With historically high fertilizer prices, manure nutrients are more valuable than ever,” says Natalie Rector, Michigan State University Extension nutrient management educator. Manure contains nitrogen (N), phosphorus, potassium and many micronutrients. But because it doesn't come in a fertilizer bag with a guaranteed analysis, some producers don't make use of this resource.
How much do you know about manure?
Manure spread during the winter and not incorporated leaves very little N for the next crop.
Winter-applied manure has N value. Much of the N from manure spread during cold weather — on soil that contains moisture — is held in the soil and is available in the spring.
If manure is spread during hot weather on dry soils (such as in August on wheat or oat stubble), it loses much of its ammonia to the air. This volatilization process is less likely to occur during cold weather.
Manure also contains organic N, which breaks down over time. As soon as soils warm up in the spring, a portion of the organic N is released and is readily available to the growing crop, even from manure that was surface-applied during cooler weather. Potentially, both the ammonium and the organic N can be retained and available from wintertime applications.
Manure spread in March and April will not be available to crops in June.
The N in manure comes in several forms, including ammonium (NH4-N) and organic. The ammonium form is readily available. As the soil warms in the spring, 25-50% of the organic N converts to N and also becomes readily available to the growing crop.
Manure is too variable to be a reliable source of crop nutrients.
Manure is more variable than purchased fertilizer, but it can be managed for efficient crop production. Manure tests estimate the amount of N, phosphorus and potassium that can be credited against fertilizer recommendations. Agitating manure in storage before hauling it to the fields improves nutrient uniformity.
Take several manure samples while emptying a storage system to see how much the nutrients vary between the first and last loads. You may find it is not as variable as you think.
It's important to spread manure as uniformly as possible. If an applicator spreads over a consistent distance at a consistent speed and avoids random skips and overlaps, manure nutrients will be consistent.
Manure N is in a form that isn't available to plants.
Crops can't tell if N is coming from fertilizer, livestock manure or legume crops such as soybeans, clover or alfalfa. Manure contains several forms of N, inlcuding organic and ammonium, and all forms of manure nitrogen ultimately convert to plant-available forms of N.
Manure increases soil's organic matter and tilth, but shouldn't be considered a nutrient source. Full rates of fertilizer should be applied to assure good yields.
Manure is a valuable nutrient source that should be credited against fertilizer recommendations. There's a wide range in manure nutrient value, so it's important to take samples as you empty manure pits or during daily hauling. This will provide useful information for making the best decisions at side-dressing time. Straw-packed manure may have less N than non-bedded manure.
Manure application rates have a major effect on the amounts of nutrients provided to the field. To use manure to meet fertilizer needs, test soil and manure and calibrate manure application spreaders.
When manure is spread on a field for the first time, the manure will be of little nutrient value.
From the first time it's spread on a field, manure has nutrient value. Spreading on the same field over three years can result in a significant amount of N being “slow released.” After the third year, manure spread two or three seasons earlier will still be releasing N. Applying more manure the third year may provide sufficient N for a high-yielding corn crop. Use a pre side-dress N test to tell how much N is available.
To learn more about manure management strategies, visit www.rootzone.msu.edu.
Natalie Rector is a Michigan State University Extension nutrient management educator in East Lansing. Contact her at firstname.lastname@example.org.