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**Conservation Tillage Fact Sheet**
- **Water Use/Yield Relationships for Central Great Plains Crops**
 **D.C. Nielsen - USDA-ARS, Akron, CO** **The Fact: Knowing and using these relationships can help you    make wise cropping decisions.**
**The single most importantfactor determining crop yield grown in rotations in the central Great Plains is availablewater. When nutrients are not limiting, and pests and weeds are controlled, therelationship between water used and final grain yield is usually a simple linear function.Knowing these relationships for various crops can be a valuable aid for producers makingcrop choices for rotations in their cropping systems.**

**The amount of water used bya crop is sometimes referred to as evapotranspiration or ET. It is comprised ofevaporation (the water that evaporates from wet soil and plant surfaces following a rain),and transpiration (the water that moves from the soil profile through the roots and stem,and out through the leaves to the air). Good residue management prior to the crop growingseason can increase the amount of water available for transpiration by increasinginfiltration rates of the soil, and by decreasing evaporation rates from the soil surface.**

**As water is taken up by roots, the plantcombines water with carbon dioxide from the air to synthesize plant parts: leaves, roots,stems, and grain. The more water the plant can take up, the more of these components theplant can make.**
**An example of the    relationship between ET and grain yield for corn and for wheat is shown in Fig. 1. The    equations at the top of the figure define the lines that run through the data points. The    first number in each equation is the slope of the plotted line. The steeper the slope, the    more responsive crop yield is to additional water. The point where the line intersects the    Cumulative ET axis of the figure shows how much ET is necessary before any grain can be    produced.**Fig 1.

Corn and wheat grain produced (lb/a)

as a function of water used (in).

- **For corn, the relationship tells us that    582.2 lb/a (10.4 bu/a) of grain are produced for each additional inch of water used after    about 9 inches of water use. Winter wheat, on the other hand, is less responsive to water    than corn, producing 390.4 lb/a (6.5 bu/a) of grain for each additional inch of water used    after about 6.8 inches of water use. The water use/yield equation coefficients for other    crops are given in Table 1.**
**Table 1.**
**Water use/yield prediction equation coefficients    and inches of ET that must occur before grain is produced****Crop****Slope (lb/a per    inch ET)****Y-axis    intercept****inches    of ET before yield****Corn****582.2****-5314****9.1****Winter Wheat****390.4****-2637****6.8****Proso Millet****236.7****-818****3.5****Safflower****185.2****-1698****9.2****Sunflower****160.5****-843****5.3**
**From the slopes of therelationships in Table 1, we note that corn is the most responsive to additionalincrements of water, while sunflower is the least responsive. Proso millet requires theleast amount of water to grow the plant before grain production begins (3.5 inches).**

**An example of how thesewater use/yield relationships can be used is given in Table 2. In this example we assumethat we sampled the soil prior to corn planting and found that there were 4 inches ofextractable water in the soil profile. This is water stored in the soil from the time thelast crop was harvested to the time the corn crop is planted that will be available to thecrop during the growing season. Table 2 gives three possible precipitation scenarios:average precipitation during the growing season, and average precipitation plus or minus30%. (This range would cover about 80% of the years at Akron, CO.) Precipitation plusextractable water is the total growing season ET that is used with the equationcoefficents given in Table 1 to arrive at the predicted yield. For example, an extractablesoil water content of 4 inches and a growing season precipitation of 11.4 inches wouldgive a total water use of 15.4 inches. Using the factors given in Table 2 for corn, wewould calculate:**

**15.4" * 582.2 - 5314= 3652 lb/a or about 66 bu/a.**

**If the ET were reduced to 12inches (4" + 8"), then the grain yield would be 1672 lb/a. Under wetterconditions of 18.8 inches of ET (4" + 14.8"), we would expect to harvest 5631lb/a. Table 2 shows the results of similar calculations for other crops.**

**Table 2. Predictions of yield for various crops assuming varyingamounts of growing season precipitation and using the water use/yield relationships givenin Table1.**

**----- Precipitation (in) ------**
**Grain Yield at Given Precipitation Level (lb/a)**
**Crop**
**Extractable    Soil Water (in)**
**Avg    - 30%**
**Average**
**Avg    + 30%**
**Avg    - 30%**
**Average**
**Avg    + 30%**
**Corn**
**4**
**8.0**
**11.4**
**14.8**
**1672**
**3652**
**5631**
**Wheat**
**4**
**5.6**
**8.0**
**10.4**
**1111**
**2048**
**2985**
**Millet**
**4**
**5.5**
**7.8**
**10.1**
**1430**
**1975**
**2519**
**Safflower**
**4**
**8.3**
**11.9**
**15.5**
**580**
**1247**
**1913**
**Sunflower**
**4**
**6.2**
**8.9**
**11.6**
**794**
**1227**
**1661**
**Final Comments**

**One caution that users ofthese water use/yield relationships should be aware of is that final grain yield of a cropis influenced by the timing of precipitation as well as the amount of water used. This isone reason why the data points shown in Fig. 1 do not all fall exactly on the wateruse/yield relationship line. In fact, two of the points shown for corn in Fig. 1 at about17" of cumulative ET are about 1700 lb/a lower than the yield predicted by the wateruse/yield relationship. This happens because of corn's high sensitivity to water deficitsduring the tasseling and pollination stages. Low water availability and restriction of ETat this time reduces yield much more than a similar level of water deficit during theearly vegetative growth stage. Consequently, the water use/yield relationships given inTable 1 are valuable for approximating potential grain production for an upcoming growingseason, but will not exactly predict final grain yield due to variations in yield thatwill occur due to timing of water stress.**

**Predicting growing seasonprecipitation is virtually impossible. But a producer can know the amount of extractablewater in the soil profile by soil sampling with a hand probe, or with tensiometers orgypsum blocks. Knowledge of the amount of water in the soil in the spring along with anassumed amount of growing season precipitation in the proper water use/yield relationshipscan help the producer make a wise decision regarding the choice of a crop to use in hisproduction system.**
- Conservation Tillage Fact Sheet #2-95. Published by USDA-NRCS,    USDA-ARS, and Colorado Conservation Tillage Association.
- By: D.C. Nielsen, Research    Agronomist, USDA-ARS, Akron, CO.

**[Top][ 2002 Annual Report ][ Fact Sheets ][ Feed Back ][ Field Day ][ NRCS-Akron ][ Our Research ][ Publications ][ Related Links ][ Staff Pages ]****Questions? Comments? Contact forWeb Info RequestLast edited:****Wednesday April 14, 2004** |
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