Tuesday, November 26, 2019

The Science Behind Manure Management Plans - Nitrogen

Manure management plans are a tool used by both the farm to make sure they are getting the most from their manure and by society to ensure manures are managed in a way that is appropriate and only allows acceptable risk to environmental quality. I think we can all agree these are good things we want to occur but like most things, the devil is in the details. How do we, as a society and individual farmers, work to select the right nutrient application rates to balance both fertility decisions and impacts on yields and economics, with the potential environmental consequence?
In most manure plans, the yield goal method is used to define the maximum allowable nitrogen application rate. This approach is based on a mass balance approach where we are trying to match nitrogen application rates to removal and loss rates. In this sort of system, there is a fair amount of uncertainty in understanding where nitrogen ends up and how it moves. Changes in corn genetics have impacted how the existing factors in this equation may interact. Below I’ve constructed a partial nitrogen budget (estimate N application using the yield goal method minus the amount removed in grain) with two different grain nitrogen content. The old budget uses the 0.8 lb N/bu of grain which is what is listed on the USDA crop nutrient removal tool, while the newer partial budget uses 0.6 lb/N bu of grain which is just a bit higher than what newer research on corn nitrogen suggests is occurring. I’ve also marked two vertical lines; the gray line represents Iowa’s corn yield in 1995 (state average) while the black line represents Iowa’s corn yield in 2018.

What should we be taking away from this information? It isn’t that the yield goal method is archaic, but rather, as corn genetics have changed we need to be thoughtful about how it impacts the nitrogen use coefficients listed for different crops within the document. It is possible, at some point, these will need to be updated and deciding when, how, and what is the right factor is critical to providing a realistic yield estimate. While there is a lot to the nitrogen cycle and it can get a bit confusing, there are two things to note in this figure. The first is the two lines diverge from each other at yields get higher and the second is over the last 25 years the yields have greatly increased.

So what does this mean at the 1995 yield level? The old partial N budget application approximately matched the removal in corn grain. If we look to current yields, even at the old corn N removal rate, we were putting on around 25 lb N/acre more nitrogen than would be removed in the grain, but as grain nitrogen content has come down, this amounts to about 40 lb N/acre.

So what does this really mean? While the yield goal method is based on a mass balance approach for nitrogen, many of the factors in mass balance are hard to predict. Thinking about how our agricultural systems have changed over time and what this means for rate selection, is critical for making an informed decision. Moreover, this is one of the reasons Iowa State has switched to recommending the maximum return to nitrogen approach for rate selection.

Figure 1.  Partial nitrogen budgets (input – grain N removal) for old (corn with 0.8 lb N/bu) and new (corn with 0.6 lb N/bu) estimates. The vertical gray line represents corn yield in 1995, while the vertical black line represents corn yield in 2018. Representation for a corn soybean rotation with a soybean credit (rotation effect) of 50 lb N/acre)