The last few irrigations of the season require some of the most important water management decisions of the year.
An unneeded irrigation may mean wasting 1 to 3 inches of water and 2 to 5 gallons of diesel fuel per acre. Even if your irrigation costs are only $10/ac-in, each inch reduction is worth about $1,300 for each quarter-section pivot. For example: If a field of silt loam soil is left at field capacity, it’s missing out on 5.5 inches of off-season storage, worth about $7,200 — not even accounting for the leaching loss of nutrients. Nitrogen loss has been shown by research to be about 5 to 10 lbs/a for each inch of water that moves through the profile.
Across Nebraska, during most years, enough precipitation will be received from October through May to refill the soil profile on irrigated fields. For example, the Grand Island area gets about 14.2 inches during this timeframe, and even the Scottsbluff area will receive about 8.6 inches on average.
The final decisions on furrow-irrigated fields need to be made sooner due to the typical higher application amounts with flood, while the final decisions on the pivot or subsurface drip irrigated fields can be delayed to take advantage of any rainfall that may occur. With this in mind, Nebraska Extension publication Irrigation Scheduling Strategies When Using Soil Water Data (EC3036), and the video series How to Schedule Irrigations with Soil Water Data were developed. The resources focus on season-long irrigation scheduling and wrap up with end-of-season scheduling.
Many irrigators apply more water than necessary toward the end of the irrigation season because irrigation applications continue at a rate they have grown used to during the middle of the summer, even though the crop is using less water per day. Water use goes down because the crop is getting more mature and the days are getting shorter and cooler. The average crop water use rate for corn drops from around 2.1 inches per week at silking to only about 1.2 inches per week by the full dent stage. This 40% reduction requires irrigators to adjust their thinking about how much water needs to be applied each week.
So, calculating the amount of water needed from rain and irrigation for the crop to reach maturity becomes important starting in early August.
Knowing approximately how much plant available water is remaining in the active root zone is critical for calculating the last few irrigations and will be referred to as the "remaining available water." The best method for determining the amount of remaining soil water is to use a soil water monitoring system. Table II (below) shows the typical amount of water that will be in the different soil types and can be used to help estimate the volume of water. Remember to use the top 4 feet of soil for the active root zone and plan to use the soil water down to 40% of plant available water (60% depletion) after the dough stage for corn/sorghum and the R5 stage for soybeans/dry beans.
Predicting how much rain the field will receive is difficult, but if you do not plan for the possibility of rain, it is easy to apply too much irrigation water. Figures 2 and 3 show the long-term weekly average rainfall for August and September in Nebraska.
Method to Estimate Late-season Crop Water Needs
To determine how much water from rain and irrigation will be needed to mature the crop, use the method described below and information from Table 1 (below).
Step 1. Based on the current crop growth stage (identified in columns 1 and 2), look up the approximate days to maturity in column 3 and the water use to maturity in column 4.
Step 2. Add the approximate days to maturity to today’s date to estimate the crop maturity date.
Step 3. Subtract the water use to maturity from the remaining available water (as determined by soil water sensors or monitoring). If the number is positive, it indicates adequate soil water to mature the crop; however, if the number is negative, it indicates the additional amount of water the crop will need from rain and irrigation to reach maturity.
The following sample calculations illustrate how to use this method.
Part 1
Determine how much water from rain and irrigation will be needed to reach crop maturity, assuming the field is located in central Buffalo County, the date is Aug. 10, and the corn crop is at the R4 (dough) stage. For this example, the remaining available water is 2.25 inches if the top 4 feet of soil is allowed to dry to 40% of available water-holding capacity.
- Referencing Table 1, corn at the R4 stage should take approximately 34 days to mature and will use about 7.5 inches of additional water to reach maturity.
- Add 34 days to today’s date to get the approximate day of maturity, using Aug. 10 as the example.
| Today’s date | 8/10 |
| Days to maturity | + 34 days |
| Predicted maturity date | = 9/13 |
- Subtract the water required to reach maturity from the remaining available water.
Remaining available water 2.25 inches Water required to mature crop – 7.50 inches Remaining water at maturity – 5.25 inches - The calculation shows that 5.25 inches of water will be needed from rain and irrigation to mature the crop without limiting yield.
Part 2
Determine the average rainfall per week for the remaining weeks of crop growth and add it into the prediction of how much water may be needed from irrigation. This number should be recalculated each week until the crop is mature.
