Some seed lots of some corn hybrids being delivered this spring are uncommonly small. The smallest I have seen is a 100,000 kernel bag that weighs 32 lbs. (equivalent to a 25-lb. 80,000 kernel bag). This translates to about 3100 seeds per lb. or almost twice as small as what many farmers would prefer to plant. The accompanying article in the Apr 24 issue of Pest & Crop by Peter Thomison of Ohio State University, titled "Does Seed Corn Size Affect Hybrid Performance?", describes why seed size in and of itself should not be of concern relative to the potential yield of a given seed lot of hybrid corn. But, seed as small as mentioned above deserves some additional comments relative to planter adjustments and operation.
The most obvious consequence of a planting operation using such small seed will be a greater frequency of double or triple seed drops that result in seeding rates greater than what you anticipate for the transmission settings of the planter. At first glance you might worry about the plant stress resulting from such over-populations.
Interestingly enough, there are recent data from the Univ. of Illinois (Nafziger, Emerson. 1996. Effects of Missing and Two-Plant Hills on Corn Grain Yield. J. Prod. Ag. 9:238-240) that suggest that the existence of a small frequency of doubles (about 10% of the stand) not only is NOT detrimental to corn yield, but may actually result in yield increases! The latter may be particularly true if your usual seeding rate is lower than it should be for your conditions. For more information on this research, contact Emerson Nafziger (Email: email@example.com) at the Univ. of Illinois.
Nonetheless, if you feel that your usual seeding rate is already "aggressive", there may well be negative side-effects of overpopulating due to doubles and triples. Hybrids with average or mediocre stalk health or strength may be more disposed to stalk lodging at higher than normal plant populations, especially if weather stress occurs during the season.
A less important, but potentially aggravating, consequence is that the unintentional overseeding of a field(s) due to double and triple seed drops will result in fewer acres planted per unit of seed. Over many acres, your planter hoppers may run empty before the end of the planting season if you do not realize how much seed you are running through the planter.
Several options exist for adjusting a finger-pickup style metering unit for such small seed sizes. First of all, replace the brushes that help remove double seed pickups. Secondly, the tension of the finger assemblies can be increased, but is best performed by planter service technicians. If you have the time, take a sample of the small seed plus your metering units to your dealer and ask them to finetune the units to your seed. Be aware, however, that increasing the finger tension will also cause the whole planter drive system to operate a little harder and increase the odds for slippage of the drive tires. Doublecheck the planter tire pressures and keep the starter fertilizer tanks full to ensure maximum ballast to prevent tire slippage.
Another option to deter double drops with excessively small seed is to slow the planting speed a little to allow the metering unit more time to eliminate doubles and triples. This may be especially valuable if your planting speed is on the excessive side anyway. Aim for maximum planting speeds no greater than five miles per hour.
For seed corn that approaches 3000 seed per lb., you may want to consider purchasing popcorn/sunflower finger assemblies. Purchase of complete finger assemblies allows you to switch back to the normal finger assemblies rather easily when you plant more normal sized seed.
Most planters that utilize this type of pneumatic metering technology can be adjusted to accurately meter extreme seed sizes through prudent choice of both seed disc size (size of holes) and the system's air pressure. Case® planters utilize a pneumatic metering technology, but with a single metering unit (drum) that delivers seed to all the rows of the unit (two drums on larger planters). Adjustment for seed size with this planter is primarily by choice of drum size (no. and size of holes) and secondarily by modification of air pressure to the system. Study your planter manual carefully to determine how to manage unusual seed size. If the manual does not clearly outline these adjustments, contact your planter dealer.
No matter what style of seed metering unit your planter uses, take the time to both 1) calibrate the planter to the seed size prior to planting and 2) check the actual seed placement regularly during the planting operation. Both activities are recommended every year, but may be especially important when faced with the challenge of uniformly seeding unusually small seed corn.
Thanks to Bruce Reynolds (Orville Redenbacher Popcorn) and Larry Cline (John Deere) for their suggestions regarding finger-pickup metering units.