Getting the most from your manure: Dealing with Uncertainty and Variability of Manure

A couple weeks ago I wrote about how today’s spreaders are helping you control the amount of manure going down on every acre better than ever before, today I wanted to follow up on that a bit. Unfortunately, manure is a variable fertilizer – its nutrient content varies from year-to-year or farm-to farm, the availability of some of the nutrients within the manure is influenced by the weather that year, and our ability to control how uniform it is applied is dependent on both our equipment and the field conditions. Understanding the sources of variability in manure and figuring out the best ways to manage them are critical to making the best agronomic and environmental use of manure resource.

There are three main sources of variability and uncertainty when using manure as a fertilizer for crop production. These are:

·         The nutrient content of the manure

·         The availability for the manure nutrients to the crop

·         Application variability

If you don’t account for this variability, you can end up applying too much or too little manure, both of which are costing you profits. Applying too little manure can lead to reduced crop because of nutrient deficiency while too much manure wastes nutrients and could negatively impact water quality.

When it comes to handling uncertainty in the nutrient content of manure, collecting a good, representative sample is the key. By sampling, you know the NPK contents of the manure and make a more informed decision. Certainly, some uncertainty still remains as there is nutrient variation from load to load based on the effectiveness of different agitation and mixing strategies, but overall you can be much more confident about the nutrients levels that are in the manure.

The next issue is availability for the manure nutrients to the crop. Here I’m using availability to refer to both the fraction of nutrients that will be mineralized and supply due to losses that occur at the time of application. In general, this source of uncertainty is larger for nitrogen then either phosphorus or potassium, as the P and K in manures are usually readily plant available whereas some of the nitrogen compounds are more difficult to break down to the forms plants can use. Additionally, as nitrogen has a gaseous form (ammonia), whereas P and K don’t, it is much more prone to losses during application. In terms of minimizing uncertainty from these components there aren’t always easy answers. The amount of nutrients that are potentially plant available or dependent on properties of the manure, the soil you are applying to, and the weather during that growing season. The ISU publication PMR 1003 gives some recommendations on first year availability of manure nutrients based on different manure types and this is a great starting point. The rest of the variability is harder to control as the weather is unpredictable and it play a role, as cool years promote slower mineralization than warmer years. However, when it comes to ammonia volatilization injection or incorporation is one way to lower the potential variability in nutrient loss. With typical injection or immediate incorporation typically you only lose about 0-5% of the applied nitrogen (so 5% uncertainty in nitrogen supply), but if you broadcast a liquid manure you’ll typically lose 10-25%. This means that your uncertainty in the amount of nitrogen you are supplying is already 15%.

Finally, the third area to think about is variability in your application. This encompasses both the variability in your manure as it comes from the storage and the uniformity of the spreading equipment. Certainly agitation of liquid manure can help improve uniformity, but we’ve all seen that often times the last load out of a pit has a thicker consistency than the first load. With solid manure this may be even more difficult, the manure packed in the back of our storage structure might have been the first we collected after land application last year, but there usually aren’t many good ways to mix that with our freshest manures that are being produced during application.

Another part of this equation is how accurately can you hit your planned rate and how consistently do you maintain this rate? If your goal is to apply 3000 gallons an acre how close are you to this rate and how does it fluctuate as you move through the field. If you are applying using tankers how quickly does your equipment lock on to the rate and what distance does this translate to in your field where you weren’t locked on yet? If you are using a dragline system, how consistent is your flow rate? We also need to ask how uniform our spread pattern is. We discuss this in terms of solid manure application as we can see the pattern on the soil surface, but we should think about liquid spreader the same way. Does each injector get the same amount of manure, what happens if you’re on a hill and the spreader is at an angle.

To get the most value from the manure we have to do our best to minimize the variability and uncertainty in all parts of our manure application program. As you tackle and control the uncertainty and variability in one aspect of your manure system it will provide a clearer picture of the variability and uncertainty with other components of your manure system.

In addition to this there are at least two additional techniques you can use to reduce the to reduce the risk that uncertainty and variability provide in your fertility program. The first is to consider using split nitrogen application with manure in the fall (perhaps 50-75% of your planned nitrogen need) and then supplementing this in the spring. This technique helps reduce risk from uncertainty with nutrient availability in the manure as it provides a second, mineral fertilizer that is quickly available. It can also reduce the uncertainty associated with uneven application as by providing a second application of fertilizer we minimize the potential of missing the same spot twice (i.e., reducing streaking from non-uniform manure application). Another option is to focus on crop rotations – rotations with legumes (in particular alfalfa or even soybean) reduce the sensitivity of the following crop (often corn) to nitrogen rate. Often times we’ve referred to this as a “N credit” (although it may be a bit of a misnomer as the crop doesn’t always add more nitrogen than it removes). However, with legumes in the rotation we tend get soils that are more capable of buffering the nitrogen supply in the soil to meet crop needs, making it less sensitive to the nitrogen we are adding with manure.