Final irrigation of the growing season - Timing is everything

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This year has been challenging for irrigators as we started the year with extremely low levels of profile water in most places, highly variable precipitation, and periods of above-normal temperatures.  As we look towards the end of the irrigation season, producers have an opportunity to improve their water productivity by properly timing their final irrigation application. This is an important decision as an early termination of irrigation can result in reduced grain yield, primarily through reductions in the kernel weight yield component. Conversely, a late termination of irrigation results in unnecessary pumping and energy consumption, increases the risk of soil compaction at harvest due to increased soil water content, and increases the risk of water loss from the soil profile through drainage over the winter.

With the goal of matching available water to crop needs while avoiding excess, it is important to understand crop water use requirements late in the growing season. Table 1 shows anticipated water use from various growth stages until physiological maturity for corn, grain sorghum, and soybeans. It is important to note that this is total water use that could come from multiple sources, including precipitation, irrigation, and stored soil water.
 

Table 1. Anticipated water use for corn, grain sorghum, and soybeans at various growth stages.

Crop	Growth stage	Approximate days to maturity	Water use to maturity (inches)
Corn	Blister	45	10.5
	Dough	34	7.5
	Beginning dent	24	5
	Full dent	13	2.5
	Black layer	0	0
Grain sorghum	Mid-bloom	34	9
	Soft dough	23	5
	Hard dough	12	2
	Black layer	0	0
Soybeans	Full pod	37	9
	Beginning seed	29	6.5
	Full seed	17	3.5
	Full maturity	0	0

Adapted from K-State MF2174, Rogers and Sothers

 

Research in western Kansas has shown the importance of keeping the management allowable depletion limited to 45% during the post-tassel period. In other words, maintaining available soil water contents above 55%. By knowing anticipated water use from a given growth stage and the remaining soil water in the profile, producers can add just enough irrigation water to meet that demand and maintain profile available soil water content above 55% while adjusting for any precipitation that may be received.

By closely following the growth and development of the crop, one can know when physiological maturity, i.e., the black layer in corn or sorghum, has been reached. At that point, water use for the production of grain yield has ceased, and additional irrigation is certainly unnecessary.

Termination based on calendar dates

Traditionally, many producers have used a fixed calendar date to determine their final irrigation. Long-term studies conducted by Freddie Lamm at the Northwest Research-Extension Center at Colby show the potential problems in this approach. Table 2 shows silking, maturity, and irrigation termination dates for a long-term study in corn. For this study, the irrigation termination date for maximum grain yield varied from August 12 to September 21. This is a significant departure from a general rule of thumb using Labor Day as a termination date. As shown, using a fixed date on the calendar without regard to crop progress, soil water status, or ET demand would have resulted in both forfeited yield and wasteful pumping across this timeframe.

Table 2. Silking, maturity, and irrigation termination dates for a long-term study in corn.

 

Consequences of excess late-season irrigation

In the silt-loam soils common in western Kansas, water drainage out of the soil profile starts to occur when the profile water content rises above 60% available soil water. The rate of drainage loss increases rapidly with increasing water content. Late-season irrigation in excess of crop water use results in increased accumulation of water in the profile, which is subject to drainage losses. A survey of irrigated corn fields was conducted in 2010 and 2011 (Figure 1). Fields were surveyed after corn harvest across three east-west transects in western Kansas.

Figure 1. Results from a 2-year survey of irrigated corn fields. Fields were surveyed after harvest across three east-west transects in western KS.

The line at 9.6 inches of plant-available soil water (PASW) denotes the approximate water content where drainage losses would start to occur. On average, most producer fields were near this level of soil water storage indicating a good management strategy as drainage losses had been minimized while yet maintaining adequate soil water to complete grain fill.

Producer fields near the minimum observed values likely did not have adequate soil water to ensure maximum grain yields. The most concerning scenario, however, is the fields at the upper end of soil water values such as the maximum observation. The red line at 16 inches PASW represents field capacity, the point at which free drainage and significant water losses from the profile would occur. In the wettest producer fields, in all three regions, significant amounts of free drainage and water loss would have been occurring at the time of crop maturation and harvest.

Timing of the final irrigation:

1. Determine crop growth stage and anticipated remaining water use
2. Determine soil water status in the field by probe or calibrated soil sensor technology
3. Determine the irrigation strategy necessary to meet remaining crop water use while maintaining soil water content at or above 55% (limit depletion to 45%).
4. Be ready to make adjustments based on changes in ET demand, precipitation, etc.

Additional information, including a step-by-step procedure, can be found in the publication MF2174: “Predicting the final irrigation for corn, grain sorghum, and soybeans” -  http://www.bookstore.ksre.ksu.edu/pubs/MF2174.pdf

 

Special Note:  Much of the data in this article was collected by Freddie Lamm, Irrigation Engineer at the Northwest Research-Extension Center at Colby.  Freddie passed away in May 2022, just months short of completing his 43^rd year of irrigation research at the NWREC.  A tribute to Freddie’s career can be found at: https://newprairiepress.org/cgi/viewcontent.cgi?article=8336&context=kaesrr

 

Lucas Haag, Northwest Area Crops and Soils Specialist
lhaag@ksu.edu

Tags:  dryland soil moisture irrigation