Planting wheat in hot and dry soils

Amanda de Oliveira Silva, Small Grains Extension Specialist

With severe dry conditions and high temperatures in our state, it is good to consider the possible effects of high temperature and drought on wheat germination and early growth. As of September 19, soil temperature was in the 80’s F across the state and reached 97 F in some areas (Figure 1). Wheat can germinate in soil temperatures from 40 F to 99 F, but temperatures from 54 F to 77 F are optimal.

Figure 1. Soil temperature across Oklahoma. Figure courtesy Oklahoma Mesonet.

Wheat germination and emergence in HOT soils

Is the variety I am planting high-temperature germination sensitive?

High-temperature germination sensitivity is a more elaborate way of saying that some wheat varieties do not germinate well in hot soil conditions. This is not to say that the seed will not germinate, but it may not germinate until the soil temperature has lowered. Keep in mind too that this sensitivity can vary from year to year. For example, a sensitive variety like Ruby Lee may germinate fine in 90°F soils one year and only produce a 10% stand in the same soil conditions the next. When sowing early, it is best to plant varieties first that do not have high-temperature sensitivity (e.g., Duster, Gallagher). Soil temperatures typically begin to cool by about September 20 due to lower air temperatures and/or rainfall events. However, our summer temperatures seem to be sticking around for longer this year. Waiting until at least mid-September to plant sensitive varieties can help reduce the risk of this issue. A high temperature germination sensitivity rating for wheat varieties can be found in the OSU Fact Sheet (available by clicking here). An updated version of this factsheet will be published soon.

Coleoptile Length

Hot soil conditions at sowing also reduce coleoptile length. The coleoptile is the rigid, sheath-like structure that protects the first true leaf and aids it in navigating and reaching the soil surface. Once the coleoptile breaks the soil surface, the coleoptile will stop growing, and the first true leaf will emerge. If the coleoptile fails to reach the soil surface, the first true leaf will emerge below ground and take on an accordion-like appearance (Figure 2A-B). If this happens, the plant will die.

Figure 2A and 2B. Example of two different wheat seedlings in which the coleoptile failed to break the soil surface. The first true leaf emerged below the soil surface and resulted in this accordion-like appearance.

The coleoptile length for most wheat varieties today can allow for the seed to be safely planted up to 1.5 inches deep. Under hot soil conditions though, the coleoptile length tends to be decreased. Therefore, “dusting in” early-sown wheat at ¾ to 1 inch depth and waiting on a rain event may result in more uniform emergence than trying to plant into soil moisture at a deeper depth, if soil moisture is not available in the top 1 to 1.5 inches of the soil profile. A rating for coleoptile length for wheat varieties can be found in the OSU Fact Sheet PSS-2142 Wheat Variety Comparison. We are also working on updating this.

Wheat germination and emergence in DRY soils

The most important physiological requirement for wheat to germinate and sustain the developing seedling is soil water. Therefore, planting decisions should be based on a combination of available soil moisture and expected rainfall. In addition, other factors such as adequate seeding depth, sowing date, soil fertility, seed treatment, seed quality, etc., should be considered to guarantee good crop establishment. For more information, check the materials on our website.

Wheat seed needs a minimum water content of 35 to 45% of its dry weight to initiate germination, and germination will be more complete as moisture levels increase. Dry soils can still maintain a relative humidity of 99%, which can provide enough moisture for seeds to germinate. It might just take longer than with free-moisture availability. My concern with the current situation in Oklahoma is the severe drought we are in and the lack of rain in the forecast. In some cases, we could have enough moisture to start the germination process in some regions of the state, but seedling emergence and growth could be compromised if we do not see any rain soon.

What happens if the soil completely dries out before wheat emergence?

There are three phases during the germination process: water absorption, activation when the seed coat is ruptured, and visible germination when the radicle emerges, followed by the seminal roots and coleoptile. These processes will start and stop depending on soil moisture availability. Thus, if the soil dries out before the roots and shoots are visible, the seed remains viable, and germination will be paused and continue once water is available. However, if the soil dries out after those structures are emerged (approximately 4-5 days after germination has begun), the seedling may not tolerate the lack of water, resulting in incomplete or loss of stand.

What should I do then? Choose your battle!

The optimal time for planting wheat in central Oklahoma is around mid-September for a dual-purpose system or around mid-October for a grain-only system (Figure 3). With the current forecast, we are planning to wait another 7-10 days to decide on our dual-purpose and forage trials. There are different ways we can go about it, but we must remember that there is always risk involved when planting wheat in dry and hot soil conditions.

Figure 3. Forage and grain yield potential in relation to the day of the year. Every 1,000 kg/ha is equal to approximately 900 lb/acre or 15 bu/acre. Ideal planting dates for dual-purpose wheat in Oklahoma are mid-September (i.e., approximately day 260). Planting for grain-only should occur at least 2-3 weeks after dual-purpose planting (i.e., mid-October or approximately day 285).

