Several herbicides are labeled for use in alfalfa, and the best choice depends on whether you are targeting grasses, broadleaf weeds, or both, and whether your stand is newly seeded or well established. The most widely used options include imazethapyr and 2,4-DB for broadleaf weeds, fluazifop-p and sethoxydim for grasses, and glyphosate if you are growing a Roundup Ready variety. Getting the selection and timing right matters more than most growers realize, because weedy alfalfa can lose a staggering amount of feed value in the first cutting alone.
Post-Emergence Herbicides for Broadleaf Weeds
When broadleaf weeds are already up and competing with your alfalfa, the go-to sprays in conventional (non-Roundup Ready) stands have been 2,4-DB and imazethapyr. Research screening large numbers of herbicides has consistently identified those two, along with prometryn and imazapic, as safe and effective against broadleaf competitors in alfalfa.1Agronomy. Optimizing Reduced-Dose Post-Emergence Herbicide Tank Mixtures for Broad-Spectrum Weed Control and Sustainable Alfalfa Production Bentazon is another post-emergence broadleaf option that has shown good crop safety across alfalfa cultivars and strong weed suppression when paired with a pre-emergence product.2PubMed Central. Combined Pre- and Post-Emergence Herbicides for Effective Weed Control in Alfalfa (Medicago sativa L.)
A word of caution on 2,4-DB: while it is one of the oldest and most trusted alfalfa herbicides, it can cause visible crop injury to seedling stands. In trials comparing it to glyphosate in glyphosate-tolerant alfalfa, 2,4-DB plus sethoxydim caused the most visual injury and lowered initial yields at every location tested.3Agronomy Journal. Influence of Seeding Rate and Herbicide Treatments on Weed Control, Yield, and Quality of Spring‐Seeded Glyphosate‐Resistant Alfalfa Established stands tolerate it much better, so growers spraying young alfalfa should keep rates conservative and watch for leaf curl or stunting in the days after application.
Post-Emergence Herbicides for Grasses
Grassy weeds such as foxtail, crabgrass, and volunteer cereals call for a different chemistry. Fluazifop-p (commonly sold as Poast Plus or Fusilade) and sethoxydim (Poast) are the standard grass-selective sprays for conventional alfalfa. These products kill grasses without harming broadleaf plants like alfalfa. In greenhouse and field trials, fluazifop-p showed high alfalfa survival rates even at four times the standard dose, putting it among the safest post-emergence options tested.2PubMed Central. Combined Pre- and Post-Emergence Herbicides for Effective Weed Control in Alfalfa (Medicago sativa L.)
If you are dealing with a mix of grasses and broadleaves at the same time, you will likely need to tank-mix a grass killer with a broadleaf product or apply them in sequence. No single conventional herbicide does a great job on both weed types simultaneously, which is part of why Roundup Ready alfalfa became so popular so quickly.
Pre-Emergence Herbicides and Why Sequencing Matters
Spraying weeds after they emerge is only half the battle. Pre-emergence herbicides applied before or just after planting create a chemical barrier in the soil that prevents weed seeds from germinating in the first place. In alfalfa, prodiamine and s-metolachlor are two pre-emergence options that have demonstrated good crop safety. Of these, prodiamine followed by a post-emergence broadleaf or grass spray has proven especially effective in research trials.
When prodiamine was applied before emergence and then followed up with either bentazon or fluazifop-p after the alfalfa and weeds came up, weed biomass in the plots dropped by 70 to 85%. That was far better than any single herbicide applied alone, which managed only about a 24 to 48% reduction. The sequential approach did not hurt alfalfa plant height or yield compared to hand-weeded control plots.2PubMed Central. Combined Pre- and Post-Emergence Herbicides for Effective Weed Control in Alfalfa (Medicago sativa L.) Early research dating back decades supports the same general principle: combining pre-emergence products like EPTC or neburon with post-emergence sprays like 2,4-DB delivered broader weed control than any single herbicide.4Agronomy Journal. Pre‐emergence, Preplanting and Postemergence Herbicides for Alfalfa and Birdsfoot Trefoil
The takeaway for growers is that relying on a single spray pass often leaves too many weeds behind, especially in new seedings where the alfalfa canopy has not yet closed. A two-step program, one product in the soil and one over the top, consistently outperforms a one-and-done approach.
