Scheduling the Final Irrigation for Wheat and Barley
Crop production in irrigated areas is becoming more water constrained. In many areas, municipal water use is increasing with urban expansion where periodic drought episodes occur. Water rights adjustments will potentially reduce the amount of available irrigation water, particularly where sourced from groundwater. At the same time, increased irrigation (i.e., water, energy, and labor) costs as well as other production inputs have reduced the economic return for grain crops. As a result, it is now more necessary than ever to achieve the best grain yield and quality per unit of water applied.
Management of crop water stress at different grain formation stages offers an opportunity to conserve water during late-season crop development without adversely affecting yield and quality. In many irrigated areas, irrigation of spring grains continues after the kernel is mature with the belief that continued water applications increase grain weight and yield during all crop growth stages. Field experience of long-time University of Idaho Extension faculty indicates that when the final irrigation is applied to refill the soil profile of sandy-loam or silt-loam soils to field capacity at the soft dough stage, sufficient water can be stored in the soil profile to meet the crop water requirement until harvest. Water applied after these stages will either remain in the soil profile or percolate below the crop root zone, without improving returns.
Optimum irrigation management during grain formation sustains agronomic and economic productivity while reducing the water applied and the risk of water-related diseases. The purpose of this bulletin is to help growers determine the optimum timing and amount of the final irrigation application needed to sustain agronomic and economic returns for wheat and barley production.
In addition to yield, grain quality (e.g., protein, plumps, test weight) of wheat and barley is a crucial consideration for end-use quality. For example, water management during grain formation stages affects the grain quality where water applied by a sprinkler system close to grain harvest can result in water-related diseases that reduce grain test weight and quality.
Water-Use Pattern for Wheat and Barley
An average crop water-use pattern for early spring to harvest periods of spring and winter grains is shown in Figure 1. The timing difference is due to winter grain evapotranspiration (ET) beginning in the spring as soon as sufficient heat units have been received to break dormancy (i.e., greenup), while spring grain ET is delayed until the crop can be planted and emerge. Following spring grain emergence, the water-use patterns are similar, with winter grain reaching maturity a few weeks sooner. An important characteristic of the water-use curve for both winter and spring grains is the rapid decrease in ET at the end of the season. The decrease is more abrupt than for most other major crops in Idaho. This characteristic is important to keep in mind since it is easy to continue irrigating to meet peak ET even after the ET has dropped. Overirrigation at this point increases irrigation costs, does not increase yield or quality, may increase the chance for fungal diseases such as black point, and may decrease protein and test weight.
The early water-use pattern and rapid end-of-season ET decline make grains an effective rotation crop in water-short conditions, since water can be shut off on the grains early with limited yield or quality penalty and transferred to other crops such as potatoes, corn, or alfalfa, where timely application of irrigation water will provide a greater economic benefit.
Yield and Quality Response to Final Irrigation Timing and Nitrogen Application Rates
Potential grain yield is determined during tillering. The maximum number of grain heads is determined at this stage, where the crop is only 4–6 inches tall. Yield losses due to excessive stress during this stage typically cannot be recovered; the remaining yield potential can only be maintained by best management for the remainder of the season. The next critical stage is flowering, when the number of kernels per head is determined. Following flowering, excessive water stress can reduce the number of seeds per head and the weight and plumpness of the kernels. This is reflected in Figure 2, which shows lowest yield with earliest cutoff (greatest and earliest water stress).
In five-plot years (two locations) for wheat and three-plot years (one location) for malting barley, there was never a significant yield increase because irrigation after the root zone was filled at soft dough. In most years the yield was statistically equal or reduced for irrigation past soft dough. Black point was more of a problem in later irrigation cutoffs in years such as 2000, with weather conditions favoring its development (Table 1). Therefore, under full-water conditions, the best return for water applied was to stop irrigation with the root zone filled to hold 2.0–2.5 inches of usable water at soft dough. In water-short conditions, Figure 2 indicates the yield penalty that would be incurred by an earlier irrigation cutoff. The value of lost grain production could then be compared to the potential gain from adequately irrigating another crop. For example, irrigation cutoff with a full profile at early milk would give an expected yield of only 70% of full season irrigation.
