What Exactly is Agronomic NUE?

Today’s growers try to walk the fine line between producing large crops as efficiently as possible while working hard to maintain environmentally friendly practices. A term we hear tossed around quite a bit today is NUE, which stands for Nitrogen Use Efficiency. You often hear numbers like .8#/bu or 1.1#/bu associated with the concept – meaning .8 pounds of nitrogen produced 1 bushel of corn. In reality the term NUE is more involved. Often NUE is thought of as the amount of applied nitrogen (N) required to produce a bushel of corn. Research says it takes around 1.1 pounds of N to grow a bushel of corn, but that includes a combination of applied nitrogen and soil-supplied nitrogen.

Some define NUE as a combination of the ability of crops to take up N from the soil in connection with the efficiency with which crops utilize the absorbed N to create yield (Benincasa et al, HortTech June 2011). You might be surprised at all the formulas and varied opinions that exist on how to calculate NUE.

For now, let’s examine the topic from the perspective of what can be done agronomically to influence NUE.

If we think about the ability of the crop to take up N from the soil there really are two sources:

  1. Nitrogen supplied by fertilizers (inorganic)
  2. Nitrogen from the soil (organic)

To take that a little further, N from the soil can come from a couple of different places:

  1. Mineralization of soil organic matter
  2. Crop residue breakdown

It’s important to remember that N availability from soil and crop residue are available at different times of the year. Also keep in mind that organic forms of N depend on biological processes that mineralize them and make them available to the crop. There are many agronomic factors that play into these biological processes that growers can impact to help improve NUE:

  • Drainage/compaction – poor drainage and/or compaction increases the likelihood of denitrification, leads to poor aeration that slows down mineralization and poor biological activity that slows root growth and uptake, and impedes efficient plant growth.
  • pH- This has a huge impact on biological processes including mineralization and nitrification. Balance your pH to maximize these processes.
  • Residue – How we manage residue breakdown can have a large impact on the efficiency of applied N the following year. Is it tied up to break down the residue or released to the crop in season?
  • Limiting factors- Many times growers apply extra N and can’t seem to get a yield bump. In many cases the issue is that something else is limiting yield. Maybe it’s another nutrient, maybe it’s population, sunlight or compaction. Whatever the cause, simply throwing more nitrogen at the problem without fixing other limiting factors is wasteful and harmful.
  • N practices – Are you maximizing the timing, placement and method of application? New technologies emerge every day that allow for more exact application of N. They may take more time or increased management but the payoff can be large.
  • Plant stresses – Efficiency is also about making sure the plant does not waste energy due to stressful conditions and can be more efficient in utilizing the N that it does take up. Make sure you are reducing stressful events such as diseases or weeds that can cause the plant to be inefficient.
  • Measurement – New tools allow you to measure the nitrate levels in a growing crop. These soil tests give you the ability to know if you need to increase or decrease your post- planting nitrogen application rates and allow you to fine tune your NUE strategy.

The best way to increase agronomic NUE is to increase yields with efficient use of N in conjunction with solid agronomic practices. A low NUE doesn’t necessarily mean cutting N rates. Maximizing yield potential can be an effective strategy for reducing your NUE.

The bottom line is that NUE is a big issue, from a practical standpoint we want to grow the most grain with the least N possible. In many cases simply improving some agronomic practices can have a large impact on both the efficiency of uptake and the plant’s ability to efficiently utilize the N once taken up.