How to Thicken a Thin Hay Field: Overseed, Interseed, or Start Over?

By the XES Netting team — bale net-wrap manufacturer · Updated August 5, 2026 · ~14 min read

Every hay producer runs into the same field eventually. It made good tonnage three years ago, it is making two-thirds of that now, and the weeds are moving into the gaps. The question is always the same: can I fix this with a bag of seed and a drill, or am I just throwing money at a field that needs to be torn up?

The answer depends almost entirely on two things — what species is thin, and how thin it actually is. This guide walks the decision the way working hay producers argue it out, with the numbers that settle it, links to the original discussions, and the extension research that explains why the single most common "fix" is also the one most likely to fail.

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Step one: count the stand, don't eyeball it

A thin field always looks worse from the pickup than it is, and a mediocre field always looks better than it is. Before you spend money, get out and count. There are three counts worth doing, and each answers a different question.

Plants per square foot — is the stand still viable?

Drop a square-foot frame in several representative spots and count crowns. Dr. Dan Undersander of the University of Wisconsin lays out the benchmarks in "Is the Alfalfa Stand Worth Keeping?" (National Alfalfa & Forage Alliance, September 2018, accessed August 5, 2026): producers seed 60 to 120 seeds per square foot, more than 50 seedlings typically emerge, the stand thins to 25 to 35 seedlings per square foot by fall, and lands near 20 plants per square foot after the first winter. Fifteen plants per square foot going into winter from a spring seeding will still yield well the following year. Below 15, he writes, stands are low-yielding and weedy.

Missouri work cited by University of Wisconsin Extension puts the economic floor at 3 plants per square foot — below that, alfalfa simply stops paying (Understanding Autotoxicity in Alfalfa, accessed August 5, 2026). University of Minnesota Extension puts the target at the other end: "to maximize yields, growers need at least 25 plants per square foot at the onset of the first production year" (accessed August 5, 2026).

The number you compare against depends on how old the stand is, and this is where a lot of people talk past each other. Undersander's spring assessment guide is explicit about the two cases ("Assessing Alfalfa Stand Condition in the Spring," University of Wisconsin Extension, accessed August 5, 2026):

"A high yielding alfalfa stand seeded last year should have 20 plants/ft2, counts as low as 12 will produce good yield but result in shortened stand life. Stands, seeded last spring or fall with less than 12 plants/ft2 should be disked and reseeded."

— Dr. Dan Undersander, University of Wisconsin Extension

"A high yielding alfalfa stand over 1 year old should have at least 6 plants/ft2."

— Dr. Dan Undersander, University of Wisconsin Extension

Six is not a typo. An older alfalfa plant is a much bigger plant, and it throws far more stems than a first-year seedling does. That is why the count that matters on any field past its seeding year is stems, not crowns.

Stems per square foot — what will it actually yield?

On older stands, plant count stops predicting yield because surviving plants compensate by throwing more stems. Undersander's stem-count curve is the more useful number on an established field: 55 stems per square foot are needed to maximize yield. Count them in early September, shortly after a cutting, by counting cut stem ends inside a square-foot frame.

There is one more number worth writing on the back of your hand, because it decides whether overseeding is worth doing at all. From "Seeding into an Existing Alfalfa Stand" (Gildersleeve, Undersander and Wood, University of Wisconsin Extension, accessed August 5, 2026):

"Research suggests that the benefits of overseeding on yield are not seen unless the alfalfa stand is less than 5 plants/ft2 or 40 stems/ft2."

— University of Wisconsin Extension

Read that carefully, because it cuts both ways. Above 40 stems per square foot, overseeding is money you will not get back — the existing plants will simply fill the space you were paying to fill. Below it, overseeding starts to earn its keep. There is no middle ground where a light touch-up helps a little.

Root health — will it survive another winter?

Dig several plants, split the taproot lengthwise with a knife, and look at the inside. Undersander's scoring runs 0 for a clean white root through 5 for a dead one. A root with more than 50% discoloration scores 4 and is rated poor — it is unlikely to be there next spring. A root with moderate discoloration scores 2 and is still good. This is the count that separates "thin but healthy" from "thin because it is dying," and they call for completely different decisions.

What you counted	Reading	What to do
55+ stems/ft², healthy roots	Not yield-limited	Leave it alone. Fix fertility and weeds instead.
40–55 stems/ft²	Some yield loss, but overseeding won't pay	Fix fertility and weeds. Plan a rotation date.
Under 40 stems/ft², or under 5 plants/ft²	Overseeding can now add yield	Interseed a grass or cereal — never alfalfa.
Under 3 plants/ft²	Below the economic floor	Plan the rotation. Patching buys one poor year.
Root score 4–5 on many plants	Stand is dying	Don't reseed into it. Rotate.