|Crop||Crop Stage at Irrigation Cutoff|
|Late milk (%)||Soft dough (%)||Hard dough (%)|
|Two-row malting barley||3||4||9|
In a trial conducted in 2015 on malting barley at Kimberly, Idaho, final irrigation at soft dough resulted in similar returns as compared to an additional irrigation. When the final irrigation was at late boot, severe yield losses and elevated proteins for malting purposes were measured. Maximum measured yield was achieved at the same nitrogen rate (135 lbs applied N + soil inorganic N) at both the soft dough and soft dough + 7d cutoff, where additional nitrogen applications did not increase yield; proteins numerically increased at N application levels above those needed to achieve maximum measured yield (Figure 3). Nitrogen applications rates to maximize yield at the soft dough and soft dough + 7d irrigation cutoffs were within the range of current University of Idaho recommendations for irrigated spring malt barley. Thus, proper fertilizer nitrogen and irrigation management will ensure optimal agronomic and economic goals are met.
Water Use During Late-Season Crop Stages
Water use during a given crop stage will vary somewhat from year to year, depending on weather conditions. Values given in Table 2 are three-year averages for two-row malting barley at Burley, Idaho. Values for hard red spring wheat are estimated using an equation relating cumulative water use to Haun crop stage. Thus, irrigation cutoff at soft dough is applicable if at least 2 inches of usable water is stored in the crop root zone for wheat and 2.5 inches stored for barley.
|Crop stage||Two-row malting barley (in)||Hard red spring wheat (in)|
|Emergence to milk||11.6*||14.3**|
|Milk to soft dough||3.4||2.1|
|Soft dough to hard dough||1.9||0.8|
|Hard dough to harvest||0.8||1.2|
|*2000–02 average for two row malting barley at Burley, Idaho.
**Estimated from Y = 0.110X^2 - 0.00244X^3 + 0.27 where Y = cumulative water use (inches) and X = Haun crop stage of development (Bauer et al. 1989).
Usable Water Stored in the Crop Root Zone
Measured root-water extraction patterns indicate that barley and wheat can remove water from soil to a depth of at least three feet if no restrictive soil layers are present. Depth of the crop root zone and the soil texture establishes the water-holding capacity (WHC) of the root zone. Only about half of the WHC can be used between irrigations without reduction in crop yield or quality. Root-zone depth may be limited by rock, dense soil layers, seasonal high-water table, or irrigation system operation. Water that can be used by the crop for various soil textures and depths is shown in Table 3. Although many other soil textures are present in southern Idaho, these textures cover the majority of the cropped area. Heavier-textured silt loams are generally found in the Treasure Valley, the south side of the Snake River in the Magic Valley, and in some areas of eastern Idaho. Light-textured silt loams are generally found in the Upper Snake Valley (Rexburg Bench, etc.), while sandier soils generally occur on the north side of the river in the Magic Valley and the Fort Hall area.
|Root-zone depth (in)||Sandy loam (in)||Light-textured silt loam (in)||Heavier-textured silt loam (in)|
Soil texture for a particular field may be determined from the United States Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS) soil survey or by other field or laboratory tests. Fields may contain multiple soil textures, so the dominant one is usually chosen for water management purposes. Table 4 combines water required past soft dough and usable root-zone water storage to identify the depth of water that must be applied past soft dough on shallow or low water-holding soils. For example, a sandy-loam soil 12 inches deep will need an irrigation of about 1.2 inches after soft dough to supply additional water to bring the crop to harvest with no yield or quality penalty. Note that for light or heavier-textured silt-loam soils at least 24 inches deep, no additional water is needed after soft dough if at least the top 2 feet of soil is wetted to field capacity at that time.
|Root-zone depth (in)||Sandy loam (in)||Light-textured silt loam (in)||Root zone depth (in)|
It is important to note that roots will not extend into dry soil in search of deeper water; moist soil is required for root extension. For example, in heavier-textured silt-loam soils, runoff potential limits application per pivot revolution to no more than 0.75 inches. If the soil starts at field capacity, 0.75 inches of water use will dry the soil to about 70% available soil water. Adding 0.75 inches of net irrigation will re-wet the top 18 inches to field capacity. In this scenario, the effective root-zone depth is about 18 inches due to system operation, even if deeper soil is present.
