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Issue No. 11, Article 4/June 15, 2012

Crops and Water

Illinois cornfields in areas that have had some rain continue to look good, but crop ratings are still sliding as corn shows visible symptoms of drought stress in the areas that have received little rain over the past six weeks. As of June 10, National Agricultural Statistics Service reports that the average height of the corn crop is a record 29 inches, but it is rated at only 56% good to excellent, and the soybean crop at only 50% G-E.

With few other problems such as hail, nutrient deficiency (except those related to dry soil), disease, or insects, most of the perceived deterioration in crop condition is due to the ongoing lack of rainfall and the crop stress symptoms that accompany it. According to data from the Midwest Regional Climate Center, rainfall between May 1 and June 12 ranged from 2 to 5 inches in Illinois, with deficits from normal ranging from 1 to 2 inches along and south of I-80 to 5 inches or more in extreme southern Illinois. Some rainfall on June 12 in some of the driest areas of southern Illinois moved these totals up a little, but dryness persists through much of the state.

While we all sense when we're in a drought, there are ways to look at dryness from the crop's perspective, which often differs from the human perspective. One way is through an indication reported weekly by NASS about how much of the surface and subsurface soil is dry. On June 10, 26% of topsoil was listed as being "very short" on moisture, 52% was "short," and 22% was adequate. Subsurface numbers were similar, with a little more (28%) listed as adequate in moisture.

Another way to look at severity of drought is through the Palmer Drought Index, a complex measurement taking into account both rainfall (as compared with normal) and anticipated effects on agriculture. As of June 5, most of Illinois, with the exception of the northeastern part of the state, was "abnormally dry," which is the mildest form of drought. A small part of extreme southern Illinois was categorized in "severe drought," which would be expected to reduce crop yields considerably if it persisted much longer. Parts of that area that received some rainfall this week will drop out of the severe-drought category by the time the next Palmer map appears. An area north of the severe-drought area and another in central-west central Illinois were in "moderate drought."

How are crops faring in this ongoing period of less-than-normal rainfall? As we've said before, water use and photo¬synthetic rates are closely linked, so a decrease in water availability means a decrease in the crop's daily dry matter production. So while cooler weather in recent weeks has decreased water use rates, and the corn crop can have its leaves out and active longer in the day before they roll up when the roots can no longer provide water fast enough, lower temperatures also mean slower rates of photosynthesis.

As the corn crop grows and expands its leaf area, water loss rates increase as photosynthetic rates increase. Figuring out how much water the crop uses as it grows is a little complicated, but it starts with knowing how much "demand" the atmosphere has for water each day, then using a factor called the "crop coeffi¬cient" to estimate how much of this demand the crop provides--that is, how much water the crop loses through the process of transpiration. Transpiration is loss of water vapor from leaves, and its rate is linked to the rate of intake of CO2 during photosynthesis, hence the close tie between the two processes.

The amount of water that evaporates from an open pan is used to estimate demand on a daily basis. This amount increases with wind speed, temperature, sunshine, and low humidity, and under central Illinois conditions it ranges up to about 0.3 inches per day. According to the Illinois Climate Network (part of the Illinois State Water Survey), open-pan evaporation was estimated at 6.84 inches at Champaign last month. This is a very high value for May--evaporation was only 3.91 inches in May 2011. Rainfall during May 2012 was 3.55 inches.

The larger the crop plant, the more water the crop uses; the crop coefficient ranges from 0 to 1 and is the percentage of open-pan evaporation that the crop uses at each stage of development. The crop coefficient for corn rises from around 0.1 at VE or V1 to about 0.2 by V3, 0.4 by V6, 0.6 by V11, 0.7 by V15, and 0.9 by the time pollination has ended and the canopy is full. So the corn crop planted in mid-April and growing from stage V1 to V7 with the 500 GDD received in May this year would have transpired about 30% of potential evaporation in May, or about 2 inches of water. Evaporation from the soil surface would have used a little more, but total use would not have exceeded the 3.55 inches of rain in Champaign during May. Such a crop is now at about V10, with a crop coefficient of above 0.5, and so is using about 1 inch of water per week. By mid-June, such a crop has used a total of about 5 inches of water.

In areas with less rain, soils that can hold 2 to 3 inches of plant-available water per foot of depth would easily have had enough water to keep the crop growing through mid-vegetative stages. The reason this did not happen in some areas, where plant leaves are rolling on a daily basis, is that the root system has not been able to tap the water that is available in the soil. "Rootless" or "floppy" corn with poorly developed nodal roots represents one reason for this. In some fields that were planted later or into drier soils, roots were unable to get into more moist soil zones early, and once soils dried between these zones and the ends of the roots, the ability of roots to reach these zones was lost. Insect or disease problems of roots are contributing in some cases as well.

Soybean water use follows a pattern similar to that of corn, with the crop coefficient increasing steadily from about V2 through R3 or so, reaching a maximum of about 0.8, so a little lower than corn's maximum use rate. Soybeans planted in April here at Urbana have reached V6 and are about 16 inches tall; their crop coefficient of around 0.45 would mean a daily water use rate of perhaps 1/8 of an inch, and water use up to now totaling perhaps 3 to 3.5 inches. Most of the May-planted crop is only at V3 or so and is using water less rapidly.

Some soybean fields planted into dry soils have poor stands in places, and others have roots that have not yet reached soil moisture. So the soybean crop is showing symptoms of lack of water in many fields as well. Leaves don't roll in soybean as they do in corn; instead they tend to lose turgor and droop, in some cases dropping to vertical orientation. This reduces the intensity of sunlight on the leaf and the heating that comes with it when there's not enough water to keep photosynthesis going, but it also means leaves are doing no photosynthesis and so will not show any growth for the time they are in this condition. Note that soybean leaves tend to drop to vertical during the night as a natural phenomenon--this helps protect them from radiational cooling and can be a benefit on cool nights. They normally reorient to horizontal as the sunlight increases in the morning.

Though we tend to worry a lot when the weather is dry, the best indication of how the plant perceives dry weather--that is, how much water it's able to extract from the soil despite lack of rainfall--is visible in how the plant grows. Many have commented on how well the corn crop is increasing its plant height even in dry weather. In drier areas or with the crop farther behind and lacking good roots, though, the current dry period will result in decreases in plant size. If this persists, those cells that make up kernels, silks, leaves, and seeds may also be restricted in size, with direct effects on yield.

Plant height and leaf area expansion occur when water moves into cells to push out cell walls. Cell walls harden after this expansion, after which no further size increase is possible. Cells have to attract water from the rest of the plant in order to expand, so this process is quite sensitive to water availability. This is why most cell expansion--what we see as visible "growth"--takes place at night, when there is no transpiration.

True growth is an increase in plant dry weight; this happens only during photosynthesis (so in daylight hours) and is often not visible to the eye. It of course powers, through sugar formation, processes that result in cell expansion, but taking water into cells is not really "growth." Still, the ability to photosynthesize and to form grain is affected by cell expansion, so having enough water that cells can expand fully is critical.--Emerson Nafziger

Author:
Emerson Nafziger

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