Three percent of Iowa’s corn lies quietly in the ground awaiting warmer soil temperatures (see USDA- NASS report). That contrasts dramatically with an average of 28 percent for the last five years and is hardly worthy of mention when contrasted with the record of 61 percent last year.  If temperatures warm as promised later this week, field work and corn planting will resume in earnest.

We still have time to plant corn in Iowa without sacrificing significant yield potential. On average, yields within 5 percent of maximum are still attainable in all but the north central and northeast regions of Iowa if we can plant by mid-May. In past research, this area shows a greater reduction in yield as planting is delayed presumably due to the limitations in growing season length. Regardless of the exact calendar date, maximizing harvestable yield depends upon soil conditions at planting and subsequent weather during the remaining growing season. In some rare situations, later planting dates produce more yield than early planting dates.

One thing discussed with later than normal planting is whether different hybrids should be planted to compensate for the later planting dates. For May planting dates, this is not normally necessary; hybrids adjust for delayed planting

To test this, I used a computer model, Hybrid Maize, to estimate physiological maturity dates of generic hybrids with relative maturities (RM) typically grown at either Nashua (NE Iowa) or Lewis (SW Iowa) (see Table). In both situations, hybrids matured before there was a significant probability of frost. Of course, along with delayed maturity, grain moisture will be greater with delayed planting.

We still have time to plant corn without significant yield losses. Adapted hybrids adjust to later planting by shortening the time necessary to reach silking. Farmers will want to plant longer season, adapted hybrids as soon as possible followed by mid- and shorter-season hybrids. Development and final yield of these hybrids will not be largely affected unless an early fall frost occurs.

Table. Effect of planting date on days to maturity (R6) and the probability of frost.  Two Iowa locations with hybrids of typical relative maturities (RM) for those locations.