Temperatures over most of Illinois dropped to the upper 20s or low 30s on Saturday morning, May 9. This resulted in damage or even death to emerged and emerging corn and soybeans. The extent of damage was closely tied to when fields were planted.
Corn planted during the warm part of April—the first week—was up and growing (slowly) by May 1, with limited leaf area. In some fields, emerged stands were already subpar, especially in the wettest parts of the state, whether or not water stood in the field. According to NASS, 68 percent of the corn crop was planted by May 10, and 23 percent had emerged. With 8 percent planted by April 19 and 37 percent planted by April 26, we can estimate that corn planted by about April 23 had emerged by May 10. About 110 to 120 GDD accumulated between those dates in central Illinois. To match a similar GDD accumulation, that date would have been later in southern Illinois and earlier in northern Illinois.
Low temperatures on May 9 caused damage to emerged corn plants, and possibly to plants that had not yet emerged. With different low temperatures across regions, and with damage ranging from minor leaf loss to death, the only way to know if seedlings that are still alive will survive is to see if they produce new, green tissue after a few days with warmer temperatures. By early next week, we’ll know.
What we won’t know is whether or not damaged plants that produce minimal green leaf area, and that grow or regrow very slowly (these two are connected) will turn into thriving, productive plants. My suggestion is to take a “realistically pessimistic” approach to this, and to include in stand counts only those plants that look ready to make normal growth by May 17 or 18. With what we hope will be warm, drier weather coming after that, replanted fields should get off to a fast start, which will help to restore yield potential.
Soybeans planted by the end of the first week of April emerged early enough to have produced some leaf area by May 9. These mostly escaped serious injury, although in low-lying areas, where temperatures might have been even lower than those recorded, damage including leaf loss could be more serious.
Two percent of Illinois soybeans were planted by April 19, and 16 percent were planted during the week that ended April 26. Ten percent had emerged by May 10, so we can estimate that those planted before April 22-23 (the same as for corn) had emerged by the frost date. Temperatures just beneath the soil surface may have dropped low enough to cause some damage to soybean seedlings that had not yet emerged. Soil crusting in some fields might also have decreased emergence.
Soybeans planted between about April 10 and April 22 had enough time and temperature to emerge, or to be in the process of emerging, by May 9. These suffered the most damage. Reports are that soybean damage ranges widely, and is severe enough in some fields that replanting will be necessary. We will need to wait a few more days to assess the potential for regrowth, but we should be able to make realistic counts of (productive) stand by early next week. That will enable us to decide whether to replant or to supplement the early-planted stand with more seeds.
I’ll close with two take-away points:
- Be realistic when going to the field to assess stands. With few if any starts to a season like we’ve had this year, we don’t have much guidance on what to expect. But we can’t take for granted that marginal stands will—as they often have in the past—produce full yields this time. Having plants suffer early in the season is not known to increase their ability to yield well by the end of the season.
2. Use the replant guidelines posted to Farmdoc earlier today to decide whether your fields need to be replanted. That article is similar to the May 6 Bulletin article, but includes a suggestion to adjust the first planting date based on the slow start for this year’s crops.