- Use Figures 2 and 3 to determine the average rainfall per week in August and in September for central Buffalo County.
- The chart shows that, on average, central Buffalo County gets 0.60 inch of rain per week in August and 0.55 inch per week in September. The crop is expected to continue growing for three weeks in August and two weeks in September.
| Remaining available water at maturity | – 5.25 inches |
| Average August rain for three weeks | + 1.80 inches |
| Average September rain for two weeks | + 1.10 inches |
| Water need after accounting for average rain | – 2.35 inches |
The chart predicts that if average rainfall is received over the next five weeks, only 2.35 inches of irrigation would be needed to mature the crop.
Using this information for a center pivot that is set to apply one inch of water per irrigation, the suggested recommendation would be that the system should be off for the next few days to use some of the remaining water. The next inch of water should be applied within the next seven days. Then, the next irrigation should be recalculated using updated soil water readings.
The best way to use the remaining water is to delay the start of the last few irrigations a few days each time and react to any rain that might occur during the period. Slowly using the water in the lower portion of the root zone starting in early to mid-August is much better than keeping the profile full until the very end and expecting the crop to use the water all at once.
Differences in Late-season Irrigation Timing for Corn/Sorghum and Beans
Corn/sorghum and beans are somewhat different in respect to crop water use as they approach maturity. For example, if hot dry windy conditions are experienced in September, daily crop water use will increase. However, since corn/sorghum develop based on heat units, they will use more water per day but will mature in fewer days using about the average amount of water shown in the chart. Beans develop based on day length. So, with the same hot, dry, windy conditions, they will use more water per day and will still mature at about the same date, resulting in more water use than predicted in the charts. Thus, it is very important to monitor soil water in beans until they reach maturity.
For additional details and charts, see NebGuide Predicting the Last Irrigation of the Season.
| Crop Stage | Stage of Growth | Approximate Days to Maturity | Water Use to Maturity (inches) |
|---|---|---|---|
| Corn R2 | Blister | 45 | 10.5 |
| Corn R4 | Dough | 34 | 7.5 |
| Corn R4.7 | Beginning dent | 24 | 5.0 |
| Corn R5 | 1/4 milk line 1/2 milk line - Full dent 3/4 milk line |
19 13 7 |
3.75 2.25 1.0 |
| Corn R6 | Physiological maturity | 0 | 0.0 |
| Grain Sorghum Stage 6 | Half bloom | 34 | 9.0 |
| Grain Sorghum Stage 7 | Soft dough | 23 | 5.0 |
| Grain Sorghum Stage 8 | Hard dough | 12 | 2.0 |
| Grain Sorghum Stage 9 | Physiological maturity | 0 | 0.0 |
| Soybeans R4 | End of pod elongation | 37 | 9.0 |
| Soybeans R5 | Beginning seed enlargement | 29 | 6.5 |
| Soybeans R6 | End of seed enlargement | 18 | 3.5 |
| Soybeans R6.5 | Leaves begin to yellow | 10 | 1.9 |
| Soybeans R7 | Beginning maturity | 0 | 0.0 |
| Dry Beans R5 | Early seed fill | 35 | 7.0 |
| Dry Beans R6 | Mid-seed fill | 25 | 4.2 |
| Dry Beans R7 | Beginning maturity | 15 | 2.0 |
| Dry Beans R8 | Harvest maturity | 0 | 0.0 |
| Soil Textural Classification | Available water in 1 foot of soil at 100% of available water (in/ft) |
Available water in top 4 feet at 100% of available water (in/4 ft) |
Minimum balance in top 4 feet at 40% of available waterA | |
|---|---|---|---|---|
| Fine Sands | 1.0 | 4.0 | 1.6 | |
| Loamy Sand | 1.1 | 4.4 | 1.8 | |
| Sandy Loam | 1.4 | 5.6 | 2.2 | |
| Silty Clay or Clay | 1.6 | 6.4 | 2.6 | |
| Fine Sandy Loam, Silty Clay Loam, or Clay Loam | 1.8 | 7.2 | 2.9 | |
| Sandy Clay Loam | 2.0 | 8.0 | 3.2 | |
| Loam, Very Fine Sandy Loam, or Silt Loam Topsoil | Silty clay loam or silty clay subsoil | 2.0 | 8.0 | 3.2 |
| Medium Textured Subsoil | 2.5 | 10.0 | 4.0 | |
| aBased on leaving 40% of the available water holding capacity at physiological maturity. | ||||
