By Romulo Lollato, Wheat and Forages Specialist
Getting good stand establishment of wheat is the first step in establishing the yield potential of the crop. A good and uniform stand allows for better use of resources — such as light, water and nutrients — decreasing in-field yield variability and increasing the yield potential of the crop.
Simultaneously, getting good stand establishment is the first hurdle for producers as they begin the new season. If the wheat doesn’t emerge, or emerges in a spotty pattern, growers will have to diagnose the problem quickly and decide whether it would be best to wait a little longer or replant the field.
Poor emergence or stand establishment can be caused by a number of problems, such as deep planting (Figure 1), a plugged drill, poor seed quality, seedborne or soilborne diseases, seed dormancy, dry soil, soil crusting, false wireworms and low soil pH. Growers should take time to examine the evidence, looking for field patterns that can be an indication of the possible causes of poor stands.
Soil Temperature and Moisture
The ideal soil temperature for germination of wheat seed is between 54 and 77 F. Planting in soils with temperatures more than 90 F can reduce wheat germination in varieties with high-temperature germination sensitivity, although seed of varieties with high-temperature germination sensitivity might eventually germinate once a cool rain brings the soil temperature down. High soil temperatures can also reduce germination by reducing coleoptile length. If the coleoptile is too short, the first true leaf will emerge below the soil surface and will take on an accordion-like appearance (Figure 1).
This year, however, temperatures have been mild for the most part, conducive for good germination if there are no other problems. Topsoil moisture is adequate in most of Kansas, but is too dry for good germination and emergence in some areas of southwest Kansas.
Some fields have been crusted by heavy rains after planting, which can prevent the coleoptile from breaking through the soil surface. If the wheat hasn’t emerged in a timely manner and you’ve had a heavy rain after the wheat was planted, dig up some seed and look for crinkled coleoptiles. If this is the case, you can try to break up the crust with a light tillage or hope for a gentle rain. But if the coleoptile stays underground for more than a week or so and hasn’t been able to break through the soil surface, the germinated seed will start losing viability. At that point, the producer will need to consider replanting if final stands are below approximately 50% of the targeted stand, or if the emergence pattern is not uniform.
If soil temperatures are ideal, the topsoil is not unusually dry, and there has been no crusting, the most likely causes of poor stands would be deep planting, a plugged drill, poor seed quality, unusually long seed dormancy, diseases or insects.
Deep planting (deeper than the coleoptile’s ability to elongate) can slow emergence or cause stand establishment problems. Varieties differ in their coleoptile lengths, but for the most part wheat should be planted about 1-1.5 inches deep. Most varieties can emerge at slightly deeper depths if the soil is not too restrictive and temperatures are in the ideal range, but it is possible the wheat cannot emerge if it is planted deeper than 2.5 inches.
Coleoptile length is determined by the variety and soil temperature conditions. Coleoptile length is shorter at both lower and higher temperatures than the ideal range. If the coleoptile grows as long as it can and still hasn’t broken through the soil surface, the first true leaf will emerge below ground (Figure 1). Under normal conditions, this first true leaf emerges above ground. If the first true leaf has to start growing in the soil, it is very unlikely to be able to force its way through the soil and emerge. What you’ll see when digging up the seed is an intact coleoptile alongside a short first leaf that is scrunched up or crinkled. If this is the case, replanting will likely be necessary.
Another possible reason for poor emergence is poor seed quality. As long as the seed was tested for germination by a licensed laboratory and had an acceptable germination rate, seed quality should not be a problem. If germination testing on the seed lot was not done by a laboratory, poor seed quality could be a problem if other potential problems have been ruled out as the cause of poor emergence.
At times, wheat doesn’t germinate simply because the seed has an unusually long seed dormancy requirement. This is hard to identify in the field and can cause growers to replant when it’s not necessary. There are variety differences in seed dormancy requirement, although this hasn’t been tested recently. And even within the same variety, some seed will have longer dormancy than others depending on the conditions in which it was produced. If a seed lot has unusually long seed dormancy but is acceptable in all other qualities, it should eventually germinate and emerge just fine.
False wireworms can be the cause of poor emergence. False wireworms are soil-inhabiting, yellowish to orange-colored worms up to 1½ inches long. A pair of short antennae is clearly visible on the front of the head and the head region does not appear flattened when viewed from the side. They commonly follow the drill row in dry soils, feeding on the seeds prior to germination. Other insect and disease problems can attack seedlings after emergence.
Low Soil pH
Although low soil pH has been historically known to decrease overall plant growth (root and aboveground biomass), its effects on wheat stand establishment had not been reported until recently. A study in Oklahoma looking at four different varieties and a soil pH gradient ranging from 4-7.5 indicated that a minimum soil pH of 4.9 was needed to ensure an acceptable emergence uniformity and stand establishment.
Making the decision to replant the crop needs to take into account percent stand establishment as compared to the targeted stand, replanting date, rate and weed control. See the accompanying article on replanting decisions in this issue of the Agronomy eUpdate.
For more information on emergence problems, see K-State’s publication S-84, Diagnosing Wheat Production Problems.