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What to expect from the nitrogen-use efficiency trait in corn

The future demands crops that will produce more food using less nitrogen (N). Corn is one of the most productive cereals, producing a lot of carbohydrate per unit of N applied. Several plant breeding companies have set goals to substantially increase the N-use efficiency (NUE) trait of their future hybrids.

How will these differ from the hybrids of today?

Corn producers have improved N-use efficiency, already. This does not necessarily mean less N per acre. Since the 1970s in the US Corn Belt, the amount of corn produced per unit of fertilizer N applied has increased by 78 percent, while N rates have gone up 30 percent.

How has this improvement in efficiency been achieved?

Mainly by increasing yields, associated with:

• Greater N uptake, extending later into the season;
• Increased internal efficiency in the plant, yielding more grain per unit of N taken up; and
• Small reductions in the crude protein (N) content of the grain.

Where this is leading
Plant breeding companies have ramped up efforts to continue genetic improvement. Both conventional and biotech applications are being used.
What are the traits that might contribute?

• Further increases in yield and tolerance to stresses like high plant populations;
• Roots that explore the soil more quickly and thoroughly;
• Transporters that assimilate nitrate, and enzymes that convert it to amino acids more efficiently;
• Altered patterns of storage and remobilization of N within the plant; and
• Ultimately, symbiotic N fixation: however that is an unknown, and a long way off.

Updates to nutrient management in corn
These traits may require changes to the way nutrients are managed for corn. What will the right choices look like for source, rate, timing and placement?

Source – Corn will likely continue to take up N as ammonium and nitrate. Physiologically, it takes the plant less energy to make protein from ammonium than from nitrate, even though corn is efficient at using nitrate. Increasing ambient carbon dioxide also favours ammonium uptake. Corn may start showing more preference for ammonium. So perhaps we can envision using sources that slow or prevent the conversion of fertilizer into nitrate, keeping it as ammonium later into the growing season.

Rate – Plant breeding will not likely improve our ability to predict what the soil might provide, or what the weather might remove from the soil by leaching, denitrification or other loss routes. These factors will likely remain as the main determinants of the optimum rate to apply, though when yields increase, some account will have to be made for increasing plant demand for N, as well.

Timing – The corn plant needs N from start to finish. European studies show that continued N uptake beyond even a typical silage harvest date can be important for grain yield. Can we find ways to split the dose or control release for effective N uptake over a more extended period of time?

Placement – Could we envision a root trait that changes the depth from which N is captured? Roots operate most efficiently within the topsoil. It will still be important to get the applied N into that zone. But could we place other nutrients, such as P and K, in a way that helps express the full potential of a NUE trait? Can we envision a trait that proliferates roots in zones where nutrients have been banded in ways that minimize losses to water and air?

There are good reasons to expect more genetic improvement of N-use efficiency in corn. To make the most of it will require more agronomic experimentation with plant nutrition, as well.

*Dr. Tom Bruulsema is Northeast Director for the International Plant Nutrition Institute (IPNI), located in Guelph, Ontario. Reprinted from Better Crops with Plant Food, with permission of the International Plant Nutrition Institute (IPNI).