Manure Application Following Silage

Manure Application Following Silage

With silage harvest coming up quickly, manure application will soon follow. Because silage is often the first crop to come off the field, it allows for earlier manure application and thus an earlier cleanout of pens before winter. As that manure application plan develops, include best stewardship practices for optimum rates and preferred application methods in final decisions. But, wait, what do those things mean?

Agronomic rates

Agronomic rates consider what future crops will need. They are generally based on one nutrient. That may mean some of the other nutrients will be applied in excess of crop needs and others will leave the crop deficient if not supplemented with other fertilizer. Your optimum rate may be an agronomic rate, but it may be somewhere in between the two most common:

Nitrogen based rates

A nitrogen-based (n-based) rate considers how much nitrogen will be needed for the next season’s crop. For example, if a field is to be planted to corn in the spring, the n-based rate would utilize available manure nitrogen to meet all the needs of the corn to be grown next year. Because nitrogen is likely to be lost to leaching if over-applied, a n-based rate is the heaviest rate that can be applied to a field. In many cases, a n-based rate will far exceed nutrient needs for nutrients other than nitrogen.

Phosphorus based rates

Occasionally someone may choose to use a phosphorus-based (p-based) rate. A p-based rate requires more land to utilize the same amount of manure as an n-based rate. This is especially true with beef manure and when distillers grains are fed because the ratio of phosphorus to nitrogen in the manure is much higher. Because phosphorus is not likely to be leached into the soil or groundwater, a p-based rate may account for multiple (4-5) years of phosphorus need. This is particularly common if the risk of erosion and runoff are low. For example, if you were applying to a field with a corn and soybean rotation, you might apply based on the P needs for the next 4 years of crops (2 years of corn and 2 years of soybean).

Choosing an optimum rate

In many cases, a person will choose a rate somewhere between n-based and p-based. That rate will often meet phosphorus needs for a couple of years, but is not sufficient to meet nitrogen needs. It allows for nitrogen to be applied later in the season, closer to when the crop needs it. This method capitalizes on the complementary benefits of manure and commercial fertilizer and minimizes loss of nitrogen from leaching. Additionally, it allows for manure to be applied on more acres, thus gaining the benefits of manure other than nutrient value on more fields.

Knowing how much is being applied

The only way to know the actual application rate is to calibrate the manure spreader. Many people believe that’s a complicated process, but it doesn’t have to be. In many cases, calculations can be minimized and occasionally, with the proper tools, they can be completely eliminated. If you need help with your manure spreader calibration, contact myself or anyone on the manure team.

Incorporating Manure

When manure is applied on the surface of the soil, it remains exposed to the elements. This exposure can lead to nutrient losses from the manure. Nitrogen in the form of ammonia can be lost to the atmosphere, and phosphorus can be lost in runoff. To manage these two losses, a farmer may choose to incorporate the manure, essentially tilling it in. The sooner the farmer incorporates manure, the less the risk of loss. After 7 days though, especially if the weather is warm, ammonia nitrogen is already gone so there’s no nitrogen benefit for incorporation.

Before anyone decides whether to incorporate manure or not, they need to weigh the pros and cons of that incorporation. And the weight of these pros and cons are different for each farm or application.  

Reasons why someone might choose to incorporate:

  • They are using manure with a large proportion of manure N in the form of ammonium N (risk of loss is high).
  • There’s a rainfall event predicted the next day that would likely cause runoff (higher risk of loss of P).
  • They’re also seeding a cover crop and are preparing the seed bed prior to planting or after broadcasting that seed.

Reasons why someone might choose to NOT incorporate:

  • They are using manure with already low ammonium N content (loss would be minimal).
  • They’re applying to relatively flat land where risk of runoff is low (loss of P would be minimal).
  • They’re applying when there is little to no risk of rain for several days (loss from P from runoff is minimal).
  • The field where they’re applying has few or no neighbors nearby to be bothered by the odor.
  • They have no equipment or not enough time/labor to get it done in a timely fashion.
  • They have steep hills and they’re not allowed to till the land without immediately following with a cover crop (high risk of erosion).

So, as you are applying manure or are watching manure applications occuring this late summer and early fall, remember that manure is not just a waste product to be disposed of. It's packed with nutrients crops need, making it an investment for that field. The person doing the application most likely has a carefully orchestrated plan and they’ve probably thought about all of the risks and benefits of that manure application for that particular field.

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