If you decide to dust in your wheat and wait for a rainfall event to drive germination, watch your seeding depth. The optimum seeding depth to plant wheat is about 1-1.5” deep. We typically do not have as many issues with winterkill in Oklahoma as in more northern states, so I am comfortable with dusting in at about 0.75 – 1” deep. Planting at 0.5” or less is too shallow in most circumstances. Also, there is always a chance for a pounding rainfall event and subsequent soil crusting, which makes it difficult for the coleoptile to push through the soil surface and may result in poor emergence. Fields with stubble cover may be less affected and reduce the risk of soil crusting. If we receive light rain in the following weeks, that could cause wheat to emerge, but it may not be enough for wheat to continue growing. Most of the fields do not have good subsoil moisture, either.

If subsoil moisture is available and you decide to plant deeper to reach moisture, be careful with the coleoptile length of your variety, and make sure it has a long-enough coleoptile that will allow emergence if conditions are favorable. Consider increasing seeding rate to compensate for reduced emergence, which is prone to occur in this situation.

Should we wait for rain to plant then? This is a farm-by-farm call and it depends on which source of risk you find most comfortable. Personally, I would rather plant my wheat in the optimal planting window and adequate seeding depth than waiting for a rain that may take too long to happen or missing my optimal planting window. If the latter is the case, consider bumping seeding rate to try to compensate for the reduced time for tillering (especially in a grain-only system). Planting wheat at optimal time allows for more time for root growth in seedlings, helping the crop to establish more quickly under dry conditions and possibly help the plant to scavenge for water that is available deeper in the soil profile.

Are there any specific agronomic traits that could help wheat seedling growth under water stress?

Traits that will help with seedling growth in dry conditions are coleoptile length potential, which allows to plant a little deeper in moisture and good emergence (if deep planting is the practice of your choice). There are indications that sowing wheat varieties with larger seed may help to reduce the negative effects of drought during early growth (Mian and Nafziger, 1994). In general, the greater reserves of larger seed result in faster germination and crop establishment by increasing root growth and tiller production. Keep in mind, however, there are varieties with small seed size that germinate more rapidly than larger seeded varieties, owing to their differential response to available moisture.

Planting wheat in hot soils

Kickoff of the college football season and the start of wheat planting are Labor Day traditions in the southern Great Plains. Many producers are waiting to see if rain forecasted for this weekend materializes, but it is likely that forage-based wheat farmers will start sowing wheat next week whether it rains or not. This means sowing wheat into hot soil conditions which can cause wheat germination and emergence issues. Given the potential problems, there are a few questions producers should ask themselves prior to planting into soil temperatures >90F.

coleoptile length

If the wheat coleoptile does not break the soil surface, the germinating wheat plant will not survive.

Will you have to plant deep to reach moisture? That first structure protruding from a germinating wheat seed is actually not a leaf. It is the coleoptile. The wheat coleoptile is a rigid structure whose sole purpose is to “punch through” the soil surface so that the first true leaf emerges above the soil surface. If this does not happen, the first true leaf will try to extend below the soil surface, turn yellow, and take on an accordion-like appearance (picture above). Modern semi-dwarf wheat varieties have shorter coleoptiles than older, tall wheat varieties and coleoptile length is shortened even further by hot soil conditions. So it is important to plant a variety with a longer coleoptile length (e.g. Garrison or Doans) if planting deeply into hot soils. A rating of coleoptile lengths for wheat varieties can be found in OSU Fact Sheet 2141 OSU Wheat Variety Comparison Chart available at www.wheat.okstate.edu or at the direct link to the publication here.

Is the variety high temperature germination sensitive? High temperature germination sensitivity is a fancy way of saying that some wheat varieties simply don’t germinate well in hot soil conditions (e.g. 2174, Overley). The extent of the sensitivity varies by year, so Overley might germinate fine in 95F soils one year and produce a 10% stand in the same soil conditions the next. When sowing early, it is best to plant varieties that do not have high temperature germination sensitivity (e.g. Duster, Gallagher, or Armour). Soil conditions generally cool due to lower ambient temperatures or cooling rains by about September 20; however our summer temperatures seem to be arriving late this year, so it is best to know the level of germination sensitivity in the variety you are planting. A rating of high temperature germination sensitivity for wheat varieties can be found in the variety comparison chart linked above. A more detailed explanation of the phenomenon can be found in OSU Fact Sheet PSS 2256 Factors affecting wheat germination and stand establishment in hot soils (available by clicking here).

Partial funding for the research included in this blog post was provided by USDA Project No.2012-02355 through the National Institute for Food and Agriculture’s Agriculture and Food Research Initiative, Regional Approaches for Adaptation to and Mitigation of Climate Variability and Change

Planting by seeds per acre versus pounds per acre

Should we plant by seeds per acre or pounds per acre? I will certainly not settle this issue in a single blog post, but I will provide ammunition for those in both camps in the discussion below. Before the discussion begins, I must come clean by acknowledging that when I came to Oklahoma ten years ago, I was solidly in the seeds per acre camp. As a crop consultant I spent most of October every year calibrating clients’ drills each time they changed varieties or field conditions changed. I was certainly not willing to entertain the notion that this could have been wasted time. After reading the published research on the subject, however, I have moved to the pounds per acre camp and will probably remain there unless seed prices increase dramatically OR seed quality improves significantly (I will elaborate below).

