Getting rid of johnsongrass in a hay field requires a sustained campaign, not a single treatment. This perennial weed spreads both by underground rhizomes and by seed, so killing the visible shoots barely slows it down. The most effective programs combine selective herbicides, well-timed mowing, and sometimes a full field renovation where you kill everything and replant. Complicating matters further, some johnsongrass populations have developed resistance to commonly used herbicides, which means the chemical that worked five years ago may no longer do the job.
Why Johnsongrass Is So Hard to Kill
Johnsongrass (Sorghum halepense) earned its reputation as one of the most troublesome weeds in warm-season agriculture for good reason. The plant’s rhizomatous root structure, prolific seed production, and seed viability make it extraordinarily persistent once established.1Texas Tech University Institutional Repository. Herbivory and control of pesticide-resistant Johnsongrass found in Texas A single plant can produce tens of thousands of seeds in a season, and those seeds remain viable in the soil for years. Meanwhile, the rhizomes creep outward underground, sending up new shoots even after you mow or spray the existing growth.
Johnsongrass also has a chemical trick up its sleeve. The plant releases allelopathic compounds into the soil that suppress the germination and growth of surrounding plants. Research has shown that johnsongrass extracts inhibit the germination, fresh weight, and root length of both crops and other grasses, with effects more severe than those of bermudagrass.2Agronomy Journal. Allelopathic Potential of Bermudagrass and Johnsongrass and Their Interference with Cotton and Corn In a hay field, this means johnsongrass doesn’t just compete for space, water, and sunlight. It actively poisons the soil chemistry around it, weakening your desirable forage grasses and making reinfestation easier after any control attempt.
Understanding these three survival strategies together explains why a single mowing or a single herbicide pass rarely works. You have to attack the rhizomes, deplete the seed bank, and keep enough healthy forage growing to resist the allelopathic pressure. That takes planning over multiple seasons.
Selective Herbicide Options for Hay Fields
Your herbicide choices depend on what forage species make up the rest of your hay field. This is the central constraint that makes hay fields harder to treat than row crops. In a cornfield, you can use herbicides labeled for corn that kill johnsongrass selectively. In a mixed-grass hay field, the lines between “weed grass” and “desirable grass” blur, because johnsongrass is a grass too.
If your hay field is a warm-season grass like bermudagrass, you have a few selective options. Nicosulfuron and some other ALS-inhibiting herbicides have been widely used against johnsongrass in various cropping systems. However, the window of effectiveness is narrow: these products work best on young, actively growing johnsongrass, generally when shoots are under about 12 to 18 inches tall. Once the plants mature and the rhizome network is well established, foliar-applied herbicides become far less effective because they do not translocate deeply enough into the root system.
Glyphosate remains the most reliable option for killing established johnsongrass rhizomes, but it is non-selective, meaning it kills your forage too. This limits its use to spot treatments or full-field renovation, which we’ll get to below. For spot spraying, a backpack sprayer or a wiper applicator that physically touches the taller johnsongrass without contacting the shorter forage can work in fields where the infestation is patchy rather than uniform.
In fields with cool-season forages like tall fescue or orchardgrass, some graminicides (grass-specific herbicides) can selectively remove johnsongrass. Products containing sethoxydim or clethodim target grasses through the ACCase enzyme pathway. Clethodim in particular has shown effectiveness against johnsongrass, including populations that have become resistant to other herbicide classes.3Weed Technology. Target-site cross-resistance to ALS inhibitors in johnsongrass originating from Greek cornfields The catch is that these ACCase-inhibiting herbicides will also injure or kill warm-season grasses in the stand, so they work only if your desirable forage is a cool-season species or a broadleaf like clover or alfalfa.
Herbicide Resistance Is Already Here
If you have been spraying ALS-inhibiting herbicides (the sulfonylurea family, including nicosulfuron, rimsulfuron, and foramsulfuron) for several years and the johnsongrass keeps coming back, resistance is a real possibility. Research on johnsongrass populations collected from fields with long histories of ALS herbicide use has confirmed high-level cross-resistance to multiple ALS inhibitors. One study found that resistant populations were not controlled by foramsulfuron, nicosulfuron, rimsulfuron, or imazamox at their recommended field rates.3Weed Technology. Target-site cross-resistance to ALS inhibitors in johnsongrass originating from Greek cornfields The resistance traced to a single-point mutation in the ALS gene, meaning a small genetic change gave the weed full protection against an entire class of chemicals.
The practical takeaway is straightforward. If your ALS herbicides are not performing the way they used to, do not respond by increasing the rate or making more passes. You are selecting for resistant plants and making the problem worse. Switch to a different mode of action. ACCase-inhibiting herbicides like clethodim and propaquizafop effectively controlled the same resistant populations that shrugged off ALS inhibitors.3Weed Technology. Target-site cross-resistance to ALS inhibitors in johnsongrass originating from Greek cornfields Rotating herbicide modes of action is no longer optional advice; it is a necessity if you want chemical control to keep working over multiple seasons.
