Preventing GMO Contamination in Your Open-Pollinated Corn

By Tor Janson, SSE Assistant Curator
and Steve Carlson, SSE Communications Coordinator

A Promiscuous Pollinator

Corn (Zea mays) is what we around here consider a ‘promiscuous pollinator.’ That’s because it is an outcrossing, wind-pollinated crop. Each corn plant has both male and female flowers; the pollen-producing tassels at the top of the plant are the male flowers, while each ear is an inflorescence of female flowers, producing pollen-collecting silks. As the tassels shed pollen, wind carries these tiny grains to silks that have emerged from the immature ears. Each individual silk that gets pollinated results in a kernel of corn on the cob.

Corn tassel and pollen

Corn tassel and pollen

Because corn relies on wind to carry pollen from the tassels to the silks, the light pollen grains may travel a few miles before finding and pollinating a silk. Your neighbor’s corn can therefore very easily pollinate yours, making it tricky to save pure seed from your open-pollinated corn.

During the 2013 growing season at Heritage Farm, the Preservation garden crew grew out 18 varieties of open-pollinated corn from our seed vault to replenish the supply in our collection, increase the viability of the seed, and evaluate varietal characteristics. Preventing cross-pollination between these varieties can be difficult, especially considering that Heritage Farm is located in Northeast Iowa, a region known to farm almost exclusively genetically modified (GMO) feed corn and soybeans. Cross pollination with GMO corn is of particular concern for preserving historic varieties because GMO corn contains genes not historically found in the corn genome.

Preventing Cross-Pollination

There are a few ways to prevent cross-pollination among corn varieties, the most basic of which is to grow only one variety each year. But if you have an appetite for diversity, there are options. One method involves utilizing timing and can be done by simply growing two varieties that vary dramatically in days-to-maturity. For example, a 100-110 day corn variety such as Bloody Butcher can be grown next to a 70-80 day corn such as Blue Jade. This method is risky, though, and also does not account for what your neighbor is growing.

Hand-pollinating corn

Corn ears and tassels being bagged for hand-pollination.

Alternatively, the technique that allows for the most control over seed purity is hand-pollination; most corn grown at Heritage Farm is hand-pollinated. This involves putting bags over the ears before the silks emerge to prevent any pollen from finding them. Pollen is collected by tightly bagging the tassels and then used to hand-pollinate the silks on each ear. More details about this process can be found in our webinars and in the book Seed to Seed.

One final, obvious method for preventing cross-pollination is by simply utilizing distance and natural barriers. Historically, isolation distance has been the primary method used to maintain varietal purity in corn. In his book, Corn Among the Indians of the Upper Missouri, published in 1917, George Will describes 14 distinct corn varieties maintained by the Mandan Nation of North Dakota. He writes that the Mandans maintained purity by growing each variety “a couple hundred yards” apart, and by careful ear selection for their seed corn. Two hundred yards is 600 feet—more recent agricultural research confirms that at this distance, cross-pollination is generally 1% or less. Yet studies also show that at distances up to 1640 feet, it was not possible to limit cross-pollination to below 0.1% (

The Experiment

The 890 acres of Heritage Farm contain many natural barriers for pollen dispersal, such as forested areas and rugged limestone bluffs. This year we wanted to grow a larger-than-usual planting of ‘Hjerleid Blue’ corn (pronounced “zhair lide” blue), an heirloom blue sweet corn grown by the Hjerleid family of Wisconsin since at least the 1940s.

Map showing the physical distance ‘Hjerleid Blue’ was isolated

As hand-pollination of such a large planting would be especially labor-intensive, we grew this variety in a garden that we thought would be isolated enough to prevent cross-pollination from neighboring fields of conventional corn, which is likely GMO. This garden, east of our historic orchard, was at least 2300 feet (half a mile) from any corn field and was buffered by woods and elevation changes.

A blue sweet corn such as ‘Hjerleid Blue’ is genetically recessive to GMO field corn in two visible kernel traits—a wrinkled sweet kernel and a white endosperm. GMO field corn exhibits the dominant traits of a smooth kernel and a yellow endosperm. Therefore, a kernel of ‘Hjerleid Blue’ pollinated by GMO pollen would be non-wrinkled and have a yellow endosperm —it would easily stand out amongst its peers.

We thought that growing ‘Hjerleid Blue’ in our isolated, buffered garden would be sufficient to prevent any cross-pollination with neighboring GMO corn. We were wrong. From a population of over 200 plants, we found a few scattered off-type kernels on six different ears. This genetic contamination represents less than 0.1% of the population in this generation, but if those off-type kernels were planted in the next generation, those plants, with 50% GMO genetics, would introduce a far greater level of GMO contamination to the population.

