Corn planting time in a “normal” spring
The corn planted on April 5 in our planting date study here at Urbana has germinated and sent out its radicle (root), but not yet the plumule (shoot). It has accumulated some 70 growing degree days (GDDs) through April 10, but the 10-day forecast through April 19 is for accumulation of fewer than 50 GDDs over the next 10 days, so I expect emergence to take another week or so. Over the past 10 years the GDD accumulation for April at the Champaign airport has ranged from 210 to 344, with an average of 263. We’re on track to accumulate normal GDDs in April 2013. But temperatures during April don’t always increase steadily, and we can’t count on later planting to provide warmer temperatures after planting. In 2012, GDD accumulations for the first, second, and third 10-day periods in April were 109, 94, and 80, respectively.
The variability in daily accumulation of GGDs during April produces a wide range in number of days – from a week to three weeks – required for emergence. It’s not necessarily a problem for corn to take 20 days or more to emerge, as long as both emergence percentage and emergence uniformity – the time between first and last emergence within a field – are good.
Under cool temperatures, it is normal for the emergence process to take longer, with perhaps as much as four or five days from beginning to end. People often worry about this, having heard that non-uniformity of emergence costs yield. Plants that end up being behind their neighbors in development tend to stay behind, and often lose out in the competition for light, water, and nutrients. It is not the case that such plants end up as "weeds" – that is, that they reduce the yields of remaining plants. But the remaining (larger) plants don’t completely make up for the yield lost from the plants that get left behind.
It is important that we consider the cause of non-uniformity of emergence, however. Non-uniformity of emergence is a problem if differences in emergence are related to factors such as seedling damage, soil crusting, low seed vigor, etc. But uniformity of emergence is also affected by temperature (GDD accumulation rates) just as is the time between planting and emergence. So I suggest that we consider emergence as "uniform" from the plant development standpoint if it occurs over the time it takes to accumulate 20 or 25 GDD, whether that takes one day or five days. As with other aspects of corn development, basing events on “thermal time” (GDDs) works better than using time measured in hours or days. The practical effect is that, once it warms up, plants that emerged in 110 GDD and those that took 130 GDD to emerge will differ very little in their stage of development.