The recent period of unseasonably warm temperatures may have producers concerned with the possible effects on their wheat crop. Short vernalization varieties such as Overley, Everest, WB-Cedar, and others, may be released from winter dormancy and maybe have been growing for a few days during this unseasonably warm period. The consequences of an early green up on wheat yields will largely depend on spring weather conditions, and a few consequences are discussed below.
Winter wheat loses some of its winter hardiness each time warm temperatures breaks its dormancy, although some of its winter hardiness can be regained if temperatures gradually get colder again. The growing point is near the soil surface during the tillering stage and is protected against injury. Most freeze damage at this stage occurs to leaves. The leaves can become twisted and light green to yellow in color, and are burned at the tip within one or two days after freezing (Figure 1). A strong odor of dehydrating vegetation may be present several days after the freeze. Injury at this stage slows growth and may reduce tiller numbers, but growth of new leaves and tillers usually resumes with warmer temperatures.
In the jointing stage, the developing wheat head has started to move up the stem. Even so, wheat at this stage can usually tolerate temperatures in the mid to upper 20’s with no significant injury. If temperatures get into the low-20’s or lower for several hours, there can be some injury to the lower stems, the leaves, or the developing head. If it is windy during the nighttime hours when temperatures reach their lows, this increases the chance of injury. Most wheat in Kansas should not have reached jointing yet. Our weekly report indicates that none of the 23 wheat varieties tested at the South Central Experiment Field near Hutchinson has reached first hollow stem at this point.
Whether actual freeze injury takes place depends on the low temperature reached, how long the temperatures stayed that cold, temperatures gradients in the field, wind speed, canopy density, and other microclimate factors. Soil moisture is another factor that is usually important in determining freeze injury. One general rule is that producers should not make any quick decisions about the condition of their wheat crop after a freeze. It will take several days of warm weather following the freezes to evaluate the condition of the crop and its yield potential. Even if some of the main tillers are injured or killed, producers should wait to see if enough other tillers have survived to compensate for the lost yield potential.
An early green up means an early use of the much needed stored soil moisture. Wheat generally uses a relatively limited amount of water during the winter months until spring green up, and water use increases linearly with the increase in biomass from jointing until heading. The larger the wheat’s biomass or leaf area, the more water the crop will require to maintain its canopy structure. Greater water use during the winter months will reduce the amount of profile soil moisture for the spring, which might not be a problem in years with sufficient spring precipitation. However, excessive use of the current available water can play against wheat yields if the spring turns out dry.
It is still early to know whether the spring weather will favor a stripe rust epidemic (or other wheat leaf diseases) such as the one experienced in most of Kansas last growing season. Mild winter temperatures can increase the potential for a disease outbreak because of increased overwintering of the spores, but this needs to be matched by adequate moisture conditions. Texas and Oklahoma released a few reports of active stripe and leaf rust infections in the past couple weeks, which should put Kansas wheat producers on alert as states to our immediate south are generally the source of inoculum of many leaf diseases in Kansas, including stripe rust and leaf rust. Still, K-State research has shown limited yield response to early season (Feekes 5-6) fungicide applications across most of the state; thus, in most cases it is probably too early at this point to make the decision to spray a fungicide. It is advisable that producers continue to monitor the conditions in the south and actively scout their fields.