Table 5 is one tool that helps to estimate the water required to refill the root zone. Percent-available soil water may be determined by the feel and appearance method. A description may be found in University of Idaho (UI) Bulletin 833, “Estimating Water Requirements of Hard Red Spring Wheat for Final Irrigations.” USDA-NRCS also offers an excellent visual tool, “Estimating Soil Moisture by Feel and Appearance.” In addition, Table 5 relates readings from watermark granular matrix sensors or tensiometers to percent-available water. Lastly, the Washington State University Irrigation Scheduler can be used to estimate available soil water. The depth of water required to refill 1 foot of soil at the indicated moisture content to field capacity is also given for the three soil textures. For example, if a tensiometer reads 30 cbars (centibars) on a sandy-loam soil, percent-available water is 50%, and 1.04 inches of water is required to refill one foot of soil to field capacity with a center pivot or 1.19 inches is required using a hand or wheel line. The same tensiometer reading on a heavy-textured silt loam indicates that the soil is at 100 percent-available water (field capacity) and no refill is needed.
|Available soil water||Sandy loam
WHC = 1.67 in/ft
|Light-textured silt loam
WHC = 1.97 in/ft
|Heavier-textured silt loam
WHC = 2.25 in/ft
|Watermark reading||Pivot or linear||Hand or wheel line||Watermark reading||Pivot or linear||Hand or wheel line||Watermark reading||Pivot or linear||Hand or wheel line|
|(%)||(cbars)||(inch of water to refill
1 ft of soil)
|(cbars)||(inch of water to refill
1 ft of soil)
|(cbars)||(inch of water to refill
1 ft of soil)
What Is Soft Dough?
It is somewhat difficult to develop a description of soft dough that produces consistency among a number of irrigators. Many have developed their own distinct approach, which is successful for them but may not transfer well to others. Useful definitions for a number of crop stages have been developed:
- Flowering: pollen shed
- Milk: kernel liquid appears milky
- Soft dough: kernel is mealy or doughy (Figures 4 and 5)
- A rule of about 24 days after flag leaf emergence can be helpful in identifying the general time period
- Hard dough: kernel starch is firm and can be divided with a thumbnail while holding its shape
Economic Implications of Early Irrigation Cutoff
Table 6 summarizes the relative yield and quality of malting barley when irrigation is stopped at specified crop stages with a 2 foot root zone near field capacity. The yield relative to that at soft dough is the same as that shown in Figure 2. The relative price based on quality reflects less plump kernels when irrigation cutoff is at milk and reflects the tendency toward black point with irrigation after soft dough on adequate water-holding soil. Combining these factors gives the calculated crop value per acre for irrigation cutoff at the indicated crop stages. This table may be used to estimate the increase in crop value as final irrigation is scheduled at later crop stages. For example, scheduling the final irrigation at soft dough rather than milk increased the crop value from $634 to $813 or $179/acre. However, scheduling the final irrigation post–soft dough (nearly hard dough) decreased the crop value by $106/acre. Table 7 summarizes the crop value for final irrigation at specified crop stages for hard red wheat. As was the case for malting barley, irrigation past soft dough with a 2-foot root zone on a silt-loam soil at field capacity decreases the crop value relative to soft dough and incurs additional irrigation costs.
|Irrigation stopped with root zone filled to field capacity||Relative yield SD base (decimal)||Relative price based on quality (decimal)||Relative crop value (decimal)||Crop value ($/ac)*|
|Soft Dough (SD)||1.0||1.0||1.0||813|
|*Based on average contract prices of $6.50/bu or $13.50/cwt and expected nonstressed yield of 125 bu/acre.|
|Irrigation stopped with root zone filled to field capacity||Relative yield SD base (decimal)||Crop value ($/ac)*|
|Early Soft Dough||0.93||409|
|Late Soft Dough||1||440|
|*Based on average price of $4.00/bu and average yield of 110 bu/ac.|
Table 8 indicates that two pivot irrigations or one set-move irrigation could be saved by irrigation cutoff at soft dough rather than hard dough on adequate water-holding soils. An alternative method of assessing irrigation cost is shown in Table 9, which shows the effect of system type and pumping lift. For example, the typical cost of applying 1 acre-inch of water by center pivot from a canal source is about $0.69, while it would be about $2.74 for a 400-foot lift. In contrast, the cost of applying 1 acre-inch with a set-move system from a canal source is about $4.35 or $6.87 for a 400-foot lift. The difference in cost is primarily due to labor and to the extra pressure and water required to deliver 1 acre-inch net with a set-move system.
