Drought is a concern this spring to many wheat producers across much of Kansas. Symptoms of drought stress during stem elongation start as lower leaf senescence, as the crop begins to eliminate leaves that will contribute less to the final grain yield in order to decrease the crop water needs. Meanwhile, upper canopy leaves may start to roll and give the crop a blue appearance, which can also be a symptom of drought stress (Figure 1). Many Kansas wheat fields are currently showing these symptoms.
While some yield reduction may already have occurred in Kansas wheat fields, the conditions during the next couple of months will actually determine final path of the wheat crop. If dry conditions persist, senescence can expand to more important leaves in the upper canopy, which contribute more to grain yield, increasing the potential for yield loss. Additionally, extremely dry conditions during the final stages of stem elongation through flowering (anthesis) can ultimately reduce wheat yield potential due to a decreased number of florets fertilized, which directly decreases the number of grains per head. While this is not the current situation of most of Kansas wheat, the established dry conditions across the entire state are a reason for concern if this scenario is not converted in the near future.
Total precipitation during the current growing season (from October 1, a rough estimate of planting date, until April 7) ranges from 4.3 inches in most of western Kansas to about 30 inches in the far southeast (Figure 2). Most of the wheat-producing region, though, has received no more than about 12 inches of precipitation thus far this growing season.
Figure 2. Total precipitation across Kansas during the 2015-16 wheat growing season (from 1 October 2015 until 7 April 2016).
These precipitation totals represent an average water deficit of as much as 2 inches for the growing season across most of central and eastern Kansas when compared to the long-term normal (Figure 3). While the western third of the state shows a positive departure from the normal, this departure does not mean that the crop necessarily has access to this surplus water from fall rains, mainly due to poor precipitation distribution as discussed below.
Figure 3. Departure of current growing season’s precipitation totals from normal precipitation.
Figure 4. Departure of last 90 days’ precipitation totals from normal precipitation.
Figure 4 shows the departure from normal during the last 90 days while Figure 5 subtracts that 90 days departure from the departure from normal for the whole growing season. In other words, regions characterized by a relatively moist fall and dry spring, such as southwest and southeast Kansas, and most of the central region of the state, will show a greater change in precipitation departure from normal. On the other hand, regions that received precipitation within the last 90 days will show less negative departure from normal. Figure 4 shows that almost the entire state has a negative departure from normal in the last 90 days, as a consequence of the dry spring thus far.
Figure 5. Departure from the normal precipitation for the last 90 days subtracted from the departure from normal precipitation for the entire growing season.
While precipitation totals are important to characterize the growing season conditions up to this point in time, precipitation distribution plays an even more important role in determining wheat grain yield. Situations like the one illustrated in Figure 6 are not uncommon in Kansas during the current growing season, where the crop had enough moisture supply during the fall and is starting to go into drought stress now due to water shortage in the spring. Most of the precipitation in southwest Kansas was concentrated during the fall 2015 and may have been either used by now due to the long, open fall that led to lusher canopies; or may be stored in the subsoil. Many fields in southwest Kansas have good subsoil moisture but extremely dry topsoils. Under these circumstances, wheat roots may not have access to the moisture and it may not be translated into grain yield.
Figure 6. Water-logged soil during the fall (upper image) versus cracked, dry soil during the spring (lower images) in the K-State wheat variety performance test near McPherson. Fall photo taken December 3, 2015 and spring photo taken April 7, 2016 by Romulo Lollato, K-State Research and Extension.
While fall moisture is important to ensure proper wheat tillering, the amount of water needed by the wheat crop increases linearly with the increase in biomass in the spring as the wheat is released from winter dormancy and moves into the stem elongation phase. Wheat daily evapotranspiration increases from about 0.05 inches per day following vernalization to about 0.2 inches per day or more at the peak of water needs around flowering (Figure 7). Data in Figure 7 was collected near Chickasha, in central Oklahoma, during the 2013-14 growing season. The total in-season precipitation amount during this growing season was approximately 8 inches, mostly concentrated during the fall, similar to a great portion of Kansas during the current growing season. Daily wheat evapotranspiration can be greater in cases in which the crop is not limited by soil moisture stress, such as under irrigation or sufficient precipitation regimes.
Figure 7. Daily wheat evapotranspiration in inches per day measured near Chickasha, central Oklahoma, during the 2013-14 growing season. Data by Romulo Lollato, K-State Research and Extension.