'Hjerleid Blue' corn

A scan of ‘Hjerleid Blue’ corn from the 2013 grow-out, indicating the cross-pollinated kernels.


For the purposes of non-GMO food labeling, the level of contamination we experienced is acceptable. But for the purposes of saving seed, any GMO contamination is unacceptable because the contamination will increase exponentially in each successive generation. Had we grown an historic yellow dent corn in that orchard garden, we never would have known that we had introduced GMO genes into the population because it would not have been visually apparent. For the ‘Hjerleid Blue’, we were able to carefully inspect every ear and remove the few GMO-pollinated kernels.

The main lesson here is that if you are saving corn seed in the corn belt, it is extremely difficult to prevent GMO cross-pollination without doing hand-pollination. The Midwestern summer air is awash with GMO pollen. Corn-belt seed savers who want to ensure they eliminate all GMO contamination may want to learn to hand-pollinate their corn, or grow varieties where GMO contamination is visually apparent, such as white or blue corn.

Seed Savers Exchange is offering the ‘Hjerleid Blue’ sweet corn in the 2014 Seed Exchange. Become a Seed Savers Exchange member to get access to this variety as well as thousands of other rare heirloom and open-pollinated seeds.


Seed Savers Exchange is a non-profit organization located in Decorah, Iowa, with a mission to conserve and promote America’s culturally diverse but endangered garden and food crop heritage for future generations by collecting, growing, and sharing heirloom seeds and plants.

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  1. A safe distance to avoid cross-contamination would be OVER 2 miles from GMO fields.

  2. THANK YOU for this very informative article!
    Last year, we were privileged to listen in on a Webinar about hand-pollinating.
    This is so good to know.

  3. I think that you might want to double check your science here. If one of the corn kernels is infected I believe the entire batch might be. You picked off the ones that appeared to be yellow, meaning they actually physically displayed the traits. This is the phenotype of the corn, the others could still be infected and have the coding as the genotype is not always outwardly displayed. This is sort of like how you can have a batch of puppies that look like their purebred mother except for that one who bares a slight resemblance to the father. The ones that still look like the mother cannot be sold and bred as purebreds anymore.

    • Tor, Seed Savers Exchange says:

      That’s a good question. The short answer is that corn is different than most crops and exhibits visible paternal traits in the kernel in the first generation, which allows us in this case to “see” whose pollen fertilized each kernel.

      Each kernel is pollinated by a different pollen grain. Each kernel on an ear has the same maternal genes, in this case ‘Hjerleid Blue.’ It’s possible for the kernels on the ear to have multiple paternal influences. This is different than in most vegetable crops. For example, the seeds in a tomato fruit typically share the same paternal and maternal parentage.

      Corn is also somewhat unique in that visible traits found in the aleurone and endosperm layers of the kernel are influenced by the paternal genes. In most seed, what we visibly see is the pericarp, or outer covering of the seed, which is entirely maternal. In corn kernels the pericarp layer is also maternal, but it is usually clear, revealing the aleurone and endosperm layers beneath. In most seed, one cannot see the paternal influence until one grows the seed the next year. Corn is unusual in that some of the visible kernel traits are influenced by the genes carried by the pollen.

      An example that home gardeners may be familiar with: if one grows sweet corn too close to field corn, some of the kernels on the sweet corn ears will be starchy, not sweet. This is due to the paternal pollen coming from the field corn, and the fact that the sweet gene is genetically recessive to the starchy gene.

    • Jaran,

      I think you’re concerned about recessive genes — I can have brown eyes but still invisibly carry blue-eyed genes from one of my parents.

      Tor and Steve were careful to specify that the Hjerleid Blue traits they looked at were recessive, so that pollination by standard commercial varieties would always be visible.

      If my husband and I both have blue eyes, that means that we both have only the recessive blue-eyed genes. Our child will have blue eyes because we have no brown-eyed genes to pass on. If I give birth to a brown-eyed child, since I have blue eyes the brown-eyed genes must have come from the father. My husband would sit me down for a long conversation about who that father might be and the future of our marriage.

      It’s possible for me to be unfaithful with a brown-eyed man and still have a blue-eyed child however and I think this is the scenario you were thinking of. A brown-eyed man can carry both blue-eyed and brown-eyed genes. He has brown eyes because the brown-eyed gene is dominant, but he can pass the blue-eyed gene on to his child. If I hung out with only non-Hispanic whites, then if my husband were suspicious he could not rely on my child’s eye colour as proof of my fidelity. If the child has brown eyes then I was definitely unfaithful but blue eyes don’t prove anything either way.