Why pounds per acre works
The published research on wheat seed size clearly shows that larger seeds produce more vigorous plants which result in more and larger tillers per plant. Additionally, the research indicates that if you sow the same number of seed of both small and large seeds from  the same variety, the large seed will have an approximate 10% better yield potential. While not expressly mentioned in the research, it is reasonable to assume that small seed will produce less fall forage as well. For more reading on this subject consult Kansas Agricultural Experiment Station, Keeping Up With Research numbers 74 (1984) and 101 (1991).

We have established that larger seeds are better on a seed vs. seed basis; however, there are 36% more seeds in a bushel of wheat with 15,000 seeds per pound as compared to a bushel of wheat with 11,000 seeds per pound. So, when sowing by pounds per acre the additional seeds compensate for the smaller seed size and generally results in the same total number of tillers . This compensation effect means that planting by pounds per acre works for a wide range of seed sizes and varieties. This should not be taken as an excuse to plant poor quality seed, as the compensatory effect is not the same if you are sowing shriveled, light, or head-scab damaged seed.

When seeds per acre is a better option
If you are sowing high-quality, large seed, then seeds per acre might be a better option. To get this type of seed usually requires a favorable environment during grain fill, adequate fertility, and a foliar fungicide in-season followed by an aggressive seed cleaning process. I have heard Phil Needham say, and I agree, that 10% seed cleanout is a minimum. Most seed lots require 20 – 40% cleanout to achieve large-seed status. Including a gravity table in the cleaning process will increase seed quality further. The fact of the matter is that most of our wheat seed in the Great Plains simply does not consistently fall into this category. For this reason, if you are sowing  “average” quality seed by seeds per acre, then adjust your seeding rate up accordingly to ensure you are not short on tillers.

Seeds per acre may also a better option if you are managing fertility to manipulate final tiller numbers. In this scenario, farmers will sow a set number of seeds per acre and adjust topdress rate and timing to encourage or discourage tillering to reach a desired number of heads at harvest. You can do the same thing when sowing by pounds per acre, but you will need an accurate stand count shortly after emergence. When used in combination with an accurate tiller count in February, a stand count will allow you to accurately estimate potential heads per acre and adjust accordingly.

Finally, if seed costs rise significantly in the future, a switch to seeds per acre might be justified. No one would suggest planting a $300 bag of corn or cotton seed based on pounds per acre. I don’t know what the wheat seed cost threshold is for switching to seeds per acre but would assume that if we ever reach this point seed quality will increase accordingly, making seeds per acre a better option.

Planting wheat in hot soils

As is usually the case in Oklahoma, we currently have a wide range of soil moisture conditions. Soils in central Oklahoma are generally dry on top, but there is ample soil moisture below. Soils in western Oklahoma and the Panhandle are dry all the way down. A few drills have started rolling, but most producers are waiting on a “State Fair” rain to get started (for all you non-Okies, it usually rains sometime during the State Fair in early September). While moisture availability is the primary driver of wheat planting, it is not the only consideration for producers. Hot soil conditions can affect wheat germination too, and there are a few questions producers should ask themselves prior to planting into soil temperatures >90F.

coleoptile length

If the wheat coleoptile does not break the soil surface, the germinating wheat plant will not survive.

Will you have to plant deep to reach moisture? That first structure protruding from a germinating wheat seed is actually not a leaf. It is the coleoptile. The wheat coleoptile is a rigid structure whose sole purpose is to “punch through” the soil surface so that the first true leaf emerges above the soil surface. If this does not happen, the first true leaf will try to extend below the soil surface, turn yellow, and take on an accordion-like appearance (picture above). Modern semi-dwarf wheat varieties have shorter coleoptiles than older, tall wheat varieties and coleoptile length is shortened even further by hot soil conditions. So it is important to plant a variety with a longer coleoptile length (e.g. Garrison or Doans) if planting deeply into hot soils. A rating of coleoptile lengths for wheat varieties can be found in OSU Fact Sheet 2141 OSU Wheat Variety Comparison Chart available at www.wheat.okstate.edu or at the direct link to the publication here.

Is the variety high temperature germination sensitive? High temperature germination sensitivity is a fancy way of saying that some wheat varieties simply don’t germinate well in hot soil conditions (e.g. 2174, Overley). The extent of the sensitivity varies by year, so Overley might germinate fine in 95F soils one year and produce a 10% stand in the same soil conditions the next. When sowing early, it is best to plant varieties that do not have high temperature germination sensitivity (e.g. Duster, Gallagher, or Armour). Soil conditions generally cool due to lower ambient temperatures or cooling rains by about September 20; however our summer temperatures seem to be arriving late this year, so it is best to know the level of germination sensitivity in the variety you are planting. A rating of high temperature germination sensitivity for wheat varieties can be found in the variety comparison chart linked above. A more detailed explanation of the phenomenon can be found in OSU Fact Sheet PSS 2256 Factors affecting wheat germination and stand establishment in hot soils (available by clicking here).