Last year, many Southern farmers found out just how much water a corn crop requires.

“A good, high-yielding corn crop will use 22 or 23 inches of water,” says Emerson Nafziger, professor of crop production and Extension agronomist at the University of Illinois at Urbana-Champaign. “That’s the total water loss from the crop and soil during the season. When the crop hasn’t canopied, more water is lost simply from soil evaporation.

“Twenty-two inches of water is a lot. A 200-bushel corn crop uses about 600,000 gallons of water — nearly 3,000 gallons per bushel.”

On the heels of a record Mid-South corn crop, Nafziger visited the Delta Center in Portageville, Mo., on Nov. 19 to speak at the Certified Crop Advisor meeting. He stuck to the general physiology of corn plants.

“A lot of people aren’t very clear about the physiology of corn, how much water a crop needs and how water moves through the plant. Truth is, the corn crop is steaming off water every day a green crop is out there under sunshine. There are small holes in the leaves that allow water vapor to escape.”

To prevent the need for so much water, some people wonder why corn varieties aren’t bred to allow less vapor escape.

“It’s simple: in order to get carbon dioxide into the leaves — and air-borne CO2 is the basic feedstock for making yield — water vapor must be allowed to come out.”

There’s no good way for leaves to accomplish this other than the way it already is. Whenever the plant is turning sunlight energy into sugar, it’s using and losing water.

“The increase in dry matter and loss of water is a given. It has to happen. So, if you really want to save water, don’t plant a crop.”

The first time someone hears how much water corn requires, “they are usually very surprised. It seems excessive. But an acre of corn, during a hot July, can lose 7,000 or 8,000 gallons daily. When looking at it that way, it isn’t as difficult to extrapolate those large amounts.”

When water is inadequate and plants undergo stress, productivity plummets. There can be days when the crop leaves “are curled up and trying to defend themselves from the heat. But curled-up leaves aren’t photosynthesizing, so days of stress aren’t productive.”

Nafziger doesn’t think there’s a way to produce good Southern corn without at least 20 inches of water. Some areas can produce a minimal crop with 16 or 17 inches, “but it would produce a third or fourth of the yield we have (in the Midwest).”

Obviously, warmer temperatures make water leave plants faster. And warmer night temps in the South are one of the noticeable differences with the Midwest crop.

“Warm night temperatures don’t have as much to do with water directly, since the leaves don’t lose water at night. But the plant will respire away some of the sugars it made in the daytime when nights are warm, and this reduces its productivity.”

That, in turn, reduces water-usage efficiency.

“We should view every pound of dry matter in the crop as produced at the expense of about 50 pounds of water. Seeing that expense frittered away at night can be frustrating.”

Another big difference between the regions is soil-related and has to do with the supply of water. In parts of Illinois, for example, there are deep, dark prairie soils that hold a lot of water. The root system of corn is able to get to that water, normally.

“Not many Illinois farmers irrigate corn. This year, producers in some places grew corn on maybe 8 inches of rainfall between planting and harvesting. That means about 14 to 15 inches of water had to come from the soil.”

One corn-growing mystery to Nafziger is “why in Illinois a 200-bushel crop seems to come more easily than in the Mid-South, even with irrigation.

“The only things I can come up with are the warmer night temperatures and the possibility of more diseases and insects in the South. But it’s not clear that this is the whole story. If we have a decent summer weather-wise, our corn will usually yield more than irrigated corn in the Arkansas delta.”

As for Midwest irrigation, “I’m sure there are producers that consider it. But even with the hot, dry conditions that held over the southern half of Illinois in 2007, farmers still produced fair to very good yields. Our crop had come along so well early that the roots had tapped into the soil water; when the heat wave hit it didn’t seem to hurt as much.

“We do have some irrigation systems in better soils. But if we get average rainfall they may not even be turned on in a season. It’s difficult to have an investment like that if it won’t be used.”

Irrigation in Illinois will likely “always be supplemental and not routine. That may change if Illinois ever gets to the point where rainfall is routinely only an inch or two in June, July and August.

“As it is, however, we don’t have the water supply in many areas to irrigate. And I don’t think anyone would like us building a canal and draining Lake Michigan to water corn. Illinois’ yields will have to drop dramatically and stay down for several years before irrigation could be considered on a lot of acres.”

Whether irrigated or not, corn’s significant water needs aren’t trivial.

“Think about a 160-acre cornfield irrigated with only 10 inches of water. That would take more than 250,000 gallons per acre, or some 40 million gallons for the field. So switching to irrigation isn’t a little thing.

“At the same time, this should help illustrate how much water actually falls on our crops. The average rainfall here is about 35 inches. The sky is our best, most efficient irrigator.”

Nafziger says there’s much talk in the industry about drought-tolerant hybrids being developed.

“I’m not quite sure how those might be adopted in Illinois. We just don’t experience serious drought often enough for farmers here to rush down and buy those the minute they’re available.

“But it’s certainly an interesting thing to throw into the mix. Biotech, in many ways, is being held out as the solution to our problems. I doubt it will be as simple as introducing new hybrids.”

Despite the amount it uses, corn is actually good with water-use efficiency — better than most crops grown. That’s one of the best things about it.

“I’m not convinced we’ll be able to improve on that a great deal. A side issue is many people point to grain sorghum as more water-use efficient. It really isn’t that much better.”

Some Midwest corn organizations claim the country will lose corn acres in 2008. But going by the amount of nitrogen applied this fall, Nafziger says that may not be the case.

“When fall weather is good and soils are cool enough, many people put N on in the fall. A lot of N was put out in the northern half of Illinois in November. Based on that, I don’t think we’ll reduce corn acres very much.”

Like farmers in the Mid-South, Illinois producers are excited with wheat and soybeans prices. But they’re also checking the math and finding that 200-bushel corn at $4 per bushel “can blow other crops away in profitability. And that’s even with higher N prices.

“Realistically, we’re talking about N prices changing but not ruining the chances for profits. A year ago, you may have been paying 30 cents per pound of anhydrous ammonia at 170 pounds, a typical rate. That would equal about $50.”

This year, that cost is less than $20 more than that. So corn’s profitability won’t be affected a great deal, yet.

“That isn’t to say fertilizer prices aren’t a concern. But costs of producing wheat and soybeans have also risen. It’s just more expensive to put any crop in today compared to five years ago.”

On the corn-or-soybean question, “we always compare yield as a first check. In 2007, corn was favored in Illinois. The yield ratio, which we always look at, is about 4-to-1 in the state. That’s friendly to corn since 3.2-to-1 is the average.”

Farmers won’t stop growing soybeans. But there are more fields and farms shifting to continuous corn.

“The good corn-following-corn crops this year haven’t hurt those plans. I do think there’s some overconfidence with corn going into 2008, given the good yields most have had for a number of consecutive years. If we have an unfriendly year to corn, it’ll be a big blow.”

The last decade has largely been good for corn in Illinois.

“Farmers are confident they’ll see good yields. It only takes one bad year, though, to change the mindset. But corn remains king here and isn’t likely to be knocked off the throne in the foreseeable future.”

e-mail: dbennett@farmpress.com