Considerations for Estimating the Last Irrigation in the Mid-South

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Irrigation operation - soybean field

Field Facts
From Pioneer Agronomy Sciences

Key Points

Irrigation management near the end of the season should accomplish the following goals:
  1. Provide enough available soil moisture in the root zone to carry the crop to maturity and produce the best yields.
  2. Deplete the soil ‘moisture reservoir’ as far as possible to allow room for storing off-season precipitation and to minimize fuel, labor, and water expense or demands for the present season.
Although these requirements appear to conflict, the problem can be solved if adequate field information is available or can be accurately predicted. The following information is necessary to predict the date of the last irrigation:
  • Predicted crop maturity date.
  • Predicted water use by the crop to maturity.
  • Remaining usable moisture in the root zone.
  • Probability of significant amounts of rainfall before crop maturity.

Obviously, rainfall is difficult to predict. Consequently, we assume that no rainfall will occur.

Late-Season Crop Water Use

Corn and soybean plants require some moisture up to the time of physiological maturity. Physiological maturity is defined as the time when dry matter accumulation in the kernels or seeds ceases, in other words the grain stops ‘filling’. Black layer formation at the tip of the kernel is a good indicator of physiological maturity for corn. For soybeans, physiological maturity has begun when one normal pod on the main stem has reached its mature pod color.

The water requirements by growth stage for corn and soybean, given in Table 1 and Table 2, were calculated by using crop development rates and normal water use patterns for Arkansas.

Table 1. Estimated corn water use in Arkansas based on a planting date of April 1.

Days After Planting Water Use
0-30 (early plant growth) 0.05-0.10
30-60 (rapid plant growth) 0.10-0.20
60-100 (reproductive stage) 0.20-0.30
100-120 (grain fill to maturity) 0.25-0.10

Source: Arkansas Corn Production Handbook

Table 2. Soybean growth and water use in Arkansas.

Crop Development Water Use
Germination and Seedling 0.05-0.10
Rapid Vegetative Growth 0.10-0.20
Flowering to Pod Fill (Full Canopy) 0.20-0.30
Maturity to Harvest 0.05-0.20

Source: Arkansas Soybean Handbook

Soil Moisture

Since a portion of the required moisture can come from the soil moisture reservoir, the last irrigation can usually be applied one to four weeks prior to physiological maturity, depending on the soil type and its water holding capacity. Refer to the first two columns in Table 3, or better yet a county soil survey to determine the available water holding capacity for your soil. Available soil moisture, or water holding capacity, is the portion of the soil moisture than can be effectively used by the crop. Most research indicates that a plant can extract approximately 50-60% of the total moisture held in the soil.

Calculated allowable soil moisture deficits and minimum allowable balances at crop maturity for various soils are presented in the table below. If you suspect that the root zone is less than four feet deep due to hardpans or gravel layers, you will need to figure the appropriate rooting depth for your situation.

Table 3. Allowable soil moisture deficits and minimum allowable balances at physiological maturity.

Soil type Available water capacity Allowable moisture deficit in top 4 ft of soil profileª Minimum allowable balance in top 4 ft of soil profileª
  in/ft ——————— inches ———————
Silty clay loam 1.6 3.8 2.6
Upland soil loam 2.0 4.8 3.2
Bottomland silt loam 2.5 6.0 4.0
Very fine sandy loam 1.8 4.3 2.9
Sandy loam 1.4 13.4 2.2
Fine sands 1.0 2.4 1.6

ªBased on depletion of 60 percent of the available water.

The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary. Pioneer® brand products are provided subject to the terms and conditions of purchase which are part of the labeling and purchase documents.