From extreme drought and harsh spring freezes to an almost stress-free growing season; variability is the key word describing the 2020 winter wheat growing season in Kansas. Different parts of the state were exposed to different levels of stresses, resulting in very different crop conditions and yield levels.
Fall drought: Wheat needs (at least some) water to emerge
While the majority of the state started out the 2020 wheat growing season with enough precipitation to get good crop establishment, this was not the case for a large portion of southwest Kansas. Cumulative precipitation between September 1and December 31, 2019 ranged from 0.84 inches in the far southwest corner of Kansas to as much as 18.71 inches in far southeast (Fig. 1, upper panel). The majority of the wheat growing region (namely central and western Kansas) received between 0.84 and about 7.5 inches of precipitation during this period. For southwest Kansas, this precipitation amount represents a deficit of about 3.5 to 6.4 inches as compared to the long-term normal precipitation for that period. With such a limited precipitation amount in the southwest, the wheat crop was not able to germinate and emerge during the fall in approximately eight counties west and south of Finney County. The southwest Kansas crop did not emerge until December 28, 2019 when amounts of up to 1.25 inches of precipitation occurred. This was also true for some late-sown crops in the central portion of the state (Fig. 1, lower panel). While a spring-emerged winter wheat crop can still vernalize and produce grain, its yield potential is about half of a fall-emerged crop; so the wheat season in southwest Kansas already started with a reduced yield potential.
While the remainder of the state had enough precipitation to emerge in the fall, it is important to note that an increasing amount of wheat is double-cropped after a summer crop every year, especially after soybeans in central Kansas and after corn in western Kansas. These cropping systems delay wheat sowing and decrease the time for wheat tillering in the fall. There was a striking difference in fall development as affected by sowing date in the 2020 wheat crop; with early-sown fields attaining up to 6-8 tillers by December while late-sown fields only had the main tiller and perhaps an additional, secondary tiller at the same time. This difference is important because the fall-developed tillers have a greater yield potential than spring-developed tillers. Nonetheless, the winter was mild (average temperature of 34.3 °F as compared to the long term mean 31.8 °F), which allowed the wheat crop to tiller through the winter and somewhat compensate for the delayed sowing in many cases.
Figure 1. Departure from normal precipitation during the September 1-December 31, 2019 period (upper panel), and poor stand establishment in wheat near Great Bend (Barton Co) due to extremely dry fall conditions. The target wheat population ranged from about 20 to 40 plants per square foot and the actual fall emergence ranged from 5 to 12 plants per square foot. Trials were sown in the later portion of October and pictures taken on December 12 by Kavan Mark, Assistant Scientist at K-State Wheat and Forages Extension program in the Department of Agronomy.
Expansion of area under drought stress during the spring
While southwest Kansas was the most affected area by fall drought, the water deficit expanded to the northwest and north central parts of the state during winter and early spring (Fig. 2). The cumulative precipitation between September 1, 2019 and May 18, 2020 ranged from 2.85 inches in southwest Kansas, to about 7.75 inches in northwest Kansas, and up to about 11 inches in north central Kansas. When compared to the long-term normal for these regions, these total precipitation amounts correspond to a deficit of as much as 7.2 inches in north central and parts of southwest Kansas.
Wheat fields were showing signs of drought stress that ranged from rolling up of leaves that added to a blue tint to the crop in the west-central part of the state; to a very stunted crop with senescence of lower canopy and tiller loss in north central and especially southwest Kansas. Meanwhile, central and south central Kansas had a relatively moist winter and early spring, accumulating between 13 and 21 inches of precipitation during the same period. As compared to the long-term normal, these values correspond to a slight deficit of about 1 inch to as much as 5 inches above normal.
Figure 2. Departure from normal precipitation during the September 1, 2019 through May 18, 2020 period. The comparison between this figure and the upper panel in Figure 1 depicts the expansion of the dry conditions, originally restricted to southwest Kansas, to the central and north central portions of the state during the spring. Map by Kansas Weather Data Library.
Regional differences in spring development due to moisture and temperature regimes
With good moisture availability and a mild winter, the spring development of the wheat crop started on an average date in south central Kansas, with the majority of the wheat varieties reaching first hollow stem sometime in mid-March. For southwest and north central Kansas, however, wheat development during early spring was delayed due the dry conditions, leading to a significant difference in development at different parts of the state. The wheat crop was still reaching the first node in north central Kansas by the time the crop was already heading in south central and southeast Kansas.
Stem frost further decreased the yield potential
The occurrence of frost events during the spring is not unusual for Kansas and, depending on the region of the state and also on cropping system/sowing date, those freezes impacted the wheat crop in different ways. In particular, the week of April 13-17 had several frost events in which air temperatures dipped close to single digits in northwest Kansas, and below 24 °F for at least a few hours in most areas of the state, with the exception of south central and southeast Kansas. The areas most affected by these cold temperatures were north central and northwest Kansas, where dry conditions exacerbated the negative impacts of the cold spell. While the crop was mostly still between tillering (northwest) and jointing (north central) stages, which are more tolerant to cold temperatures than more advanced stages, temperatures were cold enough to cause severe tiller and leaf losses. These losses were more apparent in the late-sown crops, usually following soybeans in north central and corn in western Kansas, mostly due to a small canopy having less potential to buffer the cold temperatures, where individual fields had 50-75% of tiller loss (Fig. 3). In earlier sown fields, the tiller loss was more in the 20-40% range, and the plants were usually developed enough to compensate for this loss with secondary tillers. With the freeze events happening in mid-April and no significant precipitation occurring in these locations until mid- to late-May, the dry conditions that followed the freeze events further hampered wheat development and decreased the crop’s yield potential. In the area around Ellsworth, where some precipitation occurred following the severe freeze, these conditions led to the loss of several primary tillers, which allowed for increased weed populations at harvest and a less uniform maturity in head stages due to the development of new tillers (Fig. 4). While the crop was more advanced in central and south central Kansas, the temperatures were not nearly as cold and therefore the crop sustained no injury from the freeze. Thus, the area south of McPherson County through Cowley County, and west to Meade County, was likely showing the best yield potential for wheat in Kansas during the 2020 season.
Figure 3. Preliminary assessment of wheat freeze injury potential for Kansas following the freeze events of April 13-15, 2020 (upper panel), and symptoms of freeze damage on late-sown wheat crops in north central Kansas (lower panel). The freeze risk assessment combined an estimated crop growth stage with the magnitude and duration of cold temperatures. Symptoms of freeze damage include severe loss of leaves and tillers. Photos by Romulo Lollato, K-State Research and Extension.
Figure 4. In areas where the severe stem frost was followed by precipitation, many primary tillers were lost but this was to some extent compensated by new tillers. These situations allowed for increased weed pressure and less uniform head maturity. Photos by Romulo Lollato, near Ellsworth, KS.
Grain filling period
As the 2020 wheat crop went through the grain filling period, the occurrence of precipitation in the last ten days of May helped the crop in most of the state. The precipitation during this period ranged from 0.97 inches to 2.99 inches, which was enough rain for most of the state to ensure at least an average crop despite all the previously mentioned adversities. However, southwest Kansas only received 0.97 inches of precipitation, with a divisional average of only 7.04 inches for the September to May period. In fact, many dryland fields in the region were going through the grain fill period and were only about 10 inches tall, leading to area abandonment (Fig. 5).
Another issue faced by the crop were warm temperatures during the grain filling. The first week of June brought extremely high temperatures across the entire state, with as many as 58 hours above 91 °F (Fig. 6). While the crop in southwest and south central Kansas were already in the late dough stages by that time, the crop in north central and northwest Kansas were still at the early stages of grain fill. Those temperatures were likely enough to decrease photosynthetic rates and consequently further reduce yields in those regions.
Figure 5. Severely drought stressed wheat near Liberal, KS (upper panels), and wheat abandonment in Finney county, KS (lower panel). Photos by Romulo Lollato, K-State Research and Extension.
Figure 6. Estimated wheat growth stage on June 9, 2020 (upper panel), and number of hours with temperatures above 91 °F during the June 1-8 period (lower panel). Maps by Kansas Mesonet and Weather Data Library.
Wheat disease summary
Overall, the 2019-2020 wheat crop was spared the major disease losses of years past. This year stripe rust was first reported in Kansas in late April in Sedgwick, Reno and Pratt counties. This late April arrival was at least two weeks later than in previous seasons, one factor that may have contributed to lower levels of the disease statewide than in previous seasons. Stripe rust was most severe in southeast and southcentral Kansas, where cool wet weather was favorable for disease development in early May. Reports of trace levels of the disease, however, were made in most counties across the state (Figure 6, Panel 1).
Very warm weather during the grain fill period was not favorable for disease development and may have spared many fields late-season losses. Weather conditions were favorable for Fusarium head blight during the critical flowering period in southeast and central Kansas, although fields with economic losses were rare. This may have been due to an extended dry period at the end of the flowering window, and at the start of the grain fill period in this region.
Reports of the seed-borne diseases loose smut and common bunt were higher than typical this season, with reports of grain being discounted due to high levels of common bunt in some areas. Producers should take particular care to clean and treat seed lots that will be saved for planting in the fall that originated in fields with these diseases in 2019-2020. In addition, there were areas in central and southcentral Kansas with high levels of residue-borne foliar fungal diseases, such as Septoria tritici blotch and tan spot. These diseases were most common in fields with heavy wheat residue. These fields should be managed carefully in the 2020-2021 seasons as inoculum levels will likely be very high as additional infested wheat stubble was left behind. In north west and west central Kansas, there were intermittent reports of wheat streak mosaic, mostly in fields that were downwind of neighboring fields that had volunteer wheat in 2019.
Figure 7. Stripe rust was reported throughout Kanas by May 29, 2020 (panel 1). Map is based on observations of K-State Research and Extension, crop consultants, and wheat producers in the state. Map created by Kelsey Andersen Onofre and Erick DeWolf, K-State Research and Extension. Although disease severity remained low in many fields, some fields in central Kansas had severe foci of infection (panel 2, left) while fields in northern and western counties only had trace symptoms (panel 2, right).
Romulo Lollato, Extension Wheat and Forage Specialist
Mary Knapp, Weather Data Library
Kelsey Andersen Onofre, Extension Plant Pathologist