Farming Magazine - May, 2013


Forages: May is Corn Planting Month

By Everett D. Thomas
Corn planting can start as soon as field
and soil conditions permit, but when
you finish planting is usually more
important than when you start.

Corn planting can start as soon as field and soil conditions permit, but when you finish planting is usually more important than when you start.
Photo courtesy of marykbaird/

May is corn planting month in much of the northeastern U.S., though in some warmer areas planting can begin in April. Planting can start as soon as field and soil conditions permit, but when you finish planting is usually more important than when you start.

Soil temperatures should be at least 50 degrees Fahrenheit, with fields dry enough to work without making clods. However, corn planted the day that soil temperatures first reach 50 degrees may not yield any more than corn planted a week or two later. In fact, date-of-planting research has shown that the corn planted earliest is often slightly lower-yielding than corn planted a week later. Corn planted early takes longer to germinate and emerge, exposing it to attack by soil insects and diseases. Newer, more effective seed treatments reduce but don't eliminate this threat.

If you can plant all your corn in a week, there's no need to hitch up your planter the first day that soil temperatures reach 50 degrees. However, few farmers can plant all their corn in a week, in which case it's necessary to get an early start. While the earliest date in date-of-planting trials may not result in the highest yields, the latest planting date - typically several weeks to a month later than the first - almost always results in lower yields. And where the crop is harvested for silage, late-planted corn often has lower forage quality because it doesn't contain as much grain.

What's the latest you can plant corn and still have it mature to the desired stage? It depends. It depends on where you farm, your production practices, the relative maturity of the corn hybrids you plant, and whether you'll harvest the crop for silage or for grain. In much of the northeastern U.S., corn planted in June almost always yields less than corn planted in May. Late-planted corn often grows taller, since stalk elongation is occurring at a different time, but just because the corn is taller doesn't mean it's going to have higher yields. About 40 percent of the dry weight of a mature corn plant is in the ear.

Spreading out corn hybrid maturity

Adverse growing conditions, such as extreme heat and dry soils, can interfere with pollination, resulting in poorly filled ears. Spreading the dates your corn will pollinate, either by planting hybrids of varying relative maturities or by different planting dates, can reduce this risk. (Some Corn Belt farmers used to mix 1 or 2 percent of a hybrid of a slightly different relative maturity in the seed hopper to increase the chances of pollination even under adverse weather conditions.)

If cool, moist conditions follow your earliest planting dates, by midsummer there may be little difference in plant development between these and fields planted a week or so later. That's because corn planted into warmer soils germinates faster, and there's also the tendency of late-planted corn to do a bit of catching up.

Select corn hybrids with the relative maturity ratings necessary to ensure that your corn will reliably mature given your planting dates. Reliably means almost every year, not just in a year with an abnormally long, hot growing season. If your planting dates are later than your neighbors', you may need to use hybrids with somewhat earlier relative maturity than they do. For instance, if your neighbors start planting on May 10 and you don't start until May 25, they may be able to use 100-day relative maturity (RM) corn hybrids while you'd use hybrids that are 90 to 95 days RM.

It's OK to plant a modest acreage of one or two hybrids that mature somewhat later than your main crop. Opinions differ among agronomists as to what proportion of your crop this corn that pushes the maturity envelope should represent. It depends on your willingness to accept risk. I'm somewhat conservative on this topic and don't think that more than 10 percent of your corn crop should be planted to hybrids that are later than what will reliably mature. Some agronomists recommend planting up to 20 percent of your corn acreage to late-maturing hybrids - again, it depends on your willingness to accept risk.

Planting some of your corn crop to long-season hybrids may result in a range of maturities this fall. This can be an advantage if you harvest corn for silage, since it may avoid having the entire crop reach the desired maturity (generally 32 to 35 percent dry matter) at the same time. However, if you plant the long-season hybrids first and good weather follows planting, everything might mature at the same time anyway! That's why if your goal is a range of maturities when you start chopping, as long as you can plant your entire corn acreage in two weeks or less, don't plant your long-season hybrids first.

Another consideration is how long it takes to harvest. If your corn is custom-harvested, it may take only a few days for the custom crew to chop and ensile your entire crop. If this is the case, there's no reason to spread out maturity dates.

Hybrid selection

By now you should have all the seed corn you ordered. However, last-minute changes and underestimating seed needs may result in you needing a few more bags of seed. Seed dealer inventories will be low, and you may not have much choice for either hybrid or seed size. Genetics are a lot more important than seed size, so choose the hybrid with the best combination of relative maturity and yield potential, accepting whatever seed size is available. If this is the last corn you'll be planting, consider buying a hybrid with earlier maturity than you'd normally use. There have been big improvements in early hybrids, and you might be surprised at how well they yield, even when planted at the tail end of the planting season.

I wouldn't be all that picky on traits, particularly since you'll probably only need a few bags of seed. However, don't ignore insect resistance traits if you need them, and if you'll use glyphosate for weed control, the herbicide tolerance trait is one you can't afford to ignore.

Ev Thomas has worked as an agronomist in New York for 45 years, first with Cornell University Cooperative Extension, then with the William H. Miner Agricultural Research Institute in Chazy, N.Y., including managing its 680-acre crop operation. He continues to work part-time for Miner Institute and is now an agronomist at Oak Point Agronomics. He has written our Forages column for 15 years and has been an expert contributor on a number of other topics.