Pennsylvania Corn: Time to Switch Corn Hybrid Maturities?

Bags of seed corn. Photo credit: Dwane Miller, PennState Extension

Looking at USDA’s Weekly Crop Progress as of May 20, planting progress is certainly behind where we would normally be on this date. Statewide, corn is reported as 35% planted, compared to 56% in 2017, and 64% for the 5-year average.

The recent spell of wet weather has kept many planters in the shed, and some growers are beginning to question whether or not to switch out hybrids with earlier maturities. However, many of us can recall a year when that 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.

AgFax Weed Solutions

Keep in mind that hybrids may reduce their GDD requirements by 100-150 GDD’s in late planting situations. Table 1.4-1 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 points difference between a 106-day and 100-day hybrid.

So, are we there yet? Is it time to pull the trigger? If you’ve selected hybrid maturities that are within reason for your area of the state, and we have good planting conditions within the next week, I’d say stick to your guns.

Once we flip the calendar into June, if the seed isn’t in the ground yet, you may need to rethink your original hybrid decisions.

The Latest

Send press releases to

View All Events

Send press releases to

View All Events