By Liz Stahl
With good field conditions, many acres of soybean and corn were planted across southern MN by mid to late April. Corn and soybean in some of these fields were just starting to or have emerged. The frost warnings issued yesterday (May 5) were not a welcome development. According to the NDAWN and MN Department of Agriculture’s Agriculture Weather Network (Figure 1), although some areas dodged freezing temperatures, minimum temperatures down to 24°F were recorded in parts SW, SC, SE, and NW Minnesota. What does this mean for crops that were planted early and are emerging or have emerged?
When can I expect crop emergence?
Check on crop emergence. It takes about 90 to 120 cumulative growing degree units (GDUs) for corn emergence and about 130 to cumulative GDUs for soybean emergence (base of 50 degrees F). To calculate GDUs, often referred to as growing degree-days (GDDs), take the average daily temperature ((max temperature + min temperature)/2)) and subtract 50. In the calculation, use 86 for temperatures above 86°F, and use 50 for temperatures below 50°F.
Assessing frost injury and making replant decisions
Soybean: Temperatures usually need to be at or below 28 to 30°F for several hours to kill soybean tissue. Even if air temperatures hit 28°F, emerged soybeans may still not freeze. Wait three to five days before assessing stands as it takes some time to tell if plants will recover from frost injury.
The article Spring frost damage on soybean discusses how to assess frost injury. If injury is detected, the article Soybean damage and replanting addresses considerations when making replant decisions.
Source : umn.edu