The last two weeks have certainly presented people, livestock and the 2015 wheat crop with challenges. Many record lows were set over the area over the last two weeks and to add insult to injury, many record low highs were set. While it wasn’t pleasant for us, our pets, and livestock, it shouldn’t have caused much harm. The question on many wheat farmers’ minds is what did this severe and long early cold snap do to the 2015 wheat crop? Much of the answer involves conditions other than temperature and the development of the wheat.
• The growing point for wheat is below the soil surface at the depth the wheat was planted. Hopefully at least an inch below the soil surface and protected from the air temperature extremes. The deeper the seed the more slowly the temperature changes. The above ground vegetation can suffer damage now and not really affect yield. It’s the temperature of the soil where the growing point is located that matters. Soil temperature is determined by a variety of factors including soil moisture, residue cover, and vegetative cover.
• Soil moisture is extremely important in determining how quickly soil cools. Wetter soils have a higher heat capacity and therefore will cool more slowly. Residue cover (no-till or minimum tillage) also serves to moderate soil temperatures and slow cooling by reducing evaporation while also acting as a blanket to slow heat loss. A well-developed wheat crop serves a similar function and acts to insulate the growing point.
• How quickly the severe cold occurred matters if the wheat didn’t have sufficient time to “harden off”, develop its cold tolerance. This is accomplished by gradually decreasing temperatures that signal the wheat to change the composition of its cells to lower the freezing point of cell contents by increasing the concentration of solutes, primarily carbohydrates. Essentially, the plant develops antifreeze for its cells. Think of it as the difference between putting diet soda and regular soda in the freezer. The regular soda has a lower freezing point than the diet.
• Drought stressed plants with inadequate plant moisture are more susceptible to damage than unstressed plants. They are better able to adapt and withstand the cold.
• Degree of development of the plant matters. Plants that are well stooled, developed tillers and top growth well, are better able to withstand the cold and protect the growing point. Also well-tillered plants can lose tillers with minimal or no yield loss. Thin, poorly tillered wheat is more vulnerable to freeze damage.
• Unstressed wheat plants are better able to withstand the cold as they can respond more rapidly and effectively to extremes.
Having gone over all the above, unless the case is quite extreme, we won’t know until late winter or well into spring. There may be no damage, minimal damage, damage that shows up as development proceeds, or plants that fail to green up that are dead.