No. 3 Article 8/April 8, 2005

Does Corn Seed Size Really Matter?

On the cover of the March 2005 issue of Farm Journal was the rather startling headline "Seed Size Matters," against a background photo of corn seed. Corn seed size and grade (the shape-size combination related to plantability, originally with different planter plates) have been the topic of research for decades, and the overwhelming conclusion of those (including me) who have done such research is that seed size and shape do not have consistent effects on seed quality. Good-quality seed will produce similar stands and yields, regardless of its size and shape.

What could have changed to make seed size "matter" now? A look at the Farm Journal article showed that their "proof" consisted of finding a seed lot (hybrid and company not identified) where one grade (small rounds) had subpar cold and accelerated aging test scores. That grade apparently (and predictably) produced lower plant stands than the other grades, though how much lower was never stated. This reduced stand went on to produce lower yields in "replicated" tests, of which we are given a summary of a "representative" trial that gives only seed "stress" test numbers and yields, not stands. This hardly forms the basis for the sweeping generalization that "seed size matters."

The article appears to have been aimed to some extent at promoting a seed testing service that offers a test that is slightly different from the usual cold test. The cold germination test, which typically uses soil to provide microbes that can attack seeds, is not standardized among laboratories, mostly because what soil is used is not standardized. Standardizing soil among labs might be possible but is not desirable, given that soils should more or less represent soils the seed will be planted in and this could not be the same for all locations. Virtually all corn seed is tested using a cold test, but such a test is not required by law and results for the same seed will not be the same from different labs. Cold scores might be available on request or even on tags, but there's no standard of what cold scores should be and the cold test is used primarily by companies in deciding whether or not to sell seed.

The accelerated aging test consists of maintaining seed in a "tropical" environment for a few days before the standard warm test. This measures the physiological soundness of seeds and will often give lower numbers than the warm germ test, especially for seeds that have been stored for a long time under less-than-ideal conditions and seeds that have deteriorated due to production or handling problems. It is thus a "stress test" like the cold test but does not try to duplicate what conditions the seed might be under in the field.

The standard warm germination percentage is the number required on the seed tag and is standardized, such that its value for the same seed should be similar among laboratories. Under good planting and emergence conditions, emergence is often close to the warm germ, while under cold, wet conditions, it may be closer to the cold or accelerated aging germination score. Some lots can also have "abnormal" seedlings, from seed usually measured as germinable under the warm test but probably more prone to emergence problems, especially when soil conditions are less than ideal.

Are small rounds generally defective as seed compared to other grades? Careful research has not shown this to be the case, and so the general answer is no. While seed lots and grades may have quality problems that can affect seed performance, such problems usually reflect genetics, production, or handling conditions and are not closely tied to seed grade. Rounds come from the ends of the ear, however, and are shaped such that their germ (embryo) can be exposed to mechanical stress more than flats. Seed with physical damage due to insects or birds can be separated out and discarded during processing, and seed with mechanical damage can be identified using germination tests.

Different types of germination tests are run on different lots and grades after processing, so quality problems seldom escape detection. If the warm germination test is low, seed is discarded, since field germination percentage cannot normally be higher than the warm test score. Grades or lots identified as having problems by one of the stress tests may also be discarded, though if the warm germination is high, such seed might be held back for later planting or sent to areas where soils are warmer at planting time and seed is less likely to undergo stress after planting.

Producers can certainly send seed samples for further testing by independent labs, but such third-party tests rarely identify problems. Mistakes occasionally happen, but if a company fails to do the best job possible to sell only top-quality seed in today's competitive environment, it doesn't stay in business long. That's probably the best protection we have.--Emerson Nafziger

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