Improving success of variable rate manure

Chryseis Modderman
June 04, 2019
By Chryseis Modderman
Precision agriculture means using variable rates based on management zones, and it has been gaining popularity over the past decade. Variable rate planting and commercial fertilizer application are the most common types of precision agriculture, but manure may soon be joining their ranks.

Many research studies show that variable rate manure application works, and that it’s economical; but there are also imperfections that make it complex. Luckily, in addition to the constant improvements of precision manure applicator technology, there are some practical methods that can increase the success of variable rate manure applications.

Precision application of manure is tricky because manure, itself, is not precise. It can be difficult to be certain exactly what nutrients are present in manure for a couple reasons. For one, some of the nutrients in manure are in organic forms, meaning they are not immediately available to the crop. Over time, those organic forms will break down into forms that can be used by plants; but the pace of that transformation is difficult to estimate since it is a microbial process that varies based on the environment. While tools and calculators exist to help estimate how quickly organic forms will become plant-available, they can never be 100 percent accurate.

Another issue is that manure nutrient content is not uniform. This is especially true for solid manures as the nutrient content in one area of a stockpile will certainly vary from another area. And no matter how well a liquid manure pit is agitated, some variability will still exist. In addition to spatial variations, manure nutrient content varies over time. Nutrient content from storage and handling to what is actually applied can drastically vary.

Also, using variable rates might be a challenge because manure application sometimes occurs during a time crunch. Perhaps harvest was late, or planting was early, or the ground was too wet, or impending ground freeze came too soon; or perhaps storage was in danger of overflowing. Whatever the case, when application must be done on a tight schedule, it is easier to have a “just get it on the field” mentality; which leaves no time for the planning that precision agriculture takes.

Now that we have the doom and gloom out of the way, what can be done about these challenges?

Properly sampling manure for testing is a practical way to better understand the actual nutrients applied. To get the best representation of the manure, many samples should be taken during application and mixed well. Keeping detailed records of sample analyses will ensure planning for future applications is done with the most accurate information possible. Speaking of sampling, soil should also be sampled regularly to determine which areas of a field need different nutrient management.

While it might not be a practical option for all farmers, composting can remove some of the inherit uncertainties in manure. Through the composting process, manure is broken down in size and is made more uniform. With composted manure, you can be reasonably certain that the first load contains roughly the same nutrient content as the last.

Finally, a little planning goes a long way. While inclement weather can’t be foreseen, you can minimize a possible time crunch by using down time (such as in the winter or before the ground thaws) to plan and prepare for variable rate applications. Calculate how much manure will need to be spread to sustain storage until future applications can be made, make or update detailed management zone maps for each field, and decide what rate will be applied in each management zone. That way, even when the environment causes delays, you’ll have done all you can to hit the ground running with manure applications.

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