Interestingly, the resistant johnsongrass plants themselves did not appear to have any obvious growth penalty compared to susceptible plants. Research comparing resistant and susceptible populations found no statistically significant differences in aboveground biomass or tiller number.4PubMed Central. Comparison of Molecularly Identified Resistant and Susceptible Johnsongrass (Sorghum halepense L.) Populations at ALS Gene, in the Absence and Presence of Field Crops That means resistant plants are just as vigorous as regular johnsongrass. They will not die out on their own once herbicide pressure is removed. Once resistance is in your field, it stays.
Mowing as a Management Tool
Mowing alone will not eradicate johnsongrass, but strategic mowing can weaken it substantially over time and prevent the seed bank from growing. The key is frequency and timing. Johnsongrass stores energy in its rhizomes to fuel regrowth after each cutting. If you mow often enough that the plant never fully restores those energy reserves, the rhizome network gradually shrinks and weakens. This typically means mowing every two to three weeks during the growing season, which is more aggressive than most hay-cutting schedules.
For hay producers, this creates a tension. You want to let your desirable forage reach a certain maturity before cutting so you get decent yield and quality per cutting. Johnsongrass, on the other hand, benefits from those longer intervals. A practical middle ground is to time your hay cuttings to coincide with the stage just before johnsongrass produces seed heads. This prevents seed production while still allowing reasonable forage growth. In the Deep South and similar climates, johnsongrass can produce seed heads as quickly as six to eight weeks after a cutting, so a four- to six-week cutting interval works better than the typical two-cut or three-cut schedule many hay producers follow.
If your field has a heavy infestation and you can afford to sacrifice a season of hay production, mowing every two to three weeks through the entire growing season, combined with herbicide applications to weakened regrowth, is one of the more effective non-renovation approaches. The frequent defoliation drains the rhizomes while the herbicide finishes off stressed plants that are struggling to recover.
Tillage and Field Renovation
For severely infested fields, the fastest path to a clean stand is a complete renovation. This means killing the entire field, working the soil, and replanting your forage from scratch. It is expensive and you lose a season or more of production, but it is sometimes the only realistic option when johnsongrass makes up a significant percentage of the stand.
A typical renovation program starts with one or two applications of glyphosate in late summer or early fall, when johnsongrass is actively moving sugars down into its rhizomes for winter storage. Spraying at this time means the herbicide rides the plant’s own transport system deep into the root network. Wait two to three weeks, then assess. If green shoots are regrowing, spray again. Once the field is thoroughly dead, you can either till or no-till drill your new forage.
Tillage has a complicated relationship with johnsongrass. Disking or plowing can chop rhizomes into pieces, and each piece is capable of sprouting a new plant. However, repeated tillage in hot, dry weather can desiccate those fragments before they resprout. The sequence matters: herbicide first to kill the living rhizomes, then tillage to bring fragments to the surface and dry them out, then another herbicide pass on any survivors before planting.
If you are replanting with a competitive warm-season grass like bermudagrass, the thick sod that bermudagrass forms once established creates significant competition against johnsongrass seedlings trying to emerge from the soil seed bank. Bermudagrass will not eliminate the problem on its own, but it gives you a forage stand that can hold its ground far better than a thin or open stand. Cool-season grasses, which go dormant or thin out in summer heat, leave more room for johnsongrass to exploit.
Prussic Acid and Hay Quality Concerns
Any conversation about johnsongrass in a hay field has to address toxicity. Johnsongrass, like its relative grain sorghum, can accumulate dangerous levels of hydrogen cyanide (prussic acid) in its tissues, particularly in young regrowth after cutting, after a frost, or during drought stress. Prussic acid poisoning can be fatal to cattle and other livestock.1Texas Tech University Institutional Repository. Herbivory and control of pesticide-resistant Johnsongrass found in Texas
For hay specifically, the risk is lower than for fresh grazing because prussic acid is a gas and dissipates during the drying and curing process. Hay that is properly dried before baling generally does not retain dangerous levels of cyanide. The danger is greater with green-chopped forage, silage, or if cattle graze a field of fresh johnsongrass regrowth directly. Still, if your hay field has significant johnsongrass contamination, you should be cautious about harvesting during drought stress or cutting very young regrowth that has not had time to mature past the high-cyanide stage.