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Why you can't thicken alfalfa with alfalfa

This is the single most common mistake on a thin alfalfa field, and it is also the most expensive, because the seed is not cheap and the failure does not show up until the following spring. The mechanism is autotoxicity: established alfalfa releases water-soluble compounds — medicarpin is the one most conclusively implicated — that delay germination and, more damagingly, stunt root growth in new alfalfa seedlings.

The number that settles the argument comes out of Missouri work summarised by University of Wisconsin Extension. Jennings and Nelson measured the "zone of influence" around an old alfalfa plant at an 8-inch (20 cm) radius. Inside that circle, new seedling density and yield are strongly inhibited — and they quantified it. In their 2002 paper in Agronomy Journal ("Zone of Autotoxic Influence around Established Alfalfa Plants," 94:1104–1111), seedling density and dry matter yield within 20 to 25 cm of an established plant averaged 70% and 44% of the control respectively. Yield was cut by more than half inside that circle.

The same paper does the arithmetic for you. They note that the zone of influence is equivalent to a stand density of just 8 plants per square metre — about 0.74 plants per square foot. That is the density at which those circles stop overlapping and a new seedling finally has clear ground. It is roughly four times thinner than the 3 plants per square foot at which the field has already stopped being economical. In other words, by the time a stand is thin enough for interseeded alfalfa to establish, it is already thin enough that you should have torn it up.

The demonstration numbers from University of Minnesota Extension make the same point without the geometry. In their trial, the untreated control carried more than 35 plants per square foot. Where whole alfalfa plants had been incorporated into the soil, the new seeding came in at 0.3 plants per square foot. Where only the roots were incorporated, 1.7. Their recommendation is blunt: allow at least one year between terminating a stand and reseeding alfalfa, and "don't thicken alfalfa with alfalfa."

Producers arrive at the same conclusion the hard way:

"Personally, I've never had too much luck drilling alfalfa into an established field (be it straight grass or mixed) - you may have autotoxicity issues from the existing alfalfa, or it's possible that your regrowth will shade out the new seeding alfalfa and kill it off. Your mileage may vary, but that's my experience."

— MrLuggs · HayTalk, "Overseeding Alfalfa into Alfalfa"

Notice the second half of that. Even where autotoxicity is mild, the existing canopy is a serious problem on its own — an alfalfa seedling that germinates under a regrowing alfalfa crop is in deep shade at the exact moment it needs light most. Autotoxicity and shading are two separate failure modes, and interseeding alfalfa into alfalfa gets both.

Undersander's summary is more measured and lands in the same place: while alfalfa can theoretically be interseeded into alfalfa one year or less after the initial seeding, it seldom provides good results.

One caution about the folk version of this rule. You will hear that alfalfa "goes toxic to itself" at a certain age:

"It is my understanding Alfalfa becomes toxic to itself between 5 - 7 years. Then you plant something else for a year then you can reseed Alfalfa."

— gradyjohn · HayTalk, "Thin stand"

The rotation half of that is right. The age half is not — autotoxicity is a function of established alfalfa being present, not of the stand reaching a birthday. The Michigan State work that produced the modern reseeding-interval recommendations (Tesar, "Delayed Seeding of Alfalfa Avoids Autotoxicity after Plowing or Glyphosate Treatment of Established Stands," Agronomy Journal 85:256–263, 1993) ran its trials on 1-, 4- and 6-year-old stands and found the effect present across all of them. A two-year-old stand can be strongly autotoxic. The practical rule is about how long the stand has been established, and it is measured in months, not years.

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The exception: patching a failed seeding

There is one real window where seeding alfalfa into alfalfa works, and it is narrow: a failed or spotty new seeding, patched before the stand is established. University of Wisconsin Extension states it cleanly in "Late summer alfalfa seeding": you can reseed immediately into a failed spring seeding, or into one from the previous fall. If the alfalfa stand is two or more years old, they recommend waiting two years before reseeding alfalfa.

The forum draws a broadly similar line, though as we'll show in a moment it draws it a good deal more generously than the research does. Here is the argument as it played out over a thin first-year Indiana stand:

"Probably won't have a serious problem with auto-tox on such a young stand. I would cut it close and drill the alfalfa seed soon after baling, and probably would cut it again at about 21-28 days depending upon existing stand density. That should give the new germination a pretty good chance."

— Vol · HayTalk, "Overseeding Alfalfa into Alfalfa"

"I vote with Mike, less than 18 month old stand, low auto toxicity, no till some seed."

— r82230 · HayTalk, "Overseeding Alfalfa into Alfalfa"

"I'm afraid you will have problems with auto toxicity at this point; I would check with an agronomist before spending money on alfalfa seed. I have seen successful reseeding during the summer of a crop planted that spring."

