How much yield potential did the latest round of precipitation add to the wheat crop?
What is yield potential?
Yield potential of a crop is defined as the grain yield achieved under optimum management conditions, where the crop is limited only by weather and soil physical constraints rather than by diseases, pests, nutrition, or any suboptimal management practice.
Therefore, yield potential is often limited either by water or by solar radiation. Previous studies looking at 30-year weather data and wheat yield potential have shown that about 70-80 percent of the variability in wheat yield potential in western Kansas and Southern Plains is explained by plant-available water at sowing plus in-season precipitation. This means that wheat yields are often limited by water in this region. Meanwhile, a large proportion of the wheat yield potential variability in eastern Kansas is explained by cumulative solar radiation between anthesis and physiological maturity. In other words, cloudy days in the eastern region often limit the amount of incident solar radiation during a critical period of grain yield determination and this often becomes a more significant factor in determining yields than water.
How efficiently can wheat use available water?
In regions where yield is limited by water, water is generally used more efficiently and yield potential can be calculated using a water-use efficiency coefficient. This is true for regions where growing season precipitation is generally less than about 13-15 inches, and precipitation distribution plays a very important role in determining the crop’s water-use efficiency.
Previous studies performed in the Southern Plains and other water-limited regions of the world, such as Australia, have shown that wheat can yield as much as 8.3 bu/acre for every additional inch of precipitation in the growing season when water is used most efficiently (Figure 1). This very effective use of water occurs only when precipitation is very timely and falls during critical periods for grain yield determination -- such as stand establishment and tillering, spring greenup, and grain filling.
Although 8.3 bu/acre/inch is the potential transpiration efficiency of wheat, this extremely high water-use efficiency value only occurs when all conditions (management- and weather-related) lead to increased grain yields. This is very seldom observed under field conditions. For instance, any limitation due to suboptimal management, such as disease or pest incidence, nutrient deficiency, or weed pressure, will decrease the efficiency with which wheat uses the available water. Likewise, heat stress during later stages of grain filling will result in shriveled grains and will reduce the crop’s water use efficiency.
As a consequence, wheat water-use efficiency values in the 3 to 5 bu/acre/inch are more often observed under field conditions. A long-term study in Tribune evaluated wheat water-use efficiency during the 1974-2004 period, and indicated that average water-use efficiency in the region during the mentioned period was 3.8 bu/acre/inch. Likewise, another study looking at 11 site-years in central Oklahoma resulted in water-use efficiency ranging from 2.9 to 4.8 bu/acre/inch, meaning that an average water-use efficiency of about 4 bu/acre/inch may be a fair number to use for wheat in the region.
Figure 1. Wheat grain yield versus growing season precipitation. Blue dots represent county-level yield data and yellow triangles represent variety performance test data. Transpiration efficiency (TE) is calculated using a linear regression approach of the most efficient observations. Figure adapted from Patrignani et al. (2014), data represents wheat production in central and eastern Oklahoma.
How much precipitation did Kansas get?
Total precipitation during April 15 – April 21 ranged from 0.8 inches in eastern Kansas to as much as 9.23 inches in west central Kansas (Figure 2). Although the wheat crop was already showing signs of drought stress in some regions of the state, such as yellowing of lower leaves and leaf curling which may have slightly reduced its yield potential, this precipitation was very timely for many regions of Kansas. Wheat crop development is anywhere from jointing stage in the northwest region to heading and anthesis in the southeast. For wheat in the more advanced stages of development, this precipitation may be directly translated into grain yield, provided other yield-limiting factors are controlled.
Figure 2. Weekly precipitation total from April 15 – April 21. Source: K-State Weather Data Library.
How much yield potential can be expected from the latest round of precipitation?
Based on the average wheat water-use efficiency of approximately 4 bu/acre/inch, wheat yield potentials in Kansas may have been improved anywhere from about 3 bu/acre in the east to as much as 36 bu/acre in west central Kansas, where the precipitation total approached 9 inches. If some of this precipitation was subjected to runoff, the potential yield increase will be reduced.
The timeliness of this precipitation event, though, may actually result in higher-than-average water-use efficiency by the crop in certain regions of the state. In the south central and southwest regions, where wheat is further along in development, as well as in the central and west central portions of the state, this precipitation matched very critical growth periods of the crop. Thus, the average water-use efficiency of 4 bu/acre/inch may be conservative in these regions, as the wheat water-use efficiency has been shown to be as great as 8.3 bu/acre/inch. Whereas the potential 8.3 bu/acre/inch may not be attained due to disease or weed pressure, the wheat crop could very well respond with a yield potential increase of more than 4 bu/acre per inch of precipitation received due to the timeliness of the rain.
It is very important to keep in mind that the recent precipitation events also increased the risk of foliar diseases, especially stripe rust. Heavy disease pressure will decrease the attainable yield of the crop, reducing the efficiency with which the crop will use the available water. Producers are encouraged to be proactive in protecting their crops in years such as this, where the disease inoculum is already present and environmental conditions are conductive to the disease. For more information on disease pressure and fungicide options, see eUpdate 564 article: Special Edition: Stripe Rust Alert.
Romulo Lollato, Wheat and Forages Specialist
Alan Schlegel, Agronomist, Southwest Research-Extension Center, Tribune
Mary Knapp, Weather Data Library
Chip Redmond, Weather Data Library