Dusted-in wheat and spotty stands this past fall was a more common occurrence than we want to see in Oklahoma. Going into winter though, I thought we would get precipitation at some point to get the remaining seed to germinate, similar to the situation in northwestern Oklahoma last growing season. Unfortunately that did not happen, and the first water some of this seed has seen came with the late February rain. Now the remaining seed is germinating and emerging, begging the question what is a realistic expectation for this late-emerging wheat crop?
Will these plants produce a head?
In order to produce a head, winter wheat must be vernalized, which means it requires exposure to cool temperatures to trigger reproductive development. Winter wheat plants that do not go through vernalization will continue to grow vegetatively (i.e., produce leaves and tillers) but will not joint and produce a seed head (Figure 1).
How cold and for how long?
In the literature, you will often see that winter cereals require exposure to cooler temperatures (33° to 51° F) for six weeks. However, the exact temperature and time period differs by variety. A general rule of thumb is varieties that are more winterhardy and later maturing tend to require lower temperatures for a longer period of time (i.e., a stronger vernalization requirement) than less winterhardy and earlier maturing varieties. Vernalization requirements for winter wheat varieties adapted to the southern Great Plains may range from 120 to 1080 hours (5 to 43 days) below 45° F (Neely, 2016).
Since this is a rare problem in Oklahoma, we do not have much data on variety-specific vernalization requirements. I spoke with Dr. Carver, and he feels exposure to constant temperatures at or below 45° F for three weeks should be sufficient for most winter wheat varieties grown in Oklahoma. If that time decreases to two weeks, though, there is a possibility that we may run into vernalization issues for some varieties. It is important to keep in mind that the vernalization clock starts clicking once the seed imbibes water and sprouts. Some good news is we have been experiencing cooler temperatures since the rain, and cooler temperatures are still in the forecast.
Figure 1. Example of a wheat variety which was able to vernalize (left foreground) versus a variety that was not able (right foreground). Notice these differences in varieties from the foreground to the background. This photo was taken by Bryan Simoneaux at a Texas winter wheat variety trial.
Is it all just temperature dependent?
In addition to vernalization, many varieties have a photoperiod signal that can tell the plant to switch to reproductive growth regardless of the temperature. Therefore, if we do not get enough time spent with cooler temperatures to satisfy vernalization, the plant will still initiate reproductive development once the daylength has become long enough. However, the plants in this scenario will likely be two weeks or more behind in development compared to normal.
Do we have information on varieties?
While we do not have variety-specific vernalization requirements at the moment, parts of south Texas will experience vernalization problems now and then. This happened at the Wharton variety trial location in 2016. Some varieties at this location were able to produce grain while others did not. Looking at these results may give us an indication which varieties grown in Oklahoma this year might have a higher probability of producing heads (Table 1).
Table 1. The 2016 Wharton, TX wheat variety trial results in which mild winter conditions resulted in vernalization issues for some varieties. Varieties that were able to still produce grain are those ranked 1-16.
What forage or grain yield potential do I have?
To dive into this question, we do have some data from Kansas that can provide some guidance. Researchers at Kansas State University conducted a seven-year study (1985-1991) near Garden City, KS where they planted winter wheat every month from October 1 to April 1 (Witt, 1996). They used one variety (TAM 107) and a seeding rate of 80 lbs/acre for each planting date. Table 2 summarizes the data from this study.
Wheat planted on all dates through March 1 was able to produce grain each year. Wheat planted on April 1 did not joint and produce heads. Relative to the October 1 planting date, the wheat planted on March 1 was:
- the lowest yielding;
- was the shortest statured (5 in. less);
- had the most delay in heading (26 days later);
- had the shortest grain-filling period (9 days fewer).
- was the last to ripen (17 days later);
- produced the fewest heads per plant (58% fewer);
- produced the fewest kernels per head (33% fewer) and the fewest kernels per plant (73% fewer);
- and produced the smallest seed weight (43% less) and the lowest test weight (59% less).
Table 2. Wheat response to delayed planting dates near Garden City, KS from 1985-1991 (Witt, 1996).
While there was variability in grain yield among the years, the average relative grain yield for each planting date compared to the first planting date was: October 1 = 100%, November 1 = 77%, December 1 = 59%, January 1 = 57%, February 1 = 41%, March 1 = 16%, and April 1 = 0%.
Forage data was not collected in this KSU study, so it is hard to say that the percent decrease in forage yield would be similar to the grain results. We can make some educated guesses on what the forage potential might have been, though. The number of heads per plant in the March 1 planting date was 58% less than the October 1 planting date (i.e., 58% less tiller production), and plant height was 20% less for the same comparison. So, it may not be out of the question to say that there was probably a 50-75% reduction in forage yield. Unless your seeding rate was significantly increased to compensate for less tiller production, the bottom line is that there is a low probability that late-emerging wheat will generate much tonnage.
In Oklahoma, especially southern Oklahoma, we warm more quickly than the location for the KSU study. Their March 1 planting date is probably more like a February 15 planting date in Oklahoma. Again, this is all estimation, but when taking this and the Kansas data into consideration, the February emergence dates for some of our Oklahoma wheat puts us in vernalization limbo. Only time will tell us whether or not our wheat was exposed to enough cooler temperatures to trigger reproductive development. Again, the good news is we are still getting these cooler temperatures. If we do run into the scenario where we do not get enough cooler temperatures but still go through reproductive development because of the photoperiod component, our forage or grain yield will likely be even lower, as growth and development will be extremely delayed.
We’re working on it
To help us gather more variety specific information on whether heads and grain will be produced or not, we planted 36 different wheat varieties at Stillwater on March 6. I will post pictures and updates of this study to keep you informed as to what we are seeing and likely outcomes.
Final thoughts:
- If you have crop insurance, contact your agent to discuss your options.
- To get a ballpark estimate on what your forage or grain yield potential might be, you need to first assess your stand. Ideally, we need 60-70 heads per square foot to maximize grain yield potential. In areas that typically have lower yield potential, we can likely lower that number to 50-60 heads per square foot. To help you with your tiller counts and yield estimates, you can find more information in fact sheet PSS-2149: Estimating Wheat Grain Yield Potential.
- Wheat that was established prior to the February rain will have had enough time under cooler temperatures to vernalize and go through reproductive development. For most of these acres which did receive rain, we still have a chance at producing full or close full yield potential if the wheat tillered or if you increased your seeding rate to compensate for the late planting.
- For those who had spotty stands prior to the rain, we can still get an estimate on potential here too. You will need to estimate the percent of the field with an established stand, and then obtain a plant/tiller count as described above. Again, these established plants may still have full yield potential. For the plants now emerging, I doubt we will get much, if any, additional tillers produced. Also, the amount of grain produced by that single head will likely be less (refer to Table 2 for an estimate on how much less), unless the variety has a low vernalization requirement (Table1 may give indication to varieties with a lower vernalization requirement).
- With wheat that was planted very late and is just now emerging (after cotton in southwest Oklahoma for example), again, I doubt there will be additional tillers produced. The grain yield potential here may be similar to the results described in the Kansas study. If a producer increased their seeding rate to compensate for less tiller development, the amount of grain yield reduction may not be as much depending on the seeding rate, but it will still likely not reach close to full yield potential.
Sources:
WORLD OF WHEAT 