— Gearclash · HayTalk, "Overseeding Alfalfa into Alfalfa"

The disagreement there is real, and it maps onto a real variable: how much reproductive growth the existing stand has already made. An earlier operator put the six-month version of the rule most simply:

"Generally not a problem for the first six months or so as it has not had time to build up to toxic levels"

— sedurbin · HayTalk, "Overseeding Alfalfa"

Timing within that window matters too. One Virginia producer's split result is worth reading twice, because it separates the autotoxicity question from the competition question:

"I had good success drilling alfalfa into a stand planted in early spring the following fall immediately after last cutting. I have had much less success drilling into existing stand in summer when I think rapidly growing hay and grass suppressed seedlings"

— keezletowner · HayTalk, "Overseeding Alfalfa into Alfalfa"

Same field age, same seed, opposite results — the difference was whether the existing crop was growing hard at the time. Patch a young stand right after the last cutting, when the canopy is off and regrowth is slowing, not in the middle of a June flush.

Be honest about what the research does and doesn't say here

We want to be straight with you, because this is the one part of the article where the forum consensus runs ahead of the published work. What extension services actually state is narrower than "under 18 months is fine":

• Reseeding into a genuinely failed seeding is supported. That is the Wisconsin position quoted above — a stand that never established is not an established stand, so there is nothing there to suppress the new seed.
• Patching a young stand that did establish is not supported by any published trial we could find. Jennings and Nelson's own summary of the literature is that "interseeding alfalfa to thicken declining alfalfa stands is seldom successful due to autotoxicity," full stop — with no age carve-out. Their measurements found seedlings still suppressed inside the 20–25 cm zone even a year after the old stand had been killed with herbicide.
• The general rule is one year. Penn State Extension puts it plainly in "Successful Forage Crop Establishment" (accessed August 5, 2026): producers should not attempt to seed alfalfa back into an alfalfa field within one year of when the old alfalfa was killed, and rotating out for a minimum of one year allows the chemical to decompose.

There is one important nuance that explains why some operators get away with much shorter intervals. The waiting period is really about how completely the old plants are destroyed and how much rain has moved through the soil since. Tesar's Michigan trials found that seedings made at least two weeks after plowing, or three weeks after a glyphosate burndown, matched the untreated checks in both density and yield. That is a fully incorporated, fully terminated stand on a fine loamy soil with good moisture — not a no-till drill run through a living crop. The Minnesota demonstration numbers cited earlier tell the other half of that story: full-plant incorporation still produced only 0.3 plants per square foot in their trial, roots-only 1.7.

So the working rule we would give a customer: patch a failed seeding freely; treat anything that actually established as a one-year rotation, and if you are going to gamble, gamble on a stand under six months old, seeded right after a cutting, on a soil that has had real rain since. Grass, as the next section covers, has none of these caveats.

The other reason summer patching fails: moisture

"I'd be more concerned about the seed germinating then the ground dries out before roots go down and it dies off summer seeding.But f it keeps raining like it has it could be fine."

— swmnhay · HayTalk, "Overseeding Alfalfa into Alfalfa"

Wisconsin Extension makes the same warning in stronger language for late-summer seedings: never seed into dry soil, because a half-inch rain is enough to germinate the seed and not enough to keep the seedling alive. That is the most expensive failure in this whole article — you pay for the seed, you pay for the pass, and you get a field of dead seedlings.

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What to seed into thin alfalfa instead

If the alfalfa is thin and too old to patch with alfalfa, the answer is to change species. Grass does not care about alfalfa's autotoxins, it fills the gaps that weeds would otherwise take, and it gives you a hay crop off the same acre while the alfalfa finishes its run.

Undersander's recommendation is specific and worth following exactly: seed a grass into the thin alfalfa stand — ryegrass or festolium for short-term, one-year benefit next spring; tall fescue or meadow fescue interseeded either in fall or the following spring if the stand is to be kept two or more years.

For a stand that has already dropped below 6 plants per square foot, Wisconsin Extension gives rates and an explicit exit plan in the spring-assessment guide cited earlier:

"If plant density if [sic] less than 6, oats (2 bu/a) or Italian ryegrass (10 lbs/a) can be over seeded to increase yield this year. Stand should be turned over immediately or at end of year."

— Dr. Dan Undersander, University of Wisconsin Extension

Read that last sentence as part of the recommendation, not a footnote. Overseeding a cereal or an annual ryegrass into a failing alfalfa stand is a way to get one more crop off the field while you organise the rotation — it is not a way to keep the field. Wisconsin's costing on that decision, based on 2001 Wisconsin custom rate guidelines, put seed at $5.00 to $16.00 per acre and planting at another $5.00 to $20.00 per acre. Those figures are two decades old and today's numbers are considerably higher, but the ratio is the point: buying one more year off a dying stand is cheap relative to a full reseeding, and that is the only case in which it makes sense.