Glyphosate in Roundup Ready Alfalfa
If you planted a glyphosate-tolerant (Roundup Ready) variety, glyphosate is the simplest and most flexible weed-control option. It kills virtually all emerged grasses and broadleaves in a single pass, which removes the need to tank-mix separate grass and broadleaf herbicides. Field trials spraying glyphosate before each of four harvests at rates ranging from 0.75 to 3 pounds of acid equivalent per acre found no meaningful crop injury during the establishment year or the two years that followed. Even an accumulated dose of 9 pounds per acre over three years caused no reduction in alfalfa yield or nutritive value at any cutting.5Forage & Grazinglands. Evaluating Glyphosate Treatments on Roundup Ready Alfalfa for Crop Injury and Feed Quality
Long-term data on established Roundup Ready stands confirms this durability. Over a seven-year period, forage yield was not hurt by herbicide treatments, and stand density declined at a natural rate regardless of how often the field was harvested or sprayed.6Weed Technology. Glyphosate-Resistant Alfalfa Response to Harvest Frequency and Weed Management Annual yields with glyphosate tended to be similar to or higher than conventional herbicide programs, and forage crude protein content was at least as good as with other treatments.3Agronomy Journal. Influence of Seeding Rate and Herbicide Treatments on Weed Control, Yield, and Quality of Spring‐Seeded Glyphosate‐Resistant Alfalfa
The catch is that repeated glyphosate-only management can select for resistant weed populations. Palmer amaranth, already one of the most aggressive weeds in warm-season agriculture, has developed glyphosate-resistant biotypes in Roundup Ready alfalfa fields.7Nativa. BORON NUTRITION AND HERBICIDE STRATEGIES IN MANAGING GLYPHOSATE-RESISTANT PALMER AMARANTH IN Roundup Ready® ALFALFA Rotating modes of action or mixing in a conventional herbicide periodically is the best insurance against losing glyphosate effectiveness in your stand.
Herbicides to Avoid or Handle With Care
Not every herbicide that kills weeds in other crops is safe to spray on alfalfa. In oat-alfalfa interseedings, post-emergence applications of pyridate, thifensulfuron, and tribenuron were too injurious to the alfalfa, the companion oat crop, or both.8Weed Technology. Weed Control in Oat (Avena sativa)-Alfalfa (Medicago sativa) and Effect on Next Year Corn (Zea mays) Yield Even herbicides with generally good safety profiles, like nicosulfuron, can cause significant growth inhibition and biomass loss at higher doses, so staying within label rates is critical.2PubMed Central. Combined Pre- and Post-Emergence Herbicides for Effective Weed Control in Alfalfa (Medicago sativa L.)
Selectivity in alfalfa often comes down to the plant’s ability to break down the herbicide before it does lasting damage. That metabolic race is why the same product can be perfectly safe at one rate and harmful at twice that rate, and why environmental stress (drought, cold nights, insect pressure) can tip the balance toward crop injury even with a normally safe herbicide. If your alfalfa is under stress, it is usually better to delay spraying a day or two until conditions improve rather than risk stunting a weakened crop.
Timing Around Dormancy
Established alfalfa stands in winter-dormancy regions offer a unique spraying window. Applying herbicides while the alfalfa crown is dormant but winter annual weeds like downy brome and cheatgrass are actively growing lets you hit weeds hard with products that would hurt actively growing alfalfa. Research in this space found that simazine, pronamide, secbumeton, and metribuzin applied in early February effectively removed weedy brome species and increased alfalfa forage yields.9Weed Science. Controlling Weeds in Dormant and Nondormant Alfalfa
Timing is everything here. Some of those same herbicides become problematic once the alfalfa breaks dormancy. Metribuzin, terbacil, hexazinone, and simazine all significantly reduced alfalfa yields when applied after green-up, even to weed-free stands. Pronamide was the exception, causing no yield loss when sprayed after dormancy ended. Diuron, terbacil, and hexazinone applied in late March, before full canopy expansion, also showed no adverse yield effects in the same trials.9Weed Science. Controlling Weeds in Dormant and Nondormant Alfalfa The practical lesson: if you plan a dormant-season application, get it done while the alfalfa is truly dormant. Once you see new growth at the crown, that window has closed for most products.
