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Is it Time to Switch Corn Hybrid Maturities?
By: Dwane Miller, Penn State University - 06/07/2019

Who doesn't remember the challenges of getting into the fields during the 2018 planting season? Are we in the same boat for 2019? Looking at USDA's June 2 Weekly Crop Progress Report, planting progress is still behind where we would normally be on this date. Statewide, corn is reported as 74% planted, compared to 73% in 2018, and 83% for the 5-year average. Some parts of Pennsylvania are experiencing a prolonged period of wet weather again in 2019. This has kept many planters in the shed, and some growers are beginning to question whether to switch out hybrids with earlier maturities. However, many of us can recall a year when that our June planted corn seemed to yield the best of all. It's important to remember that planting date is only one factor when it comes to a successful corn crop. Although timely planting is important, it is just as critical to avoid planting in poor soil conditions. Sidewall compaction and poor seed to soil contact can result in less than desirable stands and substantially limit root growth.

According to the Penn State Agronomy Guide, in most areas, switching to a shorter than adapted hybrid maturity should not be considered until at least the last week of May. If your hybrids are on the longer side of maturity for your area, these hybrids probably should have been in the ground by mid-May. If you are analyzing your hybrids, or are considering switching, you must be able to approximate the number of growing degree days (GDD's) left in the season before a killing frost. While many growers and seed dealers discuss hybrid selection in the relative days to maturity (95 day, 108 day, etc.), you may also see GDD information on your seed tag, or in the seed catalog. A corn hybrid with an approximate relative maturity rating of 90-95 days will require 1,600-1825 (GDD's) to reach black layer formation. Compare that with a 111-115 day hybrid which requires about 2,500-2,724 GDD's. Keep in mind that hybrids may reduce their GDD requirements by 100-150 GDD's in late planting situations. Figure 1.4-1 and table 1.4-1 (see below) of the Penn State Agronomy Guide lists the relative maturity ratings and their approximate GDD requirements.

A good reference tool when making a decision is the online reference map for corn hybrid latest planting dates from Penn State. You select a map based on GDD's, and locate your area of the state. From there, based on the color coding of the map, you can obtain the recommended latest planting date. For example, if I had a 2,200 GDD hybrid (approximately 104-day corn), and I live in northern Berks County, the latest recommended planting date is June 10-17. This is using historical climatological data, and assuming a 25% risk of fall frost before maturity. You will need to adjust to earlier planting dates if you're not comfortable risking the crop being hit by frost prior to maturity.

Another key factor in a decision to switch maturities is the ability for that crop to have reached physiological maturity, and the desire to harvest dry grain. Research done by Purdue University's Bob Nielsen revealed that typically a one-day difference in relative maturity rating equaled 0.5 percentage points difference in grain moisture at harvest time. For example, there would be about 2-3 points difference between a 106-day and 100-day hybrid.

Additionally, some good related reading on delayed planting (including a GDD calculator tool for delayed planting) can be found on the Purdue Agronomy Website.

So, when is it time to pull the trigger? I would advise that we are at a point where considering to switch maturities makes agronomic sense. If you've selected short season hybrids that are within reason for your area of the state, and the planters are rolling this week, stick to your guns. If you've got mid to long season hybrids and the seed isn't in the ground yet, it may be time to implement "Plan B".

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