Producers land on nearly the same prescription independently, with rates:

"I'd interseed some grass into it early next spring.

Maybe 5# annual ryegrass and 5# orchardgrass.

Or put it to beans one yr and RU the crap out of it and start over."

— swmnhay · HayTalk, "Thin stand"

The logic behind that mix is the same logic Undersander uses: the annual ryegrass produces this year while the orchardgrass, which establishes slowly, sets up the years after. For genuinely bare spots rather than a uniformly thin stand, the same operator goes heavier and hedges the species:

"If just some spots 15-20 lbs.Depends if you have any stand there?

I prefer to add some grass like orchard incase the alfalfa doesnt take,better then weeds!So maybe 5 lbs orchard and 10-15 lbs alfalfa."

— swmnhay · HayTalk, "Thin stand"

Read that as insurance rather than as a contradiction of the autotoxicity rule. In an open spot with no live alfalfa nearby, there is no zone of influence — that is precisely the ground where seeded alfalfa has a chance. Adding grass to the mix means that if the alfalfa does not take, you still get forage instead of foxtail.

One thing that does not work is mechanical stimulation of the existing stand. Running a chisel or a disc through a thin alfalfa field is a persistent piece of folklore:

"digging may prune the roots and put out a few more shoots but it surely won't fill in bare spots like interseeding would."

— swmnhay · HayTalk, "Thin stand"

A thin stand is thin because plants died. Root pruning does not create plants.

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Thin grass fields are a different problem

Everything above is about alfalfa. Grass hay fields have no autotoxicity problem at all, so overseeding is genuinely a live option — and the failures come from competition, not chemistry.

The single most useful observation in the entire corpus on this point came from a producer who overseeded almost every field he had in one season with a brand-new no-till drill, and then compared them:

"Generally I noted the less competition the better the stand-duh! By explanation- paddocks in better shape, did not show robust rows of new seedlings, those overgrazed, great rows."

— Hayman1 · HayTalk, "Overseeding Existing Stands"

That is the whole game in one sentence: the fields that looked like they needed help least were the ones that took the new seed best, because they had been grazed off. Same drill, same seed, same day — the variable that decided the outcome was how much existing canopy the new seedlings had to fight.

There is one important species caveat. Overseeding a different grass into an established stand of a competitive grass usually does not work, no matter how carefully you do it:

"Usually very little of the orchard actually establishes in the fescue in an overseed situation and a planted mix becomes mostly all fescue after a year or two. They still call it a orchard/fescue mix but is really fescue with a sprinkling of orchard."

— FarmerCline · HayTalk, "Looking towards fall reseeding hay fields"

If your fescue field is thin, drill fescue. If you want orchardgrass instead of fescue, that is a kill-and-replant project, not an overseeding project. The same goes the other way: don't expect timothy drilled into a solid grass sod to take over.

Is filling holes even worth it if total tonnage per square foot is fixed? That argument gets made:

"A square ft of ground, given the nutrients, oxygen, moisture, etc remain the same, will produce x amount of growth per unit of surface area. Light interception, temperature and soil conditions are what matters. All the rest is myth."

— Hugh · HayTalk, "Overseeding Existing Stands"

And gets answered twice, correctly:

"But if someone wants finer stemmed hay, more plants per square foot is a benefit even if the total production didn't change."

— 8350HiTech · HayTalk, "Overseeding Existing Stands"

"My philosophy has always been that is there is a hole or there are holes in your grass stand, they will grow weeds. If you want to minimize the weeds, fill the hole with the desired grass."

— Hayman1 · HayTalk, "Overseeding Existing Stands"

Both are right. A square foot of ground will grow something. The only decision you control is what. See weed control in hay fields for what usually moves into the gaps if you leave them.

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Timing: the late-summer window and frost seeding

There are two workable windows for thickening a hay field, and one that gets used because it is convenient rather than because it works.

Late summer, right behind a cutting — the strongest window

This is the window most producers use, and the reasons are good: the canopy is off, soil is warm, and weed pressure is falling instead of rising. The constraint is frost. Wisconsin Extension's rule for late-summer alfalfa seeding is that you need at least 45 frost-free days of good growing conditions after seeding, and their guide to the practice puts it as a calendar rule as well (Rankin, "Establishing Alfalfa During Late Summer," University of Wisconsin Extension, accessed August 5, 2026):

"As a rule of thumb, seed alfalfa 6-8 weeks before the average first killing frost in your area. Seeding too early increases the risk for hot, dry conditions to occur during germination and seedling development. Seeding too late will not give the plants enough time to develop adequate root and top growth before winter."

— University of Wisconsin Extension

The hard cutoff in that guide is worth quoting too, because it is more definite than most extension language: "After September 1st, your chances of successfully establishing alfalfa are very low." In Wisconsin the recommended window is August 1 to 15, earlier in the northern half of the state.

If you want one number that explains why "I'll get to it next week" is expensive, it is this one, also from that guide: in northern Pennsylvania, alfalfa yield in the first production year was reduced by 158 pounds per acre for every day seeding was delayed after August 1. On a 40-acre field, a two-week delay is over three tons of first-year hay — several hundred bales you never make.

Grasses behave the same way, and Wisconsin has the trial data. Undersander seeded six forage grasses at staggered late-summer dates across three sites over three years ("Fall Seeding Grasses," University of Wisconsin Extension, accessed August 5, 2026). The conclusion:

"However, delaying late summer seeding from mid August to mid September generally resulted in 1 ton/acre less yield the next year."

— Dr. Dan Undersander, University of Wisconsin Extension

Their recommendation is simply to seed grasses as early as possible during the month of August. Slow-establishing species — reed canarygrass especially — need to be in by early September. In only one trial out of nine did a November seeding produce a stand the following spring.

Producers use the same idea with a bigger safety margin:

"I try to have it done by the last of August."

— Swv.farmer · HayTalk, "Best time to overseed"

"Most recommendations for fall planting say to plant about eight weeks before the first possibilities of frost"

— Thorim · HayTalk, "Best time to overseed"

Work backward from your own average first-frost date, not from a calendar rule of thumb someone in another state uses. Two more things Wisconsin flags for late-summer seedings: do not use a cover crop, and do not take a fall harvest in the seeding year. Both steal the light and root reserves the new seedlings need to reach winter.

Frost seeding — cheap, and more mechanism than magic

Frost seeding means broadcasting seed onto frozen or freeze-thawing ground in late winter and letting the soil work it in. It costs almost nothing, and it is unpredictable, which is a fair trade on a field you are not sure is worth a drill pass.

Before you plan around it, three things from Undersander's "Frost Seeding of Forages" (University of Wisconsin Extension, accessed August 5, 2026) will save you a bag of seed.

First, species selection is not optional.

"Frost seeding works best for legumes and grasses that germinate rapidly and at low temperatures. Red and white clover is the easiest to establish but we have also had some luck with birdsfoot trefoil. Frost seeding alfalfa is not recommended because it does not germinate at as low temperatures as the clovers. Frost seeded Italian (annual) ryegrass and orchardgrass have high success rates. Other grasses have been frost seeded with varying success."

— Dr. Dan Undersander, University of Wisconsin Extension

That rules out the most common frost-seeding plan we hear: broadcasting alfalfa in February to thicken a thin alfalfa field. It fails twice over — on autotoxicity, and on germination temperature. Red clover, orchardgrass and Italian ryegrass are the species that actually work.

Second, be honest about the odds and what the practice is for.

"Farmers have reported that frost seeding works 60 to 70 % of the time. … Frost seeding can be acceptable for pasture improvement but not for establishing pasture or hay fields."

— Dr. Dan Undersander, University of Wisconsin Extension

Third, the mechanical constraints. Frost seeding is not recommended on sandy soils, because freeze-thaw does not crack sand the way it cracks heavier ground. The optimum is to broadcast early in spring after the snow is gone but while the ground is still frozen — not onto snow, which washes the seed off during melt, and not before March, which just feeds birds and rodents. Wisconsin's own frost-seeding season is mostly March. And residue has to be off the field first, ideally by grazing hard the previous fall, so the seed can physically reach soil.

The clearest field-level explanation of the mechanism came from a Manitoba producer:

"I believe that for frost seeding to be effective the seed has to go down after the frost and before the snow. This puts the seed on the ground where it gets pressed to the earth by the weight of the snow. Then when the melt comes the seed can be drawn into the earth as it sucks up the moisture from the melt. If you spread seed on top of the snow you will have most or all of it run off with the early melt."

— Heyhay..eh · HayTalk, "Overseeding Alfalfa"

That single detail — seed goes down after the frost and before the snow, never on top of it — is the difference between a stand and a wasted bag. He reported renovating a 17-acre field on roughly a five-year cycle using these methods, and getting 2.5 tons per acre plus two months of grazing for 35 head out of it in a poor year.

A related window is the brief spring thaw when the surface is soft but the subsoil is still frozen:

"Somtimes the freeze/thaw leaves the ground REALLY soft here in spring. Theres like a 2 week window where a guy could use a conventional drill and have good results, especially if a packer was used, or if a rain would put the seed in the ground a little."

— Rodney R · HayTalk, "Overseeding Alfalfa"

Mid-season, into a growing crop — the window to avoid

Drilling into a hay crop that is actively growing means your seedlings germinate into deep shade with a mature root system already drinking the available moisture. That is the situation keezletowner described above as his failure case, and the one Wisconsin warns about with the half-inch-rain problem. If the only time you can get a drill in the field is mid-June, it is usually better to wait for the window after the next cutting.

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Getting the seed in the ground

More overseedings fail on mechanics than on agronomy. Undersander puts a number on it in "Getting a Good Forage Stand" (University of Wisconsin Extension, accessed August 5, 2026):

"More that [sic] 90% of forage stand failures are due to one of three reasons: 1) low soil pH, 2) loose soil, or 3) seeding too deep. Each cause of stand failure is preventable by the farmer."

— Dr. Dan Undersander, University of Wisconsin Extension

Nine failures in ten are one of three preventable things. Here is how to check each one.

1. Depth. Small forage seed goes shallow — a quarter inch. Undersander's target for alfalfa is a quarter to a half inch, never deeper than half an inch (three-quarters on sandy soils), and he gives a field diagnostic that costs nothing: after drilling, you should be able to see 5 to 10 seeds per square foot lying on the soil surface. If you cannot see any seed at all, it went in too deep. Grass seed is even less forgiving.

Which is why this deserves to be a checklist item every time you borrow or rent a drill:

"I rented the no-till drill from the local NRCS office and couldn't use it. Five of the 11 depth gauge wheels were frozen and stuck at between 1" and 1.5" -- way too deep for grasses. Took it back and told them fuggedaboutit! Check the gauge adjustments before you bring it home--can't move them--don't use it--you'll just be wasting your seed."

— rjmoses · HayTalk, "Thin stand"

He notes orchardgrass needs to be about a quarter inch. A rental drill set for soybeans will bury every seed you own an inch and a half down, and you will not find out until the following spring.

2. Seed-to-soil contact. Broadcast seed sitting on top of thatch does nothing.

"I think a conventional drill would work fine especialy if it has press wheels directly over the row for good seed-soil contact."

— swmnhay · HayTalk, "Overseeding Alfalfa"

If your drill has no press wheels, a cultipacker pass behind the drill does the same job. Wisconsin's guidance for no-till seeders in established sod is the fuller version: heavy down pressure, coulters running ahead of the disk openers, working depth control, and press wheels.

There is also a free test for whether the ground is firm enough to seed at all, and it is the oldest one in forage agronomy: stand on the field. If your shoe sinks more than a quarter inch into the soil, it is too loose to seed. That is Undersander's own phrasing, and it is the reason stands are so often thicker in the wheel tracks and headlands than in the middle of the field — those strips got packed. A corrugated roller works far better than a smooth one here, because a smooth roller rides up over clods and stones while a corrugated one follows the contour and packs consistently.

3. Don't till a stand you are trying to keep. Tearing up existing sod to "open it up" usually costs more established plants than the new seed replaces:

"Locals with experience have told me not to disc but just overseed after 2nd cutting. (We only get 2 cuttings here)"

— slowzuki · HayTalk, "Best time to overseed"

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Take the competition away first

Given that competition is the number-one cause of overseeding failure, the cheapest thing you can do is remove it before the drill goes in. The standard method is mechanical and costs nothing:

"When you plan on overseeding, you kick cows on pasture for much longer than normal and you scalp hayfields on the last cut for year."

— BWfarms · HayTalk, "Overseeding Existing Stands"

"On your last cutting I would cut as low as you can go giving the new grass a better opportunity."

— IH 1586 · HayTalk, "Looking towards fall reseeding hay fields"

Note the deliberate exception this makes to normal practice. Most of the year, cutting a grass hay field short is a mistake — it costs regrowth and shortens stand life. The last cutting before an overseeding is the one time you want it scalped, because everything you leave standing is shade over your new seedlings.

There is also a chemical version, used carefully:

"There are plant growth regulators to foliar feed after taking a cutting off to slow growth. Mostly used for golf courses for minimizing mowing, but works well for slowing hay fields to allow for over-seedings to stand a chance. Look at research reports and trials before choosing the right product."

— PaCustomBaler · HayTalk, "Overseeding Existing Stands"

Take the last sentence of that seriously, and read your label. Any suppression product used on a forage crop has to be cleared for that use and for the grazing or harvest interval you intend. This is worth a call to your extension agronomist before it is worth a tank mix.

One more piece that producers routinely skip: competition removal is not a one-time job done before seeding. Undersander is emphatic that the seedlings need light kept off them through the whole establishment year — graze or mow whenever the established grass reaches 8 to 10 inches. People hold off because they assume mowing will hurt the new seedlings; his answer is that the shade does far more damage than the mower does.

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Check fertility before you blame the stand

A stand that thins early is often telling you something about fertility rather than about age or variety. Every experienced producer in these threads reaches for a soil test first:

"On top of the other great suggestions, I would suggest that you get a soil sample to help ensure that your fertility and PH levels are in good shape."

— r82230 · HayTalk, "Best time to overseed"

Two specifics are worth knowing before you spend on seed.

pH. Undersander puts the alfalfa target at 6.8 and warns that below 6.0, germination itself is inhibited. Wisconsin's late-summer seeding guidance is to lime 6 to 24 months ahead of seeding, which means that if your pH is low, the seeding you are planning for this August should probably have been limed last year. Lime now and seed next year rather than seeding into acid ground.

Potassium. This is the nutrient most often behind an alfalfa stand that thins before its time:

"As a rough guess I would say the crop has not applied enough potash and that is a reason the stand is thinning early."

— hay wilson in TX · HayTalk, "Thin stand"

The same Texas producer gives a tissue-test benchmark most people never think to use — reading potassium off the hay analysis you are already paying for:

"With good misture 1.70% K appears to work. For our normally a drought climate we need at least 2.20% K better yet 2.70% K in the hay analysis. In a dry drought Here we really need more than 2.70% K for the stand to survive well through the summer of drought."

— hay wilson in TX · HayTalk, "Thin stand"

The word "misture" there is his typo for moisture. His point is a good one and applies well outside Texas: hay potassium is a direct readout of whether the stand has what it needs to survive summer stress, and it comes free with the forage test you already run.

The soil-test side of the same question has been quantified. Alfalfa removes roughly 58 pounds of potash for every ton of forage you haul off, which on a 5-ton field is close to 290 pounds of K₂O a year walking out the gate on a wagon. Wisconsin's guidance is that alfalfa yields plateau at about 120 to 140 ppm soil test potassium, that holding that level typically takes around 200 pounds of K₂O per acre applied annually as topdress, and that there is little to no yield response to topdressing once soil test K exceeds 150 ppm (Kelling, Peters, Rankin and Undersander, "Potassium in Forages," University of Wisconsin Extension Focus on Forage, accessed August 5, 2026). Below that plateau you are buying yield; above 150 ppm you are buying nothing. Cross-check whatever you find against the removal rates in our guide to fertilizing hay fields — a field that has been hayed for years without potash replacement will thin no matter what you seed into it.

Potassium also shows up in a place most people do not connect to fertility: winter injury. Undersander's spring assessment guide lists a fall application of potassium alongside adequate soil pH and more winter-hardy varieties as the management levers for reducing bud kill — the damage that forces plants to build new buds in spring, delaying growth and cutting first-cutting yield.

Undersander also flags two problems worth ruling out before you blame the stand: herbicide carryover from a previous crop, which can carry 12- to 24-month replant intervals, and sulfur deficiency, which has become more common as atmospheric sulfur deposition has fallen.

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When to stop patching and start over

At some point the arithmetic stops favoring the patch. The signals are:

• Under about 40 stems per square foot on an established stand, or under 5 plants per square foot — the point at which Wisconsin says overseeding finally starts to add yield, and also the point at which the stand has very little life left.
• Under 12 plants per square foot in a stand seeded last spring or fall — Wisconsin's advice there is direct: disk it and reseed. A weak seeding does not grow into a strong stand.
• Root scores of 4 or 5 on a meaningful share of the plants you dig — those plants are on their way out regardless of what you seed around them.
• Weeds already established in the gaps. Seeding into a weedy stand feeds the weeds, not the seedlings. Wisconsin specifically flags stands carrying a heavy dandelion or quackgrass load as better candidates for rotation than for overseeding.
• You are on the second patch. Two renovations on the same field usually means the underlying problem is fertility, drainage, or pH, not stand density.

There is also a clock running that has nothing to do with how the stand looks. In the University of Wisconsin alfalfa variety trials, yields peaked in the first and second production years and then fell — averaging 15% lower in the third production year and almost 30% lower in the fourth (Undersander, "Alfalfa Yield and Stand," University of Wisconsin Extension, accessed August 5, 2026). Their conclusion is worth sitting with if you are trying to nurse a fourth-year field through one more season:

"All economic analysis would clearly indicate that it is more profitable to plan on shorter rotations and replant alfalfa fields oftener than to attempt to keep a low yielding field for another year or two."

— Dr. Dan Undersander, University of Wisconsin Extension

The number that makes the decision easier than most producers expect is the rotational credit. Undersander notes a 15 to 20% corn yield increase when corn follows alfalfa, and observes it should more than pay for the cost of reseeding. That is the part people leave out of the comparison — you are not choosing between "spend nothing" and "spend on a reseed," you are choosing between a thin hay crop and a corn crop that carries a yield bonus plus the residual nitrogen credit.

The forum version of that same decision, from a Minnesota producer, is two lines long:

"Or put it to beans one yr and RU the crap out of it and start over."

— swmnhay · HayTalk, "Thin stand"

Two practical notes on the termination itself. Undersander reports that fall herbicide applications are more effective than spring for killing alfalfa, applied when temperatures are above 50°F. And the rotation interval matters: the 1996 multi-state survey Wisconsin summarises found recommended intervals between alfalfa crops most often at 6 or 12 months, with the full range running from 2 weeks to 24 months depending on state. The short intervals carry a real cost — in the Missouri trials, plots rotated after only 2 or 3 weeks had 13 to 20% lower plant density and up to 8% lower yield than plots rotated after 18 months, and the ranking established in the seeding year persisted for three years.

Soil type shifts the math too. Wisconsin notes autotoxins move faster through sandy soil — more severe in the short term, but they leach out — while they persist longer in a silty clay loam, which takes roughly 50% more water to flush. Irrigators in Kansas and Nebraska water heavily after killing an old stand for exactly this reason. If you farm heavy ground, take the longer interval.

When you do start over, our guide to establishing an alfalfa stand covers the new-seeding side in detail, and hay yield per acre by forage type will tell you what the renewed field should be producing once it is up.

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Frequently asked questions

Can you overseed alfalfa into an existing alfalfa field?

Only into a failed or spotty new seeding that never really established. Once alfalfa is established it releases autotoxic compounds that suppress new alfalfa seedlings within roughly an 8-inch radius of each plant — inside that zone, published trials measured seedling density at 70% and yield at 44% of the control. The gaps only open up once the old stand is thinner than about 0.75 plants per square foot, which is far thinner than the point at which the field has stopped paying. Penn State Extension's rule is not to seed alfalfa back into alfalfa within one year of killing the old stand. Patch failed new seedings; interseed grass into everything else.

How thin is too thin for a hay stand?

It depends on the stand's age, and the two numbers are very different. A stand seeded last spring or fall should carry 20 plants per square foot; 12 will still yield well but shortens stand life, and under 12 Wisconsin Extension says to disk and reseed. A stand over a year old only needs 6 plants per square foot, because older plants throw far more stems. On established fields count stems instead: 55 stems per square foot are needed to maximize yield, and overseeding does not add yield at all until you drop below 40 stems or 5 plants per square foot. Below 3 plants per square foot, alfalfa is no longer economical.

What is the best thing to overseed into a thin alfalfa field?

Grass, not alfalfa. Ryegrass or festolium gives a short-term, one-year lift next spring. Tall fescue or meadow fescue is the better choice if you plan to keep the field two or more years. Grass is unaffected by alfalfa's autotoxins, fills the gaps weeds would take, and adds tonnage while the alfalfa finishes its run.

When is the best time to overseed a hay field?

Right behind a cutting in late summer. Wisconsin Extension's rule is to seed alfalfa 6 to 8 weeks before your average first killing frost, and it warns that after September 1 the chances of establishing alfalfa are very low. Grasses follow the same clock: delaying a late-summer grass seeding from mid-August to mid-September typically cost a full ton per acre of yield the following year in Wisconsin trials. Frost seeding in late winter is the cheap alternative, but only for clovers, orchardgrass and Italian ryegrass — not alfalfa. Avoid drilling into a hay crop that is actively growing.

How deep should hay and forage seed be drilled?

About a quarter inch, and never deeper than half an inch. A free field check: after drilling, you should be able to see 5 to 10 seeds per square foot lying on the soil surface. If you cannot see any seed, it went too deep. Always check the depth gauge wheels on a rented drill before you leave the yard.

Does discing or chisel plowing thicken a thin alfalfa stand?

No. Tillage may prune roots and push a few extra shoots out of surviving plants, but it does not create new plants and it will not fill bare spots. A thin stand is thin because plants died. Either interseed a different species into the gaps or plan the rotation.

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The thing that makes this decision hard is that a thin field looks like a seed problem and is usually a species, timing, or fertility problem. Count first. If the stand still has the plants, fix the fertility and leave it alone. If it is thin but the species is wrong for patching — which on alfalfa means almost always — change species instead of fighting chemistry. And if the count is below the floor, take the rotation and the corn yield bump instead of buying one more mediocre year.

One last thought from the bale side of this. Whichever way the decision goes, the point of the exercise is more tonnage off the same acres, and more tonnage means more bales coming out the back of the baler in a season that is already too short. That is exactly when wrap performance stops being a line item and starts being a bottleneck — a roll that breaks or fails to cover the shoulders costs you time you do not have in a narrow window. If you are rebuilding a field to make more hay, size your net wrap for full edge-to-edge coverage so the extra bales you worked for actually keep.

The XES Netting team manufactures bale net wrap for round balers and writes these guides so forage operators can find clear, source-cited answers. Every farmer quote in this post is verbatim, with a link to the original discussion — go read the threads in full.

Featured photo: Alfalfa field (Medicago sativa) by Mahdikarimi70, CC BY-SA 4.0, via Wikimedia Commons.