Comparing Water Use of Wheat and Corn

Evaluating crop water requirements might offer insight for
cropping decisions heading into the 2007 growing season.

By David Boehm and Greg Fox

Most of the Northern Plains region is sitting on low soil moisture levels due to reduced rainfall in 2006. Last season’s crops found a way to draw out what soil moisture was available to produce better than expected yields.

The question now is whether there will be enough moisture in 2007 to recharge soil water levels and produce a good crop.  This may be especially pertinent in light of the expected increase in corn acres.  Looking at crop water requirements might offer some insight for cropping decisions.

We know that wheat is a cool, short-season crop that takes advantage of early growth and moisture. Wheat needs good weather (sufficient moistures plus optimum growth temperature of 75-77 F, with minimum of 37-39 F and a maximum of 86-90 F) from May through July, about 90+ days. 

Corn can survive brief exposures to adverse temperatures, according to Purdue, such as temperatures ranging from near 32-41 F to 95-112 F, with optimal daytime ranges between 77-91 F.  Corn needs May through September, or 150+ days to produce a crop. This is 2+ months of additional risk and energy use compared to wheat.

The important consideration here is plant biomass production.  An annual grain plant is a factory that has to be built every year.  The corn plant is a massive factory that if built and fed liberally, can no question out-produce wheat, but that is the big “if.” It takes about 11” of water to build a corn plant before it starts to produce a single seed. Conversely, wheat requires only 5” of moisture to reach the heading stage.  Citing studies at NDSU, Terry Gregoire, area extension specialist, reports that after wheat heading, each additional inch of soil moisture or rain beyond that first 5” of moisture produces 4-7 bushels of grain.

Assuming there is an average of 6-8” of stored soil moisture available at early spring from western N.D. to western Minn., May, June and July usually provide an additional 6-9” of rain from west to east for wheat production.  This 12-17” of water is plenty to produce a good wheat crop considering heat and disease stress.

Corn receives an average regional rainfall from 12” in the west to 15” in the east during May to October.  The 6-8” of stored soil moisture plus 12-15” rainfall is enough to produce good corn yields, providing there are enough heat units to develop corn, which was not the case in 2004. 

Using average annual rainfall data, the region receives from 15-20” of moisture, not all which is absorbed. These comparisons suggest that both corn and wheat produce well with average available water and adequate growth temperatures.

If we may use a baseball analogy, corn is a “swing for the fences” crop. Wheat can be a good utility infielder. It doesn’t have the home run yield potential of corn, but as a lower water user doesn’t have the strike-out risk either.  As many growers prepare to swing for the fences, we must be mindful that from a moisture situation, we’re already behind the count.

Drought monitor maps and 2007 drought outlook forecasts do not paint a favorable picture for moisture this season.  Mark Seeley, extension climatologist with the University of Minnesota, announced concern for soil moisture levels in a Red River Farm Network interview. “For many western parts of Minnesota and the eastern Dakotas, (soil moisture levels) are at the lowest level for this time of the year since 1989.” He reports that less than 3” of subsoil moisture exist in many areas in the Red River Valley, where 6-8”of moisture is more typical.

Leon Osborne, professor of atmospheric sciences at the University of North Dakota, has said at a number of grower meetings this winter that judging by weather trends looking into the long-term, 2007 could see a repeat of last summer’s weather, which would be all the more troubling for those with less subsoil moisture.

Osborne believes crop producers in the Northern Plains will start the season with adequate moisture. But he is concerned that if higher temperatures and less moisture settle in later in the growing season, finishing off crops may be challenging in some areas. “We might not see a repeat of extreme drought like last summer, but with limited subsoil moisture, this year we’ll have less of a buffer against limited precipitation.”

According to Gregoire, loam soils can hold 2.25” of available water per foot of soil (1” per foot in sandy soils, and 2” per foot in clay soils).  Three feet of loam soils can hold nearly 7” of water, but since not all rain is absorbed, more than 7” of spring and summer rainfall is needed.  The message here is that for areas with low soil moisture, abundant spring rainfall will be needed to recharge soils.

There are other considerations as well, such as soil moisture recharge for the 2008 crop. The earlier harvest of wheat will allow more time for fall soil moisture recharge. Consider too that no matter how the crops perform, there are bills to be paid, and the direct costs of corn are higher compared to wheat (see table).

Certainly, there are risks inherent with producing any crop. That’s where management comes into play.  Of course, you’ll want good crop insurance. A good soil test when possible will show your current soil moisture level and soil type which can indicate water-holding capacity. This will also help you determine nitrogen levels and placement.

Although we can’t predict the summer weather, consider your average rainfall with regards to current soil moisture levels. Take a strong look at your crop rotation. As Joel Ransom, NDSU Extension Agronomist for Cereal Crops suggests, crops that follow sunflowers, safflower, corn and sugarbeets will start out at a disadvantage, especially if there is less moisture recharge.  Evaluate your moisture situation, prospective crop returns, and plan rotations accordingly.  The good news is that a number of cereal and broadleaf crops offer a promising price outlook in 2007 – if we may conclude with a mixed metaphor, it may not be necessary nor prudent to put all of your acres in one basket.

Comparison of Estimated Direct costs, 2007

Region of ND	Spring Wheat	Corn for Grain
Northwest	$77.45	$104.08
North Central	$78.56	$133.68
Northeast	$96.51	$160.27
North Valley	$108.20	$163.62
Southwest	$70.76	$92.31
South Central	$71.87	$147.51
East Central	$88.46	$153.81
Southeast	$95.36	$190.59
South Valley	$107.03	$200.66
Source: NDSU Extension Farm Management Planning Guides for 2007.  www.ag.ndsu.edu/pubs/ecguides.html

Boehm is northern business manager for AgriPro Wheat, with an MS in Crop Science; Fox is a plant breeder with Westbred LLC, with a PhD in plant breeding.