Why Weed Control Has Such a Big Effect on Forage Value
Growers sometimes wonder whether spraying is worth the cost, especially in established stands where the alfalfa looks thick enough to compete. The answer almost always is yes, because weeds do not just reduce tonnage; they wreck the nutritive value of whatever you do harvest. In spring-seeded alfalfa with poor or no weed control, weed biomass accounted for roughly a quarter to over half of the total harvested biomass at first cutting. That weed-contaminated forage had lower crude protein, higher fiber, and a reduced relative feed value compared to clean stands.10Agronomy Journal. Weed control and weed biomass influenced first cutting forage accumulation and nutritive value of spring‐seeded alfalfa
Herbicide-treated alfalfa, by contrast, generally has higher crude protein than untreated controls. In one multi-location trial, every herbicide program tested produced forage with more protein than the weedy check, and in vitro digestibility ranged from 750 to 860 grams per kilogram of dry matter across all treatments with few differences among them.3Agronomy Journal. Influence of Seeding Rate and Herbicide Treatments on Weed Control, Yield, and Quality of Spring‐Seeded Glyphosate‐Resistant Alfalfa Separate work on combined herbicide strategies showed that the best treatments maximized fresh fodder yield while pushing crude protein above 20% and weed-control efficiency near 70%.11Turkish Journal of Agriculture – Food Science and Technology. Synergistic Herbicide Applications Enhance Alfalfa (Medicago sativa L.) Yield, Weed Suppression and Forage Quality When you price hay by relative feed value rather than by the ton, the economic return on a well-timed herbicide pass can be substantial.
Choosing a Program for New Seedings vs. Established Stands
New seedings are far more vulnerable to both weed competition and herbicide injury than mature stands. The alfalfa seedlings are small, their root systems are shallow, and weeds that germinate at the same time can shade them out within weeks. This is where a sequential herbicide program pays the biggest dividends. Applying a pre-emergence product like prodiamine at planting, then coming back with a post-emergence broadleaf or grass spray once the weeds and alfalfa have emerged, gives you layered protection without piling too much chemistry onto a young crop at once.
For conventional new seedings, a common approach is a pre-emergence soil-active herbicide followed by 2,4-DB or imazethapyr once broadleaves appear, with fluazifop-p or sethoxydim added if grasses are present. For Roundup Ready seedings, a single glyphosate pass once the alfalfa has at least two trifoliate leaves usually handles both weed types. In either system, the goal is to keep the canopy as clean as possible through the first cutting so the alfalfa can establish a dense crown and start shading out future weed flushes on its own.
Established stands are more forgiving. The thick canopy itself suppresses many weeds, and the perennial root system can tolerate moderate herbicide stress. Dormant-season applications become an option, and spot spraying problem patches rather than blanket-treating the whole field can save money. Even so, stands that have thinned from age, winterkill, or insect damage often need a full herbicide pass in spring to prevent weeds from colonizing the gaps.
How Herbicide Selectivity Works in Alfalfa
You might wonder how a chemical can kill a weed standing inches away from an alfalfa plant of roughly the same size. In most cases, it comes down to metabolism. Many of the herbicides used in alfalfa belong to a class called ALS inhibitors, which block an enzyme involved in making certain amino acids. Both the weed and the alfalfa absorb the herbicide through their leaves and roots, and it moves through the plant’s vascular system. The difference is that alfalfa can break the herbicide molecule down into harmless byproducts before it accumulates enough to cause damage, while susceptible weeds cannot.12Planta Daninha. Herbicide selectivity in alfalfa crops Grass-selective herbicides like fluazifop-p work on a different principle: they target an enzyme found only in grasses, so broadleaf crops are inherently immune.
This metabolic selectivity is why label rates exist and why exceeding them is risky. At the recommended rate, the alfalfa’s detoxification machinery keeps pace. Double or quadruple the dose, and the plant’s enzymes can be overwhelmed, leading to growth inhibition even in a normally tolerant crop. Environmental stress slows the plant’s metabolism further, which is another reason stressed alfalfa is more prone to herbicide injury.
Putting Together a Rotation of Modes of Action
Whether you grow conventional or Roundup Ready alfalfa, rotating herbicide modes of action from year to year (or even cutting to cutting) is the best way to keep resistance from developing. A practical rotation for a Roundup Ready stand might be glyphosate for the first two cuttings, then a conventional broadleaf product like imazethapyr or 2,4-DB later in the season when weed pressure is lower. For conventional stands, alternating between ALS-inhibiting herbicides and non-ALS options such as 2,4-DB or bentazon achieves the same goal.
Cultural practices amplify whatever herbicide program you choose. Higher seeding rates close the canopy faster, cutting into the light that weed seedlings need. Timely harvest before weeds set seed reduces the soil seed bank over time. And maintaining soil fertility and proper pH keeps the alfalfa competitive, which is ultimately the best long-term weed suppression strategy of all. Herbicides are the sharpest tool in the shed, but they work best as part of a system rather than as a standalone solution.