Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Raising wheat or other small grains on irrigated land has not been a common practice in southwest Michigan. However, with high wheat prices and the expansion of production of short season vegetable crops in the area, many producers are trying double cropping on irrigated wheat fields. Late spring to early summer soil moisture deficits can dramatically reduce small grain yield potential on our non-irrigated loamy sand to sandy loam soils. Providing water through irrigation can raise yield levels to better than we might expect in heavy rainfall years on these soils. We tend to see fewer problems with wheat leaf diseases such as septoria and powdery mildew, and lower incidence of fusarium head blight in wheat produced under irrigation during dry years. Many producers report that they can achieve target yields on irrigated sandy soil in the 80 bushel per acre range. Even with excellent timed irrigation, it is hard to reach the yield levels that producers routinely raise in the Saginaw Valley and Thumb regions in Michigan, where soils are heavier soils and the climate is cooler than in southwest Michigan.

Irrigation scheduling is the term given to 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. (Evapotransporation or E.T).  The evapotransporation rate for crops are established by raising plants in a container of know 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 inches of water per day at flowering for a 75ºF average temperature day. This means that a one inch irrigation or rainfall event would last a little more than five days during this critical period. The accompanying chart shows how wheat E.T. increase, peaks and declines as the crop matures and how crop water use increases as average daily temperatures increase.

Wheat water usage in inches per day

The complete table and discussion can be found at the University of Minnesota website.
Click here to view larger image.

While there may be lower incidence of leaf diseases and fusarium head blight (scab) in irrigated wheat in dry years, unanticipated rainfall following irrigation can lead to more disease challenges. 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 (Feeke’s Growth Stage 10.5.1), which unfortunately occurs during the period of the highest crop water usage. Fields planted to commercial or seed corn the previous year may be more at risk of fusarium head blight development because infected corn residue may serve as the inoculum for the disease. The University of Minnesota article that references wheat crop water usage in this article recommends that producers consider irrigating as the head develops, but suspending irrigation while the wheat is flowering to ensure that adequate soil moisture is available but to avoid applying water during this critical period for fusarium head blight development. 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. Producers that raise irrigated wheat should be aware that applying irrigation water can increase the potential for these diseases, and should be prepared to apply fungicides if wetter conditions occur following irrigation, particularly during flowering. There are excellent resources available on controlling wheat leaf diseases and fusarium head blight at the MSU Field Crops Small Grain website at: http://fieldcrop.msu.edu/Default.aspx?crop=3&catagory=15

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 water used by the crop is considered a withdrawal. Overviews of irrigation scheduling tools that are available in the Michigan area are available at the St. Joseph County MSU Extension website at: http://www.msue.msu.edu/stjoseph. Follow the irrigation link in the left column.