Today's corn hybrids more efficiently use nitrogen (N) to create more grain, according to 72 years of public-sector research data reviewed by Purdue University researchers. Tony Vyn, a professor of agronomy, and doctoral student Ignacio Ciampitti looked at N use studies for corn from two periods: 1940-1990 and 1991-2011. They wanted to see whether increased yields were due to better N efficiency or whether new plants were simply given additional N to produce more grain.

"Corn production often faces the criticism from society that yields are only going up because of an increased dependency on nitrogen," says Vyn, whose findings were published in the early online version of the journal Field Crops Research. "Although modern hybrids take up more total N per acre during the growing season than they did before, the amount of grain produced per pound of N accumulated in corn plants is substantially greater than it was for corn hybrids of earlier decades. So, in that sense, the efficiency of N utilization has gradually improved."

Vyn and Ciampitti's analysis covered about 100 worldwide studies. Of those, 870 data points were taken from the earlier period through 1990, and 2,074 points were taken from studies after 1990, when transgenic hybrids started hitting the market. All studies involved analyses of total N uptake and grain yield by corn plants at maturity, usually in response to multiple N application rates.

Grain yields in these research studies averaged about 143 bu. of corn/acre over the last 21 years compared with an average of 115 bu. in the previous 50 years. Those studies showed that in the earlier period, 1 lb. of nitrogen taken up by corn plants from soil and fertilizer sources produced about 49 lbs. of grain. In the more recent period, that same amount of nitrogen accumulated in the above-ground plant parts produced about 56 lbs. of grain.

About 90% of the corn data points examined in Vyn's study evaluated N rates between 0 and 250 lbs./acre. Over both periods, the average rate of N fertilizer distributed in experimental fields was nearly the same: 124 lbs./acre in the earlier period vs. 123 lbs. in the later period.

Vyn says genetic improvements have led to corn plants that require less space around them, allowing growers to squeeze more plants into an acre. Research fields from the modern era averaged about 28,900 plants/ – about the average final plant populations in Indiana cornfields in 2011 – compared with 22,800 plants/acre from 1940 to 1990.

"The maximum individual plant N uptake stayed exactly the same despite the average gain of 6,000 more plants/acre," Vyn says. "The modern plants are just more efficient at taking N up and utilizing it than they were before."

Vyn and Ciampitti are working toward methods to increase grain yields further by investigating the contribution of N to plant biomass and yield formation processes in high-yielding hybrids under a wide range of N inputs and production stress factors. Knowing that modern hybrids are sustaining a reasonable quantity of N uptake even under progressively higher plant densities is a good start, Ciampitti says.

"We are getting clues on how plants have already improved N use efficiency, and we will use that to push for further increases," Ciampitti says. "We finally feel like we're shedding some light on what traits plant breeders should select for to increase N efficiency even more."

Vyn and Ciampitti plan to further investigate how water use efficiency and N use efficiency are tied together, as well as how plants can achieve more tolerance to environmental stresses.

Dow AgroSciences, PotashCorp and the U.S. Department of Agriculture National Institute of Food and Agriculture funded their work.