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Manure Minute: Don’t fear the unknown: Manure’s lack of uniformity


May 30, 2021
By Chryseis Modderman | University of Minnesota Extension

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One of the biggest complicating factors when it comes to using manure as a nutrient source is manure’s lack of uniformity. When you buy a bag of urea, you know you’re getting 46 percent nitrogen all throughout. But when it comes to manure, you just can’t guarantee the same level of certainty.

But that doesn’t mean it has to be a great manure mystery.

Manure nutrient content can change and vary spatially and over time. Here are a few examples: a stockpile or pit that is open to the air will lose nitrogen to the atmosphere as a gas making the outer layers less nitrogen-rich; liquid manure will separate into solids that have a higher phosphorus content and liquid which will be more nitrogen-rich; and changes in livestock management (such as feed type) may create inconsistencies in manure stored earlier versus later.
This lack of uniformity with a product can make it difficult for you to know the exact nutrient rate being applied. Though there will always be some level of uncertainty, there are some measures that can be taken to improve uniformity, account for changes and add some predictability to your practice.

Accounting for lack of uniformity
Calculating correct application rates hinges on having an accurate measure of nutrient content. To do this, you not only need to sample your manure, but you also need to take a good sample that is representative of the manure applied.

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Before you sample, you might want to consider these ways to improve the uniformity of the manure. For liquid systems, agitate the manure well before pumping, and use a tank mixer afterward to keep the solids and liquids sufficiently mixed throughout application.
When dealing with solid manure, composting is an excellent way to reduce bulk and improve the overall nutrient uniformity within the manure.

Now, if composting is outside your comfort zone, simply turning the stockpile with a front loader will help spread the nutrient content much more evenly.

For large operations, we recommend sampling manure at application right off the spreader or during pumping to get the best estimate of what is actually being applied.

The main drawback with this approach is you unfortunately won’t get your results back until after the application. But the good news is, once you’ve sampled a few times, the nutrient content should remain similar as long as no management practices have changed.
Essentially, keeping good records remains incredibly important at all times with all aspects of manure – including, but not limited to, sampling.

For smaller farms with solid manure, sampling from the stockpile before application is acceptable, since the amount of manure is low and less likely to be highly variable compared to large volumes of manure.

As a minimum, we recommend sampling annually for the first three years of operation, and then every three to four years or whenever manure management changes. Though sampling more often, every year or at each time of application, will give you the best opportunity for accurate application.

Whether you are sampling liquid or solid manure, the key to getting a representative sample is to take many samples from the beginning, middle, and end of application or loading/pumping. Taking 20 small samples and mixing them together is better than taking three larger samples. You might even want to consider collecting separate samples by load or manure storage site.

Proper manure sampling will help you reduce nutrient uncertainty and make the best and most accurate application possible. Lack of uniformity does certainly present a challenge, but it doesn’t have to become a great manure mystery.

And, with the right tools, hopefully it won’t become one. •