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• Corn growth stages
• Soybean emergence and replant
• Weed control and crop safety with cool temperatures
• Over-wintering economic thresholds for bean leaf beetles

Once again, steady rain showers moved across the area and left accumulations from ¾ to over 3 inches. The extended forecast is calling for cool days and cold nights through Saturday or Sunday of this week. I would not expect much, if any crop development over the next 3 to 4 days. Remember that all physiological process stop in corn at 50 degrees and in soybeans at 55 degrees Fahrenheit. Crop safety to many POST herbicides is due to metabolism of the compound in the crop plant. If the plant has no physiological process ongoing due to the cool temperature, then the herbicide cannot be metabolized and crop injury may occur. Soil temperatures are predicted to drop significantly over the next thee days, slowing the emergence of recently planted soybeans. The last three-day’s and the forecasted 72-hour average soil temperature can be accessed online at: http://extension.agron.iastate.edu/NPKnowledge/

Crop Development and Update

The earliest planted corn should be at or near the V4 stage; some corn in the more southern counties will be near the V5 leaf stage by week’s end. The V4 stage has major implications as this is the time in which the ear is initiated in the growing point and the number of kernel rows is determined. Any physiological (environmental or cultural induced) stress at this time may influence the potential size of the corn ear. The V5 stage is also important, as this is the time in which the growing point moves above the soil surface

Soybean emergence: The recent (heavy) rain in conjunction with cool temperatures and lower germ soybeans may result in poor emergence in some fields. Watch for soybean seedling disease and evaluate stand establishment over the next one to two weeks. An excellent source to help evaluate soybean stands is the ISU extension publication PM-1851

Undesirable crop responses are more likely to occur when using herbicides with lower margins of crop safety. Certain herbicide labels have warnings on the label concerning increased risk of injury under cool conditions. If plans were to use one of these products, the best decision would be to delay application until more favorable conditions occur or switch to a product with a greater margin of crop safety. A partial list of products with cool temperature warnings includes Basis Gold, Accent Gold, Reflex, Lightning, Cobra, and Buctril. Read all labels to determine if restrictions or warnings are present concerning use in cool temperatures. Crop and weed size also should be considered when determining whether or not to spray. More detailed information can be obtained by going to the following website: http://www.weeds.iastate.edu/mgmt/qtr98-2/cooltemps.htm

Bean Leaf Beetle: Table one can be used to help evaluate if treatment for over-wintering bean leaf beetles is warranted. The threshold is based on economic feeding injury only; it does not account for the potential spread and injury from the bean pod mottle virus.