Heat Units and Corn Maturity

Here on May 1, 2019, we are already having thoughts and fielding questions on corn hybrid maturity and what we should do as we are faced with a slow start to planting. There are worries of longer-season hybrids not maturing on time or still being too wet at harvest. The process of hybrid placement is often dominated by finding a hybrid or hybrids that fit the environment and cropping system, and then balancing the trade-off between the increased yield from longer-season hybrids and their higher harvest moisture.

First, the hybrids that were selected to be planted were picked for a reason. The strengths and weaknesses of each situation were matched to hybrids that had the best chance to be successful in those environments. Those were selected based on an “on-time” planting, and in the case of Wisconsin and Minnesota I’m going to say a planting by May 10th-15th qualifies as “on-time”, or maybe more accurately stated as “not late”. Grain yield potential on May 15th is still 90-95% + (Table 1). As we move through May and approach June, that is where yield potential does start to drop off at about 1-2% per day. With that said, stay the course with the hybrids that were selected up through this timing at the very least as they are still the best fit with the best yield potential.

Table 1:
How planting date in MN affects corn yield, as a percent of maximum

Second, corn maturity is slightly flexible as planting is delayed. Corn is a heat-driven crop that simply needs to accumulate heat units to mature. It is day-neutral and does not react to day length changes, unlike another common crop for this region; soybeans. Despite this, corn that is planted later than ~May 1st will not require as many heat units to mature as the same hybrid that was planted prior to May 1st, according to research by Purdue University and Ohio State University. For every day after May 1st, it takes approximately 6.8 Growing Degree Units (GDU) less for a corn plant to reach maturity. As an example; AgriGold A638-84 is rated as a 108 relative maturity (RM) hybrid requiring 2725 GDU to reach black layer. However, if that hybrid is planted on May 20th, the approximate number of heat units required to reach maturity shrinks to 2589 GDU. (2725GDU - (20 days * 6.8 GDU/day)) = 2589GDU, a difference of 136GDU. Approximately 2550-2600 GDU corresponds to a 103-105 RM hybrid rating. So A638-84 planted on May 20th could be expected to mature more like a 104RM hybrid instead of a 108 RM hybrid./p>

Most of Minnesota and Wisconsin have seasons that typically accumulate between 2300 and 2700 heat units (Figure 1). Most days in May only accumulate between 8-15 GDU at most, so the difference between May 1st and May 20th is typically no more than 160-300 GDU. If you’ve “lost” 200 heat units (10 GDU/day for 20 days) by planting on May 20th instead of May 1st, but your corn crop compensated by using 136 GDU less to reach maturity, then the difference is only 64 GDU, which is equivalent to a couple summer days of heat.

Figure 1:
Approximate heat unit accumulations from May 10th to October 10; a typical growing season.

Beyond May 20th-25th the GDU gap quickly eclipses 100 GDU and the possibility of an early frost or too high of harvest moisture becomes a valid risk. At this point, switching to an earlier maturity hybrid, even one that may not fit the environment as well as the original plan, is often the best decision to manage overall performance potential and fall harvestability. Planting A629-22, a 99RM hybrid rated to reach maturity at 2490 GDU, on May 25th, would only require approximately 2320 GDU (2490 GDU – (25 days * (6.8 GDU/day)) = 2320 GDU. Compared to the original plan of planting A638-84 on May 1st and needing 2725 GDU, we now need 2320 GDU starting on May 25th, a difference of 405 GDU. Approximately 250-400 GDU have probably been “lost” between May 1st and May 25th (~10-15 GDU/day) so you are back on a similar schedule for corn maturity and harvest moisture, although yield potential is lower due to capturing less of the season.

Managing hybrid decisions and planting timing in the Upper Midwest is challenging but understanding and sticking with the original plan up until the May 15-25th timeframe often has the greatest overall chance of a successful outcome. There are other early season-factors which can ultimately influence GDU requirements of corn hybrids as well such as tillage, planting depth, and soil texture (Figure 2).

Figure 2:
Factors affecting Corn Emergence and Development, adapted from “Corn Agronomy”, Univ. of WI.

Other Sources:

https://extension.umn.edu/corn-planting/planting-date-considerations-corn http://corn.agronomy.wisc.edu/AA/A063.aspx https://extension.entm.purdue.edu/pestcrop/2002/issue8/index.html#planting

Joey Heneghan – AgriGold Agronomist, 317.691.4347, Follow me on Twitter @agold_joey

Reach out to your local AgriGold Key Account Specialist or AgriGold Agronomist if you have questions.