Considerations for irrigating wheat
Irrigating winter wheat or rye is a tool to help double crop growers.
Double cropping winter wheat or rye with soybeans or snapbeans has the potential to increase profits and soil health. One of the keys to success with double cropping mid-west is adequate soil moisture. Successful establishment of the second crop must include adequate moisture for quick germination and emergence. Irrigation can provide an essential role by ensuring adequate moisture is available to make double cropping successful.
Providing adequate water to meet the water needs of the wheat or rye crop allows you to maximize their yield, quality and helps maintain adequate soil moisture to start the second crop. Rainfall during May and June is often sufficient to achieve moderate yields and keep the crop developing although dry spells can reduce wheat yield and create a water deficit for the next crop. Careful planning must take place early in the season to determine how much irrigation water to add and at what times to reduce incidence of disease, produce high yields and create an environment conducive for the second crop to be established.
Irrigation scheduling is the term used in deciding how much and when to apply irrigation water. The over-arching principle in irrigation scheduling is to replace the water used by the plant for evaporation and transpiration (evapotranspiration, or ET). The ET rate for crops is established by raising plants in a container of known volume on scales called weighing lysimeters that record reduction in the weight of the soil in the root zone as the crop uses water and as rainfall or irrigation increases the soil root zone weight.
The maximum water use by wheat and most small grains is 0.19 inch of water per day during the grain fill period (flowering to hard dough) for a 75 degrees Fahrenheit average temperature day (0.25 inch of water for an 85 F average temperature day). This means that a 1-inch irrigation or rainfall event would last a little more than five days during this critical period. Adequate water during grain fill is necessary for high yield potential in wheat.
The accompanying chart shows how wheat ET increases, peaks and declines as the crop matures and how crop water use increases as average daily temperatures increase. For the purpose of irrigation scheduling the spring green-up of winter wheat and rye is equivalent to emergence.
A more exact method of irrigation scheduling is available through reference ET values available from your nearest Michigan State University Enviroweather station. Under the Weather banner, you will find Irrigation Tools and can sign up for the reference ET data to be sent to you as an email or text message. The reference ET value is the amount of water that is estimated to be used at your location by a reference 6-inch grass. The crop coefficient, often referred to as Kc, is used as a multiplier to make up the difference between 6-inch grass and the crop you are growing.
From the time the flag leaf is fully emerged through the hard dough, wheat has a Kc of 1.05. This means wheat irrigation will need to be 105% of our reference ET. Over the first three weeks of May 2023, reference ET was 1.3 to 1.4 inches resulting in a water use of 1.4 to 1.5 inches of water each week. Many of the sandy loam fields of this area hold about 3.5 inches of available water to three feet of depth. Without the addition of rainfall or irrigation and a deeply depleted soil moisture reserve, winter wheat and rye fields will start to show reduced growth and lower potential yield.
Spring rainfall varies between years with some areas of Michigan and Indiana receiving as little as 2 inches of rainfall in May compared to the 4.2-inch average for the area.
While there may be lower incidences of leaf diseases and fusarium head blight (scab) in irrigated wheat in dry years, unanticipated rainfall following irrigation can lead to increased disease pressure. Most of the fungal pathogens that impact wheat thrive when extended periods of leaf wetness or soil saturation occur. The most critical time for fusarium head blight infection is during flowering (Feekes growth stage 10.5.1), which, unfortunately, occurs during the period of the highest crop water usage.
Research at the University of Delaware Warrington Irrigation Research Farm (sponsored by the Delaware Crop Improvement Association) conducted from 2013-2016 showed that irrigation increased yields by 7-14% in three of the four years. There were only slight differences among the various irrigation strategies. However, researchers discovered a significant trend: adequate soil moisture levels must be maintained before flowering and at levels high enough to support the crop through flowering and early grain fill.
If possible, develop an irrigation plan that will provide adequate soil moisture but avoid applying water two weeks prior to and two weeks after flowering. This may be especially important if your irrigation system has the tendency to over-apply water to portions of the field, as may be the case with some cornering arm systems. These areas are usually confined to small arcs in the field but can provide valuable locations to scout for the detection of wheat disease problems.
If you must irrigate during the two-week periods prior to and after wheat flowering to meet water demands, scout regularly for diseases because applying irrigation water can increase the potential for leaf diseases. Be prepared to apply fungicides if wetter conditions occur following irrigation, particularly during flowering, and particularly if you have a variety that is susceptible to Fusarium head blight, also known as head scab. The risk of head scab can be monitored using the online Fusarium Risk Tool hosted by Penn State University. You can reference the Fungicide Efficacy for Control of Wheat Diseases bulletin produced by the Crop Protection Network when choosing an effective fungicide. There are also excellent resources available on controlling wheat leaf diseases and Fusarium head blight at the Michigan State University Extension Wheat Disease webpage.
There are several paper and computer systems available to help irrigators schedule irrigation applications. These tools are “checkbook type systems” that view the water in the soil root zone as your checking account. Rainfall or irrigation serve as deposits in your account and evapotranspiration is considered a withdrawal. In addition, irrigation scheduling using a soil moisture sensor technology can be found in MSU Extension Bulletin E3445, “Improving Irrigation Water Use Efficiency: Using Soil Moisture Sensors.” Overviews of irrigation scheduling tools for Michigan are available at the MSU Extension Irrigation website and MSU Biosystems and Agricultural Engineering Irrigation website.