By Kevin Bradley
The condition of the wheat crop is more evident this week with the warmer temperatures we had over the weekend and especially Monday. The wheat crop varies in development between areas in the state and may vary from field to field in the more impacted areas. The further along the wheat crop, the more it may be impacted by the cold temperatures we experienced last week. In 2007, when we had the Easter freeze on April 7th of that year, we observed great differences in the wheat from Northern Missouri to the Bootheel and SW MO. That year the freeze came several weeks later on more developed wheat, the recent freeze had extremely cold temperatures, but the growth stage is the big factor in how much cold can be tolerated. See IPM Newsletter from March 9th:
https://ipm.missouri.edu/IPCM/2017/3/Understanding_Wheat_Growth_Stages_for_Estimating_Wheat_Freeze_Injury/
Wheat in Missouri, from Central to Northern portions, was not yet jointed and appears to have tolerated the freeze very well. This year, as in 2007, the major concern is on the jointed wheat in the southern portions of the state. Below are some of the observations from 2007, and what to look for as you evaluate the wheat crop this season:
- The wheat in northern Missouri in 2007, as in 2017, was in much better condition (north of hwy 36) since the wheat was in earlier stages of development when the frost hit and could tolerate the cold temperatures better. There were some isolated fields showing lodging in Northern Missouri in 2007, but that wheat was further along when the freeze hit than in 2017.
- Wheat in central portions of the state were more of a mixed bag in 2007. Some fields showed very little damage and others had a very high percentage of weak stems and bad heads. Wheat in southern Missouri was naturally further along and in more sensitive growth stages, and the temperatures were nearly as cold.
- In 2007, earlier planted fields were generally worse. They broke dormancy sooner and were at a more vulnerable stage when the freeze occurred.
figure 1: Lodged wheat field from SW Missouri in 2007 following heavy rain after the Easter freeze.
- Lodging showed up very significantly in many fields in SW and SE MO and to some extent in parts of central MO. The lodged wheat picture (figure 1) came from SW MO where weakened stems gave way due to heavy rains after the freeze.
- High nitrogen and/or high seeding rates contributed to more injury and lodging in 2007. Succulent growth has more water, less sugars in the plant cells and the wheat was further along in growth stage. Water freezes more easily than sugar water.
figure 2: using a box cutter to evaluate stem and head damage.
figure 3: bad wheat head (left) next to a good wheat head (right)
- When evaluating stem and head damage a box cutter (figure 2), razor blade or very sharp knife works well. The growing point can be found on pre-jointed wheat near the base of the tiller. In jointed wheat the head can be found by splitting the stem open and finding the growing point above the highest joint. The growing point and the developing head should be bright white to yellow-green in color and be turgid. Freeze injury will cause this area to become off colored or brown and will look water soaked. Time and warm weather helps make a better evaluation of bad vs good wheat heads (figure 3).
- To evaluate the stems the leaf sheaths must be removed and you can see if the area below the lowest node is healthy or weakened and browning (figure 4).
figure 4: weakened stem with leaf sheaths removed showing browning.
- Weakened stems will not be able to support the plant even if the growing point is still alive. In most cases an extremely damaged stem is going to have a wheat head that is already dead or showing signs of decay.
Wheat Management Decisions
As it becomes clearer whether or not a field is worth keeping, the main question is how to handle the plant matter out there. Some growers may be able to use the wheat for hay or haylage and that would certainly be a good option. If the wheat is to be destroyed, glyphosate is an effective option but the rate should be kept high due to the very slow growth of the wheat (figure 5).Gramoxone or Liberty may also be an alternative, especially if going to corn and atrazine can be applied in combination with either of these herbicides. However, neither Gramoxone nor Liberty alone will provide effective control of most wheat stands. It will most likely be difficult to get decent coverage of lodged wheat with any herbicide treatment, especially contact herbicides like Gramoxone or Liberty. Tillage could be an alternative but is not required. For soybeans or corn that are glyphosate tolerant, then any post-emergence glyphosate application would kill surviving wheat plants.
figure 5: Influence of herbicide treatments and application timings on the control of wheat (results averaged across 3 years).
If wheat is insured then you will need to follow the requirements agreed upon with your insurance company pertaining to recrop options.