Beyond the toxicity issue, johnsongrass contamination lowers the market value of hay. Buyers of quality horse hay or dairy hay do not want johnsongrass in the bale, even if the nutritional content is not terrible. The plant is classified as a noxious weed in many states, and heavy contamination can make hay unsellable to discriminating buyers or even trigger regulatory problems when transporting hay across state lines. Research has confirmed that the species causes substantial economic losses in both row crops and hay fields.5PubMed Central. The impact of maturity stages on yield, quality, and nutritive value of ensiled Johnsongrass [Sorghum halepense (L.) Pers]
Preventing Reinfestation After Control
Even after a successful control program, the johnsongrass seed bank in your soil can keep producing new plants for years. Seeds buried just a few inches deep may remain dormant for a decade or more, germinating when tillage or soil disturbance brings them closer to the surface. This is why so many producers feel like they are fighting the same battle every year: they killed the rhizomes, but the seeds keep filling in the gaps.
A few practical habits reduce reinfestation pressure. First, keep your forage stand thick and competitive. A dense bermudagrass or tall fescue stand leaves very little bare soil for johnsongrass seedlings to establish. Fertility and soil pH management for your forage species is indirectly a weed management tool. Second, scout your field borders. Johnsongrass commonly invades from roadsides, fence rows, and ditch banks. If you clean up the field but ignore the perimeter, rhizomes creep back in from the edges within a couple of years. Third, inspect hay purchased from outside sources before feeding or spreading it on your property. Johnsongrass seed is a common contaminant in hay from infested regions, and one contaminated load can reintroduce the weed to a field you spent years cleaning up.
Herbicide-treated fence rows and a mowed buffer strip around the field perimeter create a barrier that slows reinvasion. Some producers maintain a ten- to fifteen-foot sprayed strip along fence lines specifically to prevent perennial weeds from encroaching back into managed pasture or hay ground.
Biological Control and What the Future Looks Like
Chemical and mechanical control remain the backbone of johnsongrass management, but researchers have been working on biological alternatives for decades. One promising line of work involves using fungal pathogens as bioherbicides. Greenhouse trials tested a mixture of three fungi originally isolated from weedy grasses, including one from johnsongrass itself. The mixture achieved 100% disease incidence on johnsongrass and six other grass weed species, with disease severity ranging from 83 to 100% within two weeks of inoculation. In host-range testing across 52 plant species, none of the nontarget species were harmed.6Biological Control. Control of Seven Grasses with a Mixture of Three Fungal Pathogens with Restricted Host Ranges
Those results are from controlled greenhouse conditions, and the leap from greenhouse to field is large. Fungal bioherbicides need specific moisture and temperature conditions to establish infections, and field environments are far less predictable than a climate-controlled greenhouse. No commercial bioherbicide product is currently available that reliably controls johnsongrass in a production hay field. Still, the fact that these pathogens showed both high efficacy against target grasses and safety for nontarget species suggests the concept has legs. If a commercial formulation eventually reaches the market, it could offer hay producers a tool that avoids the chemical resistance problems plaguing conventional herbicides.
Grazing management is another area attracting research attention. Some work is exploring whether strategically timed grazing by livestock could reduce johnsongrass density while minimizing prussic acid exposure risks.1Texas Tech University Institutional Repository. Herbivory and control of pesticide-resistant Johnsongrass found in Texas The idea is that cattle or goats, managed to graze johnsongrass at the right growth stage, could serve as a living mower that preferentially defoliates the weed. The research is ongoing, and producers who run cattle on their hay ground in the off-season may eventually be able to incorporate targeted grazing rotations as part of their management program. For now, the caution around prussic acid means this approach requires careful planning and likely some form of forage testing before turning animals onto a johnsongrass-heavy field.
Putting Together a Multi-Year Plan
No single tool solves a johnsongrass problem in one season. The most successful programs layer multiple tactics across two to three years. A reasonable starting template for a heavily infested warm-season hay field looks something like this:
- Year one, late summer: Apply glyphosate or an appropriate selective herbicide when johnsongrass is actively growing and at least 18 inches tall. If using glyphosate, you are sacrificing the rest of that season’s hay production. Follow up with a second application two to three weeks later on any regrowth.
- Year one, fall: Till to expose and dry rhizome fragments, or no-till drill a competitive forage species if renovation is part of the plan. Apply a pre-emergent herbicide where labeled and practical.
- Year two, spring and summer: Scout aggressively for new seedlings and rhizome regrowth. Spot-spray with a selective herbicide or glyphosate on a wiper applicator. Mow frequently enough to prevent any surviving johnsongrass from producing seed.
- Year two onward: Maintain forage stand density, manage fertility to favor your desired species, and keep field borders clean. Switch herbicide modes of action if you see signs of resistance.
The specifics will vary by region, forage species, and the severity of the infestation. Your local extension office can help you select herbicide products labeled for your situation and adjust timing to your climate. The underlying logic, though, stays the same everywhere: deplete the rhizomes, prevent seed production, establish competitive forage, and stay vigilant for the comeback that the seed bank will attempt for years to come.