Soil temperatures in Oklahoma can be hot when planting in late August to early September for forage-only or dual-purpose wheat (Figure 1). Seed that was planted into soils with temperatures above 85° F may result in delayed germination or prevent wheat seedling emergence. In addition to the soil moisture status since planting, listed below are two factors that may cause poor early stand establishment when wheat is sown into hot soils.
Figure 1. Maximum soil temperature at a 4 inch depth under bare soil over the past three weeks near Altus. We can assume that the maximum soil temperature at shallower depths was likely higher. Data is from Oklahoma Mesonet.
High temperature germination sensitivity: This 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 at all, 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 germination sensitivity (e.g., Duster, Gallagher). Soil temperatures typically begin to cool by about September 20 due to lower air temperatures and/or rainfall events. Waiting until at least mid September to plant sensitive varieties can help reduce the risk of this issue. A rating of high temperature germination sensitivity for wheat varieties can be found in the OSU Fact Sheet PSS-2256 Factors Affecting Wheat Germination and Stand Establishment in Hot Soils.
Coleoptile length: The coleoptile is the rigid, sheath-like structure which protects the first true leaf and aids it in navigating and reaching the soil surface. Once the coleoptile breaks the soil surface, it 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 usually takes 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 (P.S. we are working on updating this).
Fall armyworm was a significant pest for producers in Oklahoma last year, and this year is setting up to be the same scenario. It is out in full force already in some areas, and you may have seen or heard Dr. Tom Royer recently discuss how this insect has been active over this summer, especially on bermudagrass and fescue pastures. Wheat planting is already underway in some areas of the state. As wheat planting progresses here in September, producers need to check their wheat fields very regularly after seedling emergence. Fall armyworm can decimate large fields within a few days. Scout for fall armyworms by examining plants in several (5 or more) locations in the field. A good place to start is along the field margin as they sometimes move in from the road ditches and weedy areas, but make sure to examine the interior of the field as well. Fall armyworms are most active in the morning or late afternoon.
Fall armyworms are small (3/8-1 1/2″) and can be easily overlooked (top photo). Feeding on leaves gives a transparent (“window paned”) appearance (bottom photo). Photos courtesy of Dr. Tom Royer.
Be on the lookout for “window paned” leaves, and count all sizes of larvae. The suggested treatment threshold is 2-3 larvae per linear foot of row in wheat with active feeding. Numerous insecticides are registered for control, but they are much more susceptible when caterpillars are small. We will not get relief from fall armyworms until we get a killing frost. So make sure to keep scouting regularly, especially with this early-planted wheat!
Fall armyworm can cause significant damage like this across large areas very quickly, so scout early and scout often. Photo courtesy of B. Boeckman.
Control suggestions for fall armyworm are available in the OSU Fact Sheets CR-7194 Management of Insect and Mite Pests of Small Grains.
The 2017-2018 wheat growing season is setting up similar to last year with many producers wanting to target more of the forage side of wheat production given the low commodity prices. That, coupled with the available soil moisture from the rainfall throughout August, has prompted some producers to get the drills rolling already over the past week in some parts of the state. As planting gets going, here are a couple considerations when it comes to planting dates and seeding rates for winter wheat in Oklahoma.
The optimal window for dual-purpose wheat for most of Oklahoma is between September 10-20 (approximately day 260 in Figure 1). This window represents a trade-off between maximizing forage production while minimizing potential grain yield loss. Earlier planting dates, last week into this week for example, will provide more fall forage potential, but this is usually not recommended unless the wheat is intended to be produced for grazing, or “grazeout.” Planting dates for grain-only producers will be at least 2-3 weeks later than what is the ideal dual-purpose planting date for your area. For many areas in Oklahoma, this will be around mid-October (approximately day 285 in Figure 1).
Figure 1. 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).
Producers in forage-only or dual-purpose management should plant 1.5-2x the amount of seed that is recommended for grain-only production. For example, data collected in north-central Oklahoma has showed that increasing the seeding rate from 60 to 120 lb/acre can increase fall forage potential by as much as 500 lb/acre for a mid-September planting date (September 11 in Figure 2). The increase in forage potential by using this higher seeding rate can justify the cost of the extra seed. OSU recommends 120 lb seed/acre for most areas of Oklahoma, including irrigated fields in the Panhandle. Seeding rates for dryland fields in the Panhandle for this type of management can be lowered to 90 lb/acre. OSU recommends a 60 lb/acre seeding rate for grain-only production when planted during that optimal mid-October time. Dryland fields in the Panhandle can have their seeding rate lowered to 45 lb/acre. If planting happens to get delayed in November this year, seeding rates will need to increase to provide enough available tillers to still maintain maximum grain yield potential.
Figure 2. Fall forage yield collected in north-central Oklahoma as affected by seeding rate and planting date. Source: PSS-2178.
More information about dual-purpose wheat management can be found in the fact sheet PSS-2178 Dual-purpose Wheat: Management for Forage and Grain Production.