Positive outlook for this year’s winter wheat crop
A couple of weeks ago, this column addressed the issue of aphids being present in a number of winter wheat fields across South Dakota, the yellow dwarf viral diseases the insects often transmit to the plants and how to avoid the situation in the future. By the time the column was written, much less published in area newspapers, temperatures had dipped into the single digits over much of the state, rendering the aphids dormant if not killing them.
This drop in temperature was fairly quick, going from lows in the upper 20s and 30s to zero and below in a few days, and has raised questions as to the potential damage to winter wheat. In reality, the drop in temperature was probably a good thing for the crop, helping it to harden off as well as take care of the aphids.
While the roughly 30 degree drop in temperature didn’t feel very good to us, the important part of the winter wheat plant is the crown, which develops about 1 inch below the soil surface. While we were donning an extra layer of clothes against the cold, soil temperatures at the level of the winter wheat crowns were in the low to mid 30s at most weather stations.
Most of the winter wheat varieties grown in South Dakota can withstand temperatures at the crown level down to 0 – 5 degrees Fahrenheit when the plants are properly hardened off and soil moisture is adequate. The hardening off process begins when soil temperature at the crown level drops below 48 degrees F and requires four to eight weeks, depending on temperatures at the crown. Cold acclimation or hardening off progresses quicker when temperatures at the crown are close to freezing than when significantly above. Soil temperatures at many of the automatic weather stations in South Dakota have been below 48 degrees F since about late October and in the 30s for about the last 2 weeks.
This all points to a positive outlook for the condition of the winter wheat crop, but there are a couple of negatives. The aphids that were present in many fields took some sap out of the plants, undoubtedly weakening them. There is also a reasonable chance that some of the plants were infected with one or both of the yellow dwarf virus diseases. We won’t know the effect of these factors until spring, and the consequence of any yellow dwarf infection will depend highly on moisture and temperature in May and June. The other negative is that some wheat fields went into this cold spell with limited soil moisture. Winter wheat plants going into the hardening off phase are much better off when they are well hydrated. The snow that fell in mid-November both insulated the soil from rapid temperature change, and provided welcome moisture.
One producer recently commented that the wheat he planted into soybean residue after harvest probably won’t emerge, and may not even germinate this fall due to the cold spell. He’s almost certainly right, but unlike the spring of 2013, when many winter wheat seeds laid in dry soil until mid-April or later, most areas had some soil moisture and received enough from the recent snows to get the crop started as soon as soils warm in the spring. That should be plenty early enough for the plants to vernalize and produce viable heads.
12/16/2014 – SD Soil Health Challenge, Ramkota Hotel, Rapid City
1/8/2015 – PAT, 2 p.m., CST, SDSU Extension Center, Winner, SD (also at the Pierre and Lemmon SDSU Extension Centers, and the West River Ag Center in Rapid City)
1/12/2015 – PAT, 1 p.m. CST, Courthouse, Murdo