Impact of Manure Timing on Effective Nitrogen Supply

 Nitrogen use efficiency (NUE) is the dry mass productivity per unit N taken up from the soil. The term can be used in many complex ways to illustrate different factors, like the amount of nitrogen supplied from the soil.

I looked at applied nitrogen (from liquid swine manure) as a function of manure application timing and crop yield response. At Nashua, Iowa, we have four years of data looking at manure application timing. Treatments were early fall (October), late fall (November), and spring. We’ve presented this data as the difference in yield. Here, I look at it a different way; how effective was the application in supplying nitrogen?

The process to estimate how effective the nitrogen was at supplying the crop was:

1. Use the difference in yield between the treatments to estimate the percent of maximum yield.

2. Use the Iowa State Maximum Return to Nitrogen nitrogen response curve to estimate effective nitrogen supply.

The results are plotted in figure 1.

So what does this mean? The spring manure achieved 99% of the maximum yield. It has the same yield as the spring applied UAN. The late fall manure yielded around 85% of the maximum yield. The early fall manure yielded 65% of the maximum yield. Reading from the x-axis provides an effective nitrogen supply.

The spring application, by default, supplied approximately 150 lb N/acre (as it is used as the baseline). The late fall application had a yield equivalent to 75 lb N/acre in the spring. The early fall manure had a yield equivalent to if only 25 lb N/acre had been applied. This means that the late fall manure was only taking advantage of about 50% of the nitrogen applied. The early fall manure was only 17% effective. These numbers illustrate the important role application timing plays in using manure

Figure 1. Effective nitrogen application rate from EFM (Early Fall Manure), LFM (Late Fall Manure), and SM (Spring Manure) when all were applied at 150 lb available N/acre at the time of application.