Late Planting & Fall Killing Frost Concerns

The 2019 growing season has been anything but “normal” thus far. We have had well above normal precipitation, below normal heat unit accumulation and delayed planting. Moreover, the extended 3 month forecast published by NOAA and the National Weather Service calls for higher than normal probabilities of precipitation and lower than normal temperature probabilities thorough out the remainder of the summer (figures 1 & 2).

These facts have resulted in many sales agronomist and growers wondering if the crop will make it to black layer (maturity) prior to a killing frost? To help answer these questions I have used a decision support weather model/tool developed by the Midwest Climate Center and their affiliates to help derive some insights into this question.

To help answer this question I plugged in 3 hypothetical planting dates (May 25^th, June 5^th and June 15^th) into the U2U weather/GDD model, and three different maturities spanning early to full maturity corn hybrids. I also ran the U2U weather tool at 4 different latitudes. A latitude approximating Roseville, IL, Davenport, IA, Clear Lake/Hampton, IA and Elkhorn, WI. While I won’t take the time to carve through all the graphs and data, I will give a brief synopsis of my findings.

Fun fact, the definition of a killing frost is when temperatures reach 28 degrees F or colder. This temperature is usually cold enough to turn water within plant cells into ice crystals. These expanding ice crystals burst cells and are usually lethal to the entire plant. Hence “killing frost”.

In general I found that hybrids ranging from 104 RM to 114 RM have a high probability of making it to black layer prior to a killing frost at the Roseville, IL and Davenport, IA latitudes (figure 3 & 4) when planted by May 25^th or earlier planting dates. For hybrids planted on June 5^th, it looks like the 104 to 110 day RM hybrids will also have a good chance of making it to black layer prior to a killing frost, but very full hybrids (113 to 114 RM) may experience a killing frost prior to black layer at these latitudes (figure 3 & 4). Any hybrid (104 to 114 RM) planted on or after June 15^th at the Roseville, IL and Davenport, IA latitudes has a 50% chance or greater probability of experiencing a killing frost prior to black layer.

For the latitude’s close to Elkhorn, WI any mid and short-season hybrids (95 to 101) planted on or before May 25^th have a greater than 50% chance of making in to black layer prior to a killing frost, but full maturity hybrids (107) even when planted on May 25^th have a poor chance of making it to black layer prior to a killing frost (figure 6). If corn was planted on June 5^th only the short-season hybrids (95) hybrids have a good chance of making it to black layer before a killing frost at this latitude (figure 6). For regions close to this latitude all RM hybrids (95 to 107) planted on or after June 15^th don’t seem to have a good chance of making it to black layer if we have a normal frost date (Oct 18^th) for this region. But most full-season hybrids at this latitude are grown for silage, so a killing frost is likely not to be a concern for those silage acres given harvest is much sooner than when harvesting for grain.

While a killing frost sounds devastating to yield, a killing frost when grain yield is still rapidly accumulating during mid and early reproductive growth/development is rare. The more likely scenario is that we may experience a killing frost very late in the grain filling period (also known as reproductive growth period). A killing frost at 35 to 40% kernel moisture usually has negligible effects on grain yield, given all yield has nearly been accumulated. A rarer scenario is that we could experience a killing frost at half milk line, this could result in more severe yield losses, (10 to 15% range), slower field drying, difficulty shelling kernels from cobs and poor test weight. The best scenario for us all would be a warm dry fall.