You have to love farming in the Great Plains—or it will drive you crazy. We have gone from having a mix of wet areas in a relatively dry region, to being cold and soaked. And the forecast is for another wet week or more before we will have a chance to get into the field. We have had several inquiries concerning planting dates for specific hybrids, and when will they need to consider switching hybrids and maturities.
The issue is how many heat units do we have to work with, and how is yield affected by later planting. First, the total heat unit accumulation from a presumed planting date to the average date of first freeze in the fall is fairly well known for each locality. If you look at these tables (on the web or from your university extension group) there, typically, is about 350 heat units accumulated between April 15 and May 15—that is not a lot of growth potential between those planting dates. Compare the tables of heat unit accumulation from spring dates to first freeze, and the heat unit requirements of your favorite “full season” hybrids in the 2011 Seed Guide. It is clear that at nearly every location there is likely to be enough heat units to mature our favorite hybrids if they are planted by May 20th. When it is cold, cloudy and wet (like the predictions) we accumulate few heat units. If the daily low is 40 degrees, and the high is 60, we average the two temps, and subtract 50, giving zero heat units for such a day—meaning corn isn’t going to grow anyway. We are likely to lose fewer than 150 heat units if planting is delayed for the next two weeks. And for each two days later emergence we might see (assuming a two week delay in planting), we will likely delay flowering only one day.
Second, how much is yield affected by a delay in flowering? For optimum yields, one wants to fill grain during longer days—that is as soon after July 4th as possible. However, actual yields are GREATLY impacted by heat (and moisture) stress in the period of a week before until 10 days after silking. Better or worse weather a few days earlier or later makes much more yield difference than precise planting date—and that’s mostly random in mid-July. However, if flowering is delayed until late July, the odds of hotter, drier weather increase. Especially in irrigated corn, the “optimum” planting date implies lower yields if planted BEFORE or AFTER the optimum date. And, historically those optimums lie between April 20 and May 5 for most of us.
Then, there is the issue of hybrid yield potential, by maturity. Longer season hybrids, in general, always have higher yield potential than earlier ones. They have greater leaf area—just a bigger factory to produce grain—and greater ear size and/or number of kernels—more room to pack starch and protein. Switching to earlier hybrids, even if they have high yield potential, almost always results in putting a “lid” on yield potential. As long as we have enough time (heat units) available to mature the fuller season hybrid, we are almost always better off NOT to switch to earlier hybrids with less yield potential. Hybrids of the same “heat unit maturity” will also vary in relative flowering dates, sometimes by several days. They can also vary in drydown rate (and staygreen, which affects drying rate).
So, taken together, what does this mean? In general, I can’t recommend switching hybrids from your “normal” full season choices until, AT LEAST, May 20th. While that will be later than the optimum planting date—you will sacrifice some yield, switching to earlier (and probably lower yield potential) hybrids isn’t likely to make you money. With some, relatively, earlier-flowering hybrids, that date is probably May 25th or after. One person asked me, “What about drydown problems, like we saw in 2009?” Those problems were largely the result of a cool July and cold August, REALLY RARE OCCURANCES! Even with some drying issues, full season hybrids consistently gave the highest yields and net profits.