Friday, March 17, 2017

Estimated Yield Potential for KY’s Freeze Damaged Wheat

Carrie Knott, Extension Agronomist-Princeton, University of Kentucky
Bill Bruening, Small Grain Variety Testing, University of Kentucky
Dave Van Sanford, Wheat Breeding and Genetics, University of Kentucky
Lloyd Murdock, Emeritus Extension Soil Specialist, University of Kentucky

After several nights with temperatures near or below 24°F the jointed stems of KY’s wheat crop are probably terminated by cold temperatures. We now have to decide what to do with our freeze damaged wheat crop.

The first thing we all need to do is take a moment to realize that:

There is no reason for rash decisions; we have time to make management decisions!

Fortunately, there are advantages to this 2017 wheat freeze compared to previous spring freezes.

1. Technically spring has not begun yet.
2. There is still 4 to 8 weeks to plant corn or soybeans, without affecting corn or soybean yield.
3. Most of the wheat has many tillers this year. As long as those tillers were not at the jointing growth stage or beyond, they should be unaffected by recent temperatures and likely produce decent to acceptable grain yield.
4. The National Weather Service predicts warmer than normal April temperatures in KY. This could help the surviving tillers develop to produce decent to acceptable yields.

With winter wheat, there are many “what-if’s” that make is very difficult to provide definitive yield loss estimates. Within the last 10 years, there were two other wheat freeze events: 2007 and 2012. In 2007, Kentucky’s average wheat grain yield was only 73% of the average grain yield in 2006 and 2008. However, the estimated yield reduction of only 27% may not reflect the actual damage due to the freeze because the most severely damaged wheat fields were terminated and subsequently planted with other grain crops. In 2007 a better estimate of yield loss may come from the Wheat Variety Trial; at the six Wheat Variety Trial locations yield was less than 60% of the same locations in 2006. In 2012, Kentucky’s average yield was 87% of its adjacent years (2010 and 2013). Given the timing of the 2012 freeze and the developmental stage of the wheat the expected yield loss and wheat damage was expected to be more severe.

This illustrates the difficulty we have in estimating wheat grain yield losses following a freeze event.

To estimate the yield potential of the crop we first must verify that the tillers were not at the jointing or more advanced growth stages. For fields with tillers that were not at jointing we need to estimate the possible yield potential. The best estimates we have of yield potential of wheat at specific plant stands is Table 3.4 of the Comprehensive Guide to Wheat Management in Kentucky. It will not provide a perfect estimate of expected yield potential because the data in the table are based upon plant stands prior to Feekes 3, green-up, which generally occurs in February and is based upon fall tillers that typically produce greater yields than spring tillers. In mid-March, wheat in Kentucky should be around Feekes 4 or 5 in Central and Western KY, respectively and includes both fall and spring tillers. This year the tillers we will likely be counting will be spring tillers, which we know produce less yield. Regardless of these issues, counting the number of living tillers next week will provide the best estimate of potential yield.

Table 1. Modified excerpt of Table 3.4. Wheat yield potential based on plants per square foot from Comprehensive Guide to Wheat Management in Kentucky including estimated tiller counts.

To assess wheat freeze damage:

Figure 2. Developing wheat head of a Feekes 6 wheat
plant at 20X magnification. Photo: Brenda Kennedy
1. Wait until high temperatures are at least 40°F for 5 to 7 days. According to the projected weather forecast, most of the state will have high temperatures greater than 40°F the next 7 days. Assess freeze damage next Tuesday or Wednesday. Any earlier than that may provide an inaccurate estimate of damage. Given that we are still so early in the growing season there will be no negative effects to waiting up to 2 to 3 weeks to assess freeze damage.

Figure 3. Freeze damaged developing
wheat heads (top) and normal head (bottom).
2. Wheat stems with terminated growing points from freezing temperatures will have yellow, chlorotic growing points, limp leaves and/or a silage odor as early as 5 to 7 days with high temperatures ≥40°F. Typically, within 4 to 7 days with temperatures ≥40°F a dissected growing point will begin to show damage. A healthy growing point will be turgid, somewhat translucent and very glossy with magnification (Figure 2). Without magnification is will appear a pale greenish yellow, firm to touch and have distinctive serrated edges (Figure 3). A freeze damaged growing point will be limp and white and milky and lose serrated edges (Figure 3). A hand lens is very helpful at magnifying these subtle differences within the first 4 to 7 days after the freeze event. Striking symptoms of dead wheat plants can take a couple of weeks to fully develop depending on the daily high temperatures (Figure 4 & 5). Freezing temperatures can also cause cosmetic damage to wheat leaves, most commonly as yellow leaf tips. These typically do not affect plant survival or final yield.

To assess number of viable tillers after wheat freeze:

1. Wait about 7 days with high temperature 40°F or greater. Given that we are very early in the season this year, waiting up to 2 or 3 weeks to assess viable tillers per square foot may provide the best information when making 2017 crop management decisions.

2. Viable tillers should have a green, turgid stem and growing point. Pay particular attention to any stem damaged particularly splitting. There may be some yellow leaf tips, which are cosmetic leaf damage. The key is that the stem near the growing point is turgid, green and healthy.

3. Count the number of viable tillers per square foot and reference Table 1 for estimated yield loss.

Figure 3. Declining wheat stands following the 2007
spring freeze event. Photo: Bill Bruening.

Figure 4. Damage to wheat stands several weeks
after the 2007 spring freeze event. Photo: Bill Bruening.

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