      However, if we were living in Asia or Africa or somewhere else with a *homogeneous* brown-eyed population, then the men I might be unfaithful with have only brown-eyed genes to contribute. In that case, if the child has blue eyes then my husband can be sure it is biologically his.

      The situation of the blue corn is like that. The pollen from other fields is all homogeneous for the dominant genes of yellow endosperm and smooth skin. If the kernel doesn’t show those traits then it was not fertilized by non-Hjerleid-Blue pollen.

      Does this help?

  4. I’m not quite sure how you are calculating the percentage of cross-pollinated corn, but my calculations suggest the level of contamination you have seen is more like 0.009%, or more than an order of magnitude less than 0.1%.

    First, you had six ears that showed cross-pollination. For the sake of the argument, I’m assuming you had a yield of 200 ears from the 200 plants. This means you had a 3% cross pollination rate for the ears. Again, assuming each of the 200 ears had 350 kernels for a total of 70,000 kernels, and assuming you had a total of six cross-pollinated kernels on those ears, then 6/70,000 yields a cross-pollination rate for the kernels of 0.00857%. I suspect this low rate fits in quite nicely with the distance and geographic barriers you describe, and there is a good chance the result falls right in the middle of of the probability curve. Moreover, I’d guess that if you doubled the distance between the crops, you’d see another order of magnitude of difference, and you would probably have to plant 100 times the number of seeds to illustrate this point.

    That said, as long as we ignore any viability issues involved with pollen losing viability due to prolonged exposure to sunlight or other factors, logic suggests that there is no absolute distance at which a non GMO field could be perfectly isolated from GMO pollen. Corn upwind of your field by ten miles or a hundred miles could in theory shed pollen that could potentially cross-pollinate your field.

    If you ran the same experiment you did with the Hjerleid Blue corn a hundred more summers, it stands to reason that for one or more of the hundred summers, you would see zero cross-pollination, and on one or more summers you would see dramatically more pollination, as the weather conditions and timing of the crops during those summers will vary considerably.

  5. April Reeves says:

    Regarding the Hjerluid corn contamination: I have a science background and am a staunch anti-GMO activist, but I find a statement that I’m having trouble with so I’m hoping you can add the pertinent info.

    On the photo of cross contamination, did you get tests done to prove it? Only way you will know for sure. I too have black corn that outcrops yellow now and then but it’s not contamination. When corn is crossed with GM, that will show up in the entire plant, not just in a kernel. And it may or may not change the color.

    I have done this work for years and while I love that you put this out there, we must be real and truthful, and do the research to know for sure that corn is contaminated. I don’t want us to be seen as unreliable information. Thanks!

    • Tor, Seed Savers Exchange says:

      Hi April,
      We know for certain that the off-type kernels were not pollinated by ‘Hjerleid Blue” due to the yellow endosperm color and starchy, non-sweet, non-wrinkled kernels. We do not know for certain that those kernels were pollinated by GMO corn, but we know that it is highly probable due to the fact that most of the other corn in proximity was GMO. It would be statistically unlikely that they were not pollinated by GMO pollen. The map shows the location and type of corn that was grown in the area surrounding the Hjerleid Blue plot. Not only was the GMO corn the closest, it would be the most likely to have been carried by prevailing winds, which in Iowa during summer come primarily from the west and southwest.

  6. You could try spraying weed killer on your blue corn and see how much dies (or is it resistant like the GMO?).

  7. Thank you for the transparency in this post. It would be nice if all seed providers were so open. I would also be nice if we could know all of the GMO products in our food. But, that is a different issue, but this kind of transparency by Seed Savers is a nice breath of fresh air. Keep up the great work you all are doing.

  8. I have a question about corn and GMO. If the blue corn is recessive and GMO kernel shows will that work with Stowell’s Evergreen Corn or Bloody Butcher Corn?

    • Tor, Seed Savers Exchange says:

      Hi Tina,

      Stowell’s Evergreen will work in a similar manner to Hjerleid Blue–Stowell’s is a white sweet corn, so if you see any yellow, non-sweet kernels on the ear, you know that those kernels are cross-pollinated.

      It would be difficult to tell on a Bloody Butcher ear. The red color of Bloody Butcher is on the pericarp, or outer covering, of the kernel. This red pigment in the pericarp is maternally controlled–making every kernel on the the entire ear red, regardless of the pollen source. Furthermore, Bloody Butcher is a Dent type corn, and GMO field corn is also a Dent type corn. In the case of Bloody Butcher, it is probably best hand pollinated or grown in an area where GMO corn is not grown.

  9. Please excuse my ignorance about this GMO corn question. Put simply are you saying I could tell if there has been cross contamination if yellow kernels show up in white corn? Is there an easy way for the average joe to keep cross contamination from happening?
    Thank you