The sudden, sharp drop in temperatures across Kansas observed in late-October 2019 could have varying consequences to the wheat crop, ranging from no impact to some injury in particular fields. Average temperatures dropped from around mid-60 to 70 degrees F on October 26 to approximately -1 to 14 degrees F on October 31 (Figure 1).
Figure 1. Hourly temperature dynamics from Oct 25 through Oct 31 for Cheyenne (upper panel), and Harper (lower panel). Data courtesy of Kansas Mesonet.
The actual consequences of this temperature drop should be field-specific, depending on the region within the state and several other factors. The snowfall amount and moisture level in the topsoil will be important to help buffer possible injuries resulting from cold temperatures. When more than 2-3 inches of snow is on the soil surface, it helps protect the wheat crop by buffering temperature changes in the surface soil. Topsoil moisture is very variable across the state, with topsoil (2-inch depth) percent of saturation ranging from 17% in Garden City to 93-98% in southeast Kansas mesonet.ksu.edu/agriculture/soilmoist). Where the soil is dry, it will cause the soil to have a lower thermal buffer capacity, compared to a moist soil, and winter injury might be more pronounced.
While the average soil temperatures followed a similar trend compared to air temperatures, the minimum 2-inch soil temperatures measured across the state was about 26 degrees F in parts of southwest Kansas (mesonet.ksu.edu/agriculture/soiltemp). While these soil temperatures are relatively low, they could help buffer any negative effects of the sharp air temperature drop (Figure 2).
Figure 2. Average soil temperature at the 2-inch and 4-inch depth from 25 Oct to 31 Oct for Cheyenne (upper panel) and Harper (middle panel). Lowest 2-inch soil temperature during the last 7 days (from Oct 25) across the state. Minimum soil temperatures did not reach values lower than 26°F across the state. Data courtesy of Kansas Mesonet.
Possible exceptions could include fields planted in heavy no-till residue, where the furrow might not have been closed properly at sowing, or where there was not good seed-soil contact. Under these circumstances, the lack of furrow closure results in a less protected seedling (and in some fields, crown) which might be more exposed to cold temperatures (Figure 3). Producers are encouraged to start checking for possible injury on lower portions of the fields and especially in no-till fields with heavy residue in the near future, but no immediate damage will be apparent. The cold temperatures also will be more likely to cause injury to wheat if the plants were showing drought stress symptoms and soil temperature might have fallen below those shown on Figure 2, as dry soils will get colder more easily than wet soils. Additionally, the drier and looser the seed bed soil is, the greater the potential for the planting to be exposed to cold temperatures resulting in injury. Meanwhile, firmer and moister soils should help to minimize rapid fluctuations in soil temperatures allowing the wheat to better withstand cold temperatures.
Figure 3. Effect of soybean residue on wheat cold damage. Yellow portions of the field correspond to greater residue left by the combine at soybean harvest and resulted in poorer seed-soil contact at wheat planting. As a consequence, the plants are more exposed to colder temperatures and potential injury. Photo provided by Romulo Lollato, K-State Research and Extension.
Another factor affecting wheat’s response to the cold is whether the wheat had time to become properly cold-hardened. It is important to remember that a large portion of the Kansas wheat crop was planted late, after soybean harvest. Therefore, it is still too early to suggest that the wheat has been cold-hardened. In fact, many fields have not even emerged at this point or are just now starting to emerge.
In fields that have not yet emerged but in which seeds are already sprouted, no significant injury should be expected for two main reasons:
In fields where the crop has already emerged, temperatures of around 15 degrees F or less can injure the newly emerged wheat, and these limits decrease as the crop progresses to tillering later in the fall and become more cold-hardy. Thus, some fields in western Kansas where the crop has recently emerged, especially the northwest part of the state, could sustain some level of damage. We likely won’t know for sure until temperatures warm up and give us an opportunity to scout.
If fields were affected, the first symptom will be burndown of the wheat from these cold temperatures as shown in Figure 3. If the wheat was bigger-than-normal, the plants may look “rough” with a lot of brown, dead-looking foliage on the soil surface. That doesn’t mean the plants are dead. The important factor will be whether the crown below the soil surface remains alive. Having a well-developed secondary root system will help the plants survive. As temperatures did not drop as low in the central portion of the state, the concern with possible cold injury is not as great as fields that recently emerged in northwest Kansas.
The extent of the unusually large and rapid drop in temperatures is a concern in certain scenarios described above. In fields that were planted earlier, if the wheat did not develop sufficient cold-hardiness, it will be more susceptible to injury from the recent cold snap.
Romulo Lollato, Extension Wheat Specialist
Mary Knapp, Weather Data Library
Christopher “Chip” Redmond, Kansas Mesonet Manager