Manure Management and Nutrient Cycling: Turning a Cost Center Into a Resource

A freestall herd makes a lot of manure, and most of it is a bill: storage, hauling, spreading, permits. Yet the same pile contains most of the nutrients the herd ate. UMass Extension reports that about 70 to 80 percent of feed nitrogen, 60 to 85 percent of phosphorus and 80 to 90 percent of potassium are excreted in manure. That is fertilizer, and the farm either captures its value or pays to lose it.

What it is worth

University of Wisconsin Extension's article on using manure correctly puts it plainly: manure is a nutrient, not a waste. Using spring 2023 fertilizer prices, it valued liquid dairy manure at 10,000 gallons per acre at about $143.60 per acre, and solid dairy manure at 10 tons per acre at about $85.30 per acre. The authors note these values move with fertilizer prices. The point is not the exact dollar figure; it is the method. Credit the nutrients against next year's crop needs, and your fertilizer bill falls.

Test, do not guess

Book values vary. UMass lists typical figures for milking cows: liquid manure at under 5 percent dry matter around 28 pounds of nitrogen, 13 of phosphate and 25 of potash per 1,000 gallons, and solid manure around 10, 4 and 8 pounds per ton. Wisconsin's table gives different numbers for its own manure classes, which is exactly the problem. Nutrient content depends on ration, bedding, water and storage, so sample each source and send it to a lab. Test the soil too, so you set field-specific rates.

Nitrogen is the leaky part

Up to half of the total nitrogen in manure can be inorganic and quick to turn into ammonium, then ammonia, which is volatile. UMass gives losses by application method:

Method	Ammonia nitrogen lost
Injection	0%
Incorporated within 1 day	20%
Incorporated within 2 days	50%
Incorporated within 3 days	60%
Incorporated within 5 days	80%
Surface applied, not incorporated	100%

The same fact sheet estimates that incorporating 8,000 gallons per acre of liquid dairy manure the same day can save up to 70 pounds of nitrogen fertilizer compared with leaving it on the surface. Read the full table in the UMass fact sheet.

Nitrogen releases over time

Organic nitrogen does not become available all at once. UMass puts the release from stable organic nitrogen in dairy manure at roughly 40 to 50 percent in year one, 12 to 15 percent in year two, 5 percent in year three and about 2 percent each year after. Fields with a long manure history therefore carry credits from past applications. Count them, or you will over-apply.

Phosphorus is the water-quality problem

When manure is applied to meet a crop's nitrogen needs, phosphorus and potassium often exceed what the crop needs. UMass notes that conserving nitrogen improves phosphorus efficiency. Wisconsin points out that excess phosphorus reaching streams, rivers or lakes can cause eutrophication, and that nitrates in drinking water pose a human health risk. Risky practices include over-application, spreading before rain and spreading on frozen soil. In practice, high-phosphorus fields may need manure moved to other ground or applied at phosphorus-based rates.

Timing

• Late spring and summer are preferred. Growing crops use nutrients quickly and drier weather lowers runoff risk. Wisconsin cites a four-year Ohio State study in which spreading on standing corn before the V5 stage using dragline application did not reduce yields.
• Fall is riskier. Choose fields with a cover crop or heavy residue, and apply after soil cools below 50 degrees F but before it freezes.
• Avoid frozen ground. If you cannot, pick low-slope, well-drained sites far from surface water with winter vegetation.
• Cover crops help. UMass suggests that a winter rye cover crop planted from late August to early September can reduce nitrogen leaching and help control erosion.

Storage buys you flexibility

You can only spread when conditions are right if you can store when they are wrong. Ohio State Extension's article on water usage on dairy farms recommends at least six months of manure storage, with 8 to 12 months recommended in Ohio, and advises covering cow lots and installing gutters so clean rainwater stays away from manure. Less clean water in the pit means fewer gallons to haul and more nutrient per load. Water that contacts manure, including cleaning water, counts as manure.

A practical routine

1. Sample every manure source each year at an approved lab.
2. Soil test fields and map the high-phosphorus ones.
3. Set rates by nutrient credits, including past applications.
4. Inject or incorporate quickly, and calibrate the spreader.
5. Plan storage so you can wait for the right window.

The bottom line

Manure becomes a resource when you measure it, protect the nitrogen and respect the phosphorus. Barn layout and flush or scrape choices also shape the system; see our guide to freestall barn design. For more on field-side practices, farm.fm's farming category is worth a look, and agriculture.directory can help you locate custom applicators and testing labs.