The heat continues in Kansas, particularly with the warm minimum temperatures (Figure 1). For the 25-day window of June 1-24, parts of eastern Kansas had lows above 70 °F more the 20 percent of the time. Only the western third of the state had two or less nights with minimum temperatures above 70 °F.
Nighttime temperatures in excess of 70 °F for more than two consecutive days will increase the risk of stress to livestock. As with people, the stress is cumulative.
Figure 1. Total days with minimum temperature greater than 70 °F. Source: Weather Data Library
The heat this June can also cause problems for corn in Kansas (Figure 2). The effect of combined heat and drought stresses can reduce plant size, primarily when the plant is entering the stem elongation process. When the crop reaches the V10 (tenth-leaf) stage, nutrient and water demands (0.25 inch/day) are high. At this point, a combination of heat plus drought stresses will affect potential number of kernels and ear size. Overall mean temperatures above 90 °F, and more importantly, lower fluctuations between day and night temperatures, will produce critical impacts on plant and potential ear sizes and the yield components of corn.
Heat stress will have more of an impact on corn at this stage of growth when combined with drought stress. But even in the absence of drought stress, heat stress alone can still hasten vegetative phases and tasseling, potentially increasing the asynchrony between pollen shed and silk extrusion when corn reaches flowering time. The potential for yield reductions from stress at this stage of growth is small, however, compared with severe stress occurring right around pollination.
Figure 2. Leaf rolling in corn from the combined effect of heat and drought. This can also affect final plant size. Photo by Ignacio Ciampitti, K-State Research and Extension.
In the southeast, these conditions have occurred at a bad time. Following a very wet spring, root development has been stunted. The hot/dry conditions then combine for increased stress on these shallow roots with diminishing surface/shallow moisture. The lack of growth then prevents plants from reaching the remaining moisture further below the surface. In several cases, it was so wet that producers had to replant. This younger corn has been subjected to less than ideal moisture and increased heat for the early growth. As a result, development is also stunted, with shorter corn as observed in Cherokee County on June 23, 2020 (Figure 3). In the southwest, early-season plant growth (corn is still V6-V8) in also showing stress, even under irrigation.
Figure 3. Short corn observed in Cherokee County, Kansas on June 23, 2020 (Photo from the Weather Data Library).
Monitor the heat index and growing degree days using the Mesonet
The K-State Mesonet web site has a special page that tracks the current heat index at: http://mesonet.k-state.edu/weather/heat/
There is also a page that tracks growing degree accumulation for multiple crops. With this tool, you can pick the planting/emergence date for the start of the interval. Selecting the graph will illustrate the growing degree accumulation for this season versus normal and plant stage. You can access the page at: http://mesonet.k-state.edu/agriculture/degreedays/
The data updates every five minutes when you refresh the page and is available for all 65 stations across Kansas.
Ignacio Ciampitti, Crop Production and Cropping Systems Specialist
Christopher Redmond, Mesonet Network Manager
Mary Knapp, Weather Data Library