Corn is a cross-pollinating crop in which most pollination results from pollen dispersed by wind and gravity. Pollen drift in corn has received considerable attention in recent years as the result of the development and widespread adoption of new seed technologies containing transgenes or genetically modiﬁed organisms (GMOs). Managing pollen drift has always been a major concern in the production of hybrid seed (to ensure genetic purity of inbreds) and specialty corn (to optimize expression of value-added traits, like high oil content). Pollen drift has now become an important consideration in the production of non-GMO corn as an Identity-Preserved (IP) grain crop. Producers of IP non-GMO grain are concerned that pollen drift from GMO hybrids will contaminate, by cross-pollination, nearby non-GMO corn. Farmers growing GMO hybrids approved for export also want to avoid contamination of their crops by GMO corns that have not yet received approval in overseas markets (Nielsen, 2003a).
A signiﬁcant percentage of U.S. IP corn is earmarked for overseas markets with rigorous GMO restrictions. Japan has set a zero tolerance for seed and food imports containing unapproved GMO material. Food products containing less than 5% of approved biotech crops like corn and soybeans can be labeled as non-GMO. The European Union (EU) guidelines require that foods, including grains, containing more than 0.9% biotech material be labeled as genetically engineered. Producers of non-GMO corn need to minimize pollen contamination by GMO corn if they are to obtain premiums associated with IP grain contracts. As GMO corn acreage in Ohio increases with the wider use of insect and herbicide resistant and drought tolerant corns, the potential for contamination of non-GMO corn is increasing. If growers want to produce non-GMO IP corn successfully, they need to become familiar with some physical and biological characteristics of corn pollen, potential distances that pollen can travel, and planting practices that reduce the risk of pollen contamination by nearby GMO corn ﬁelds.
Characteristics of corn pollen affecting “drift”
Corn pollen is spherical and much larger than the pollen produced by most grasses (Burris, 2002; Gray, 2003). Corn pollen is among the largest particles found in the air. Although it is readily dispersed by wind and gravity, it drifts to the earth quickly (about 1 foot/second) and normally travels relatively short distances compared to the pollen produced by other members of the grass family. Pollen may remain viable from a few hours to several days. Pollen can survive up to nine days when stored in refrigerated conditions. However, under ambient ﬁeld conditions, pollen is viable for only 1 to 2 hours. High temperatures and low humidity reduce viability. Elevated temperatures have a greater negative impact on pollen viability than humidity, with viability greatly reduced at temperatures above 100 degrees F. At ﬂowering, 60% of pollen fresh weight consists of water; pollen longevity diminishes rapidly if the water content drops below 40%. Corn plants typically shed pollen for 5 to 6 days, whereas a whole ﬁeld may take 10 to 14 days to complete pollen shed, due to the natural variation in growth and development among plants (Nielsen, 2003b). Peak pollen shed generally occurs 2 to 3 days after 50% of the plants have shed pollen. Individual corn plants produce one half million or more pollen grains, although variation exists among hybrids and plant densities (Abendroth et al., 2011). Therefore, even if only a small percentage of the total pollen shed by a ﬁeld of corn drifts into a neighboring ﬁeld, there is considerable potential for contamination through cross pollination.
How far can corn pollen travel?
Many studies have been conducted to determine how far pollen will travel—some have evaluated the density of pollen at varying distances from a corn source, whereas others have measured pollen drