Cow manure is the safer default. Horse manure is easier to get free and better for building soil structure, provided you know what the horses were fed. That last clause carries more weight than every nutrient comparison on this page put together.
The nutrient difference between the two is genuinely small. What separates them in practice is bedding, weed seeds, and one risk that can take a vegetable garden out of production for years. We will get to that risk, because it deserves more attention than it usually gets.
If you want the short version: get either, compost it properly, and ask one specific question before you accept a load.
What we will cover:
Key Takeaway
Neither manure is meaningfully better on nutrients. The decision is about contamination risk, not fertility. Cow manure from a dairy is usually a known quantity. Horse manure is free and abundant but comes with two variables you have to check: how much wood shavings are in it, and whether the hay those horses ate was sprayed.
Close enough that it should not drive your choice. Both are low-analysis, slow-release materials whose real value is organic matter rather than N-P-K.
| Manure | Nitrogen (fresh) | Phosphorus | Potassium | Max Annual Rate |
| Dairy cow | 0.45 to 1.7% | 0.1 to 0.5% | ~0.3% | 1 lb per sq ft |
| Beef cattle | 0.35 to 2.0% | 0.1 to 0.6% | ~0.3% | 1 lb per sq ft |
| Horse | 0.5 to 0.9% | 0.1 to 0.15% | 0.2 to 0.6% | 1¼ lb per sq ft |
Sources: University of Arizona Cooperative Extension
Horse manure sits inside the cow range on all three. Any article telling you one is dramatically richer than the other is comparing a fresh sample of one against a composted sample of the other.
For application, University of Wisconsin Horticulture gives home-garden rates to supply roughly 0.2 pounds of available nitrogen per 100 square feet: 75 pounds of fresh dairy manure without bedding, 95 pounds with bedding, 200 pounds if composted, and 65 pounds of horse manure with bedding. A 5-gallon bucket holds roughly 25 pounds, which makes the mental arithmetic manageable.
Because what arrives in the trailer is mostly not manure. This is the single most useful thing to understand about horse manure, and it explains most of the contradictory advice you will read.
LSU AgCenter puts the numbers on it: pure horse manure has a carbon to nitrogen ratio of roughly 10 to 18:1, which is perfectly reasonable. Horse muck, meaning manure mixed with stall bedding, runs around 50:1. Wood shavings on their own are around 500:1. And roughly 80 percent of horse stalls are bedded with wood shavings.
So the horse manure you are actually offered is usually a high-carbon material. That is not a flaw, it just changes what it is for. High-carbon material builds soil structure and feeds fungi, and it will tie up nitrogen if you dig it in fresh. Optimal composting sits at 25 to 30:1 per University of Nebraska Extension, so shavings-heavy muck needs either a nitrogen source or a long slow composting period.
Why This Works: Two Materials, Two Jobs
Stop thinking of manure as fertilizer and the confusion resolves. Fine, well-rotted cow manure is closer to a nutrient input. Shavings-heavy horse muck is closer to a mulch and a fungal food source, doing the same job as deep wood chip mulching. In a permaculture system you want both: carbon on the surface feeding structure and biology, nitrogen where crops are actively growing. Matching the material to the job beats arguing about which is better.
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Send Me the ChartProbably, but the more useful finding is that cow manure is not clean either. University of Wisconsin Horticulture notes horse manure is notorious for viable weed seeds because horses do not digest their feed as thoroughly as ruminants do, letting seeds pass through intact.
That mechanism is real. But a survey of dairy farms found roughly 75,000 weed seeds per ton in fresh dairy manure, and about 27 percent of hard-coated weed seeds survived ruminant digestion. Being a ruminant helps considerably less than the folklore suggests.
Composting is the actual answer for both. Nebraska found that holding a pile at 140°F for three days destroyed 90 to 98 percent of weed seeds, with best results after 21 to 50 days of composting, provided moisture stayed above 35 percent. Even then, a 98 percent kill on 75,000 seeds still leaves around 1,500 viable seeds per ton, so expect some germination and plan to hoe.
Aged Manure Is Not Composted Manure
A pile that sat in the corner of a field for two years is aged. It never got hot, so it still holds viable weed seeds and potentially pathogens. Composted means the material was actively managed and reached temperature throughout. Nebraska Extension makes this distinction explicitly, and it is the reason so many gardeners get a weed explosion from manure they thought was safe.
This is the one that can take your garden out of production for years, and it applies to any manure from animals fed treated hay. If you read one section here, make it this one.
Aminopyralid, clopyralid and picloram are pyridine carboxylic acid herbicides used on pasture and hayfields. As NC State Extension explains, the forage is safe for animals to eat, but the herbicides pass straight through the digestive tract and are excreted in urine and manure. They then remain active in that manure even after it has been composted.
Persistence is the problem. NC State reports these compounds can be deactivated in as little as 30 days, but field reports show complete breakdown taking several years, and hay has retained herbicide activity after three years' storage in a dry barn. Degradation is particularly slow in manure and compost piles, which is exactly where gardeners are storing it.
The sensitive crop list is brutal for a vegetable gardener. NC State lists beans, peas and legumes generally, tomatoes, peppers, potatoes and eggplant, lettuce and spinach, carrots and the whole Umbelliferae family, sunflowers, marigolds, dahlias, strawberries, grapes and sugar beets. Symptoms are poor germination, death of young plants, and twisted, cupped or elongated leaves.
Two things make this manageable rather than terrifying. First, ask the right question: what hay were these animals fed, and was the pasture sprayed. NC State offers a useful shortcut, which is that hay containing clover, alfalfa or lespedeza has probably not been treated, because these herbicides would have killed the legumes. Second, you can test it yourself.
Sample the pile properly
Take at least 20 small shovelfuls from random points throughout the pile, including deep inside it, and mix them thoroughly. Test each source separately if you have more than one.
Fill three to six pots at 1:1
Use 4 to 5 inch pots filled with a half-and-half mix of your manure or compost and a commercial potting mix containing fertilizer. Fill several control pots with potting mix alone.
Sow three pea or bean seeds per pot
Peas and beans are highly sensitive, which is what makes them useful here. Stand each pot in its own saucer, or space them so runoff from one cannot reach another.
Grow for two to three weeks and compare
Wait until there are at least three sets of true leaves. If the controls look normal and the test pots show twisted, cupped or fern-like growth, assume contamination and do not use that material on broadleaf crops.
Source: NC State Extension, Herbicide Carryover in Hay, Manure, Compost, and Grass Clippings
If you get a positive result, the material is not waste. NC State's advice is to spread it on grass pasture, grass hayfield or a non-sensitive, non-food area, since grasses are unaffected. If it is already on your beds, till repeatedly through the season, irrigate, and grow a non-sensitive cover crop for a year or two before retesting.
Key Takeaway
Ask two questions before accepting any manure: what bedding is in it, and what hay were the animals fed. If the supplier does not know the hay's herbicide history, run the pea pot test before that material goes anywhere near your vegetable beds. Three weeks of waiting is cheap compared with a garden you cannot grow tomatoes in.
Hot, turned, and monitored with an actual thermometer. The temperature targets are specific and most backyard piles never reach them.
Nebraska Extension cites 131°F held for at least three days to destroy pathogens, and around 145°F to destroy weed seeds. For horse manure specifically, University of Minnesota Extension recommends holding 140 to 160°F for about three weeks, turning when the pile exceeds 160°F or when the temperature starts to fall.
| Target | Temperature | Duration | Source |
| Kill pathogens | 131°F minimum | 3 days | Nebraska Extension |
| Kill weed seeds | 140°F | 3 days, 90-98% kill | Nebraska Water |
| Weed seed destruction | ~145°F | Best over 21-50 days | Nebraska Extension |
| Horse manure pile | 140 to 160°F | About 3 weeks | Minnesota Extension |
| Turn the pile | Above 160°F | Or when temp falls | Minnesota Extension |
Sources: University of Nebraska Extension G1315, University of Nebraska Water, University of Minnesota Extension
Get the recipe roughly right and the heat follows: a carbon to nitrogen ratio of 25 to 30:1, moisture like a wrung-out sponge at 50 to 60 percent, and enough structure that around two thirds of the pile volume is air space. If you are working with shavings-heavy horse muck, you will need to add a nitrogen source or blend it with cow manure to get anywhere near that ratio. Our composting guide covers the mechanics in more detail.
Fresh Manure and Food Crops
The FDA identifies untreated manure, including both cattle and horse manure, as a potential carrier of enteric pathogens such as E. coli O157:H7 and Salmonella, with documented human illness linked to produce grown in amended ground. Do not put fresh manure on crops you are about to harvest, especially anything where the edible part touches soil. Compost it properly first and wash produce regardless.
Cow manure is the safer default, particularly composted cow manure from a known dairy source. Horse manure is usually free and abundant and excellent for building soil structure, but it typically arrives heavily mixed with wood shavings and carries a higher chance of viable weed seeds. The nutrient content is broadly similar between the two, so the decision comes down to what you can source, what condition it is in, and whether you can verify the herbicide history of the feed.
Not on food crops. Fresh manure from cattle and horses can carry E. coli O157:H7 and Salmonella, and the FDA has linked untreated manure to illness from produce grown in amended soil. Fresh manure also carries viable weed seeds and can burn plants. Compost it to at least 131 degrees F for three days to address pathogens, and higher still for weed seeds. If you want to use raw manure at all, apply it in autumn to empty beds well ahead of any planting.
Ask what hay the animals were fed and whether the pasture was sprayed with aminopyralid, clopyralid or picloram, sold under names like Grazon, Milestone, Forefront, Stinger and Curtail. If nobody knows, run a bioassay. Mix the manure 1:1 with commercial potting mix in a few 4-inch pots, keep control pots of plain potting mix, sow three pea or bean seeds per pot and grow them for two to three weeks. Twisted, cupped or fern-like growth in the test pots means contamination.
Longer than most people expect. NC State Extension reports these herbicides can break down in as little as 30 days under warm, moist, sunny conditions, but field reports show complete deactivation taking several years, and hay has retained activity after three years in dry storage. Breakdown is particularly slow inside manure and compost piles, so composting does not solve the problem. This is the reason the contamination can persist long after the manure looks finished.
Mostly, if the pile actually gets hot. Nebraska Extension found that 140 degrees F held for three days killed 90 to 98 percent of weed seeds, with the best results over 21 to 50 days of composting and moisture above 35 percent. But even a 98 percent kill on fresh dairy manure carrying around 75,000 seeds per ton leaves roughly 1,500 viable seeds. Expect some weeds and keep a hoe handy. A pile that merely sat and aged without heating will not have killed anything.
University of Wisconsin Horticulture gives rates to supply about 0.2 pounds of available nitrogen per 100 square feet: 75 pounds of fresh dairy manure without bedding, 95 pounds with bedding, 200 pounds of composted cow manure, and 65 pounds of horse manure with bedding. A 5-gallon bucket holds roughly 25 pounds. Arizona Extension caps annual loading at 1 pound per square foot for cattle manure and 1.25 pounds for horse manure, which is worth respecting to avoid salt and phosphorus build-up.
Very good, especially the shavings-heavy kind, as long as you use it for that purpose rather than as a nitrogen source. Its high carbon content feeds fungi and builds structure over time, working much like a mulch. Spread it on the surface rather than digging it in, since incorporating high-carbon material ties up nitrogen. This fits naturally into a permaculture approach of layering organic matter from above, and pairs well with the practices in our soil health guide.
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