Another Look at Corn Planting Date Response

As we head into May, Illinois is ahead of most of the rest of the Corn Belt, with 32% of the corn crop planted here by April 27, close to the 33% that was planted by that date averaged over the past 5 years. Only 19% of the US corn crop was planted by April 27, and states to the north and east of Illinois were in single digits. It is wet in much of the Corn Belt now, and further delays are expected.

We’ve been running planting date studies for a number of years at a number of sites in Illinois. While there’s always a lot of variability in planting date responses, we do have data sets that combine well – where there is some consistency over years and where yield levels were not wildly variable. One such set is from DeKalb and Monmouth over the past seven years – 2007 to 2013 (Figure 1). Data from several years at these sites were used to develop the corn replant model in the current Illinois Agronomy Handbook. These sites also represent the Corn Belt quite well, so results are applicable over a wide area.

Figure 1. Response to planting date combined over seven years (2007-2013) and two sites in northern Illinois.

Combining data by percentage of maximum yield for each site and year shows that highest yield comes at about April 20 (Figure 1). This is similar to what we’ve seen as the “best” planting date in most recent planting date summaries. But yield penalties from planting delays are somewhat milder here than we have previously reported. Compared to the highest yield (on April 21), the yield loss for planning on April 30 was 1% (2 bushels); by May 10 it was 4% (8 bushels); by May 20 it was 8% (17 bushels); by May 30 it was 14% (29 bushels); and by June 10 the total yield loss predicted from these data was 22%, or 49 bushels per acre.

The average maximum yield across all sites was 216 bushels per acre. Losses measured as bushels per acre per day of delay, for 10-day periods starting on May 1 were:  May 1-10 (0.6 bu/acre/day); May 10-20 (0.9); May 20-30 (1.2); and May 30 to June 10 (1.9). Thus the loss accelerates, with more loss per day of delay the later it gets. Because average temperatures rise during April and May, the per-GDD yield loss would accelerate less than the per-day loss – it would be closer to a straight line.

These results bring up an obvious question: has the planting date response really changed, or are we just looking at a “favorable” set of data? That’s not completely clear, but I do think that increasing tolerance to the stress of high populations has likely helped plants to also do better under increased weather-related stresses, especially during pollination, that are more likely with late-planted corn than when the crop is planted early. Seven years is a reasonable “sample” of years, and these results weren’t only the product of what happened in one or two “odd” seasons.

One lesson we certainly can take from this is that having planting delayed to early or mid-May does not mean “game over” in terms of yield potential. Average GDD accumulations during April range from less than 200 in northern Illinois to about 300 in southern Illinois, and amount to less than 10% of the seasonal total. In other words, losing April doesn’t greatly diminish how much season is available to produce a crop. The main reason why late planting lowers yield is due to the increased chance of water stress when pollination is delayed.

The April 1-planted corn in our planting date study here at Urbana is at the 1-leaf stage now, and not looking great with night temperatures around 40 and daytime highs in the 50s this week. Plots planted on April 1 at Perry froze before emergence, and will have to be replanted. So this year, early planting didn’t get the crop very far ahead.

Our hope now is that temperatures will start to rise and will remain at least as high as average through May. This will help both to dry the soil after rain and the crop to take off once we get it planted. But as much as we hope we can finish planting within the next couple of weeks, rain in July will, as usual, be the main thing we’re going to need for good yields in 2014.