|Treatment||Irrigation depth (in)||Estimated number of pivot irrigations*||Estimated number of solid-set or set-move irrigations**|
|Emergence to Milk||11.6||14||6|
|*Net irrigation of about 0.8 inches per revolution.
**Net irrigation of about 1.8 inches per set.
|System type||Canal water||Groundwater lift (ft)|
An alternative approach to evaluating the need for additional irrigation at various crop stages is shown in Figures 6 and 7. The crop value and irrigation costs are summarized in Figure 6. The curve for total value of the product in Figure 6 can be obtained by methods shown in Tables 6 and 7. Irrigation cost is for a center pivot with 200-foot lift. As shown earlier in Figure 2, the maximum crop value occurs when irrigation is stopped with a relatively full root zone at soft dough.
Figure 7 evaluates this same information in terms of the marginal cost of irrigation and the marginal crop value resulting from the final irrigation occurring at various crop stages. It shows that for all crop stages before soft dough, the crop value added by each irrigation exceeds the cost of that irrigation. Profit is maximum at the stage where the marginal cost of the irrigation equals the marginal value produced by that irrigation. This analysis also shows maximum profit occurring with irrigation stopped at soft dough with a full root zone (i.e., 2.0–2.5 inches of usable water in the crop root zone).
References and Additional Resources
Ashley, R.O., W.H. Neibling, and B.A. King. 1998 (revised). Irrigation Scheduling: Using Water-Use Tables. University of Idaho Extension CIS 1039. 12 p. https://www.extension.uidaho.edu/publishing/pdf/CIS/CIS1039.pdf.
Ashley, R.O., L.D. Robertson, M. Seyedbagheri, and I.C. Hopkins. 2001. Estimating Water Requirements of Hard Red Spring Wheat for Final Irrigations. University of Idaho Extension Bulletin 833. 8 p. https://www.extension.uidaho.edu/publishing/pdf/BUL/BUL0833.pdf.
Bauer, A., A.L. Black, and A.B. Frank. 1989. Soil Water Use by Plant Development Stage of Spring and Winter Wheat. North Dakota State University Agricultural Experiment Station Bulletin 519. 22 p.
Hill, S., and T. Peters (Washington State University Irrigated Agriculture Research and Extension Center). n.d. “Irrigation Scheduler Mobile” (online resource with login). http://weather.wsu.edu/is.
Robertson, L.D., and J.C. Stark, eds. 2003 (revised). Idaho Spring Barley Production Guide. University of Idaho Extension Bulletin 742. 60 p. https://www.extension.uidaho.edu/publishing/pdf/BUL/BUL0742.pdf.
Rogers, C.W., J.M. Marshall, A. Rashed, and D.W. Morishita. 2015. Southern Idaho, 2015: Spring Barley Quick Facts. University of Idaho Extension CIS 1217. 2 p. https://www.extension.uidaho.edu/publishing/pdf/CIS/CIS1217.pdf.
United States Department of Agriculture-Natural Resources Conservation Service. n.d. “Estimating Soil Moisture by Feel and Appearance.” 7 p. https://efotg.sc.egov.usda.gov/references/Public/TX/estimatingsoilmoisture.pdf.
About the Authors
Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Barbara Petty, Director of University of Idaho Extension, University of Idaho, Moscow, Idaho 83844. The University of Idaho has a policy of nondiscrimination on the basis of race, color, religion, national origin, sex, sexual orientation, gender identity/expression, age, disability or status as a Vietnam-era veteran.
BUL 912 | Published January 2017 | © 2021 by the University of Idaho