertilizing hay fields · When to cut alfalfa · Alfalfa weevil control · Establishing an alfalfa stand · Tall fescue hay guide · How to sell hay
Sources: University of Georgia Extension, Using Prowl H2O for Forage Grass Weed Control (2025); University of Maryland Extension, Fall Weed Management in Pasture and Hay (2025); Virginia Tech Extension, Control of Common Grassy Weeds in Pastures and Hayfields (SPES-58); grower experience via HayTalk, threads 100382, 101659 and 100252. Accessed July 2026. Always follow the product label and your state extension weed guide.
Featured image: "Setaria pumila" (yellow foxtail) by Krzysztof Ziarnek, Kenraiz, licensed CC BY-SA 4.0, via Wikimedia Commons.
