Suppose you have 112-day hybrids in the shed waiting to be planted. If you’re able to plant corn June 1, should you be comfortable planting them, or should you switch to earlier hybrids? If you want to hear someone’s opinion besides your seed adviser, here’s what Bob Nielsen says. He’s a Purdue University Extension corn specialist. He collaborated several years ago with Peter Thomison, his counterpart at Ohio State University, to study how later planting affects maturity rate. The pair determined that the same hybrid planted later requires fewer growing degree days to reach maturity.
Typically, if planting is delayed a month, the same hybrid will require about 200 fewer GDDs to reach black layer. However, Nielsen notes that it’s possible the actual rate of increased maturity may vary by hybrid. He also says they can’t be sure if hybrids planted outside the eastern Corn Belt react the same way.
You may still need to consider switching to an earlier hybrid by the end of May or beginning of June in central and northern Indiana, depending on what maturity you intended to plant initially, Nielsen says. And don’t rely on the “days to maturity” rating system. Maturity is more closely linked to GDD accumulation, he says.
Your seed company may publish “GDD to black layer” ratings. However, they may not specify if that’s from planting or emergence, Nielsen says. He typically deals with GDDs from planting. If your company rates GDDs from emergence, add about 115 GDDs to account for the time from planting to emergence.
Nielsen uses the Corn GDD Tool developed by the USDA-funded Useful to Usable, or U2U, project. Purdue participated in that effort to develop useful tools for farmers related to dealing with climate change.
Pick your county and planting date, and the tool estimates black layer date and graphs it compared to average first killing-freeze date. However, the tool doesn’t account for the ability of hybrids to mature faster if planted later. You can manually enter an early-maturity GDD total, hit enter, and the graph will recalculate.
Nielsen tried the tool, indicating he would plant a 112-day hybrid in Tippecanoe County, Ind., on May 31. It requires 2,691 GDDs to reach black layer, at which point it would be safe from frost. Given that information, the tool estimated black layer occurring around Oct. 23.
Then he manually entered 2,487 GDDs for planting on May 31. That’s slightly more than 200 fewer GDD units, and is the amount determined based on the formula Nielsen and Thomison developed for later-planted hybrids. Learn how to use the formula to get precise numbers.
The new projected black layer date is Sept. 30, well ahead of the usual killing-freeze date.
If you decide to switch to an earlier hybrid at some point, take agronomic characteristics into consideration. Things like disease resistance and yield potential for early-maturing hybrids may vary from company to company, Nielsen notes. You still need to factor in these genetic considerations when considering switching hybrids.
The jury is out on whether full-season hybrids are hurt worse by delayed planting in terms of yield compared to short-season hybrids, Nielsen says. Some research indicates that may be true, but recent work from Iowa found very few differences among hybrid maturities in their relative rates of yield loss to delayed planting.
“Focus on whether particular hybrid maturities are expected to mature safely when planted late and/or whether expected differences in grain moisture content at harvest merit the decision to switch to an earlier-maturing hybrid,” Nielsen concludes.