Colorado By Randy Buhler
Logan County Extension Agent
A recent phone call prompts this article.
The basis of the call was to determine if spraying a wheat field with herbicide would delay plant maturity enough to avoid frost injury.
An interesting idea, especially since wheat is advanced in maturity by about one calendar month, this year. Unfortunately, the answer is no. If anything, spraying may increase the chance for frost injury, depending on several factors.
A reminder to those who have not yet sprayed their wheat for weeds. The cutoff date for spraying 2, 4-D, dicamba (Banvel, Clarity, Rave) or a combination of the two is the jointing stage of wheat development. This may occur much sooner than you expect this year. You should be scouting your fields to determine if you have enough weed pressure to justify spraying. If the weed pressure is enough, then you must spray before jointing.
To determine jointing, look at the base of the largest tillers. If you see a bulge in the stem above the basal leaves, jointing is started. The developing wheat head will be above this joint, or node. As the stem elongates and pushes the head higher above the ground, the head becomes larger and more obvious. It still is surrounded by stem and leaf tissue, which helps protect the florets from frost. A floret is the part of the head that includes the anthers and ovary within two protective scale-like leaves.
Development of the wheat plant is most dependent on temperature. Scientists estimate that 80 to 90% of the variation in development is controlled by temperature. The wheat plant is exposed to a changing cycle of temperature. Average daily air temperature is an approximate measure of the heat energy that drives development. The concept of Growing Degree Days (GDD) provides an estimate of this heat energy accumulation. A wheat variety or cultivar can vary in its inherited rate of development. Soil moisture can further modify the plant's response time.
A semi-dwarf variety of wheat uses more heat energy to change from the vegetative to reproductive stage of growth. The TAM 107 variety may use 590 GDD, from Jan. 1 to jointing. A variety like Scout may only need 490 GDD to go to the same growth stage. The GDD for wheat is calculated on a base of 32 degrees Fahrenheit.
A common occurrence with the application of dicamba or 2, 4-D is spraddling. The tillers lay over and look like they have been driven over or stepped on. Later, the plants develop the usual upright stature and produce as normal plants. Many, many tests have been done and no yield loss was found to be significant. Significant means that by virtue of chance alone, the yields could be different by some amount. Differences of less than this amount means a not significant difference has been measured.
As for creating a stress situation in the plant to delay maturity, herbicides like 2, 4-D and dicamba are not the answer. A poll of the Colorado State University research weed scientists and wheat breeders and agronomists agreed on the lack of any evidence for maturity to be slowed enough to make any difference.
Frost avoidance can be better managed with selecting the semi-dwarf varieties that require a longer period of GDD accumulation over the standard wheat varieties. Doing the best job you can of growing your wheat is all you can do. After that, it is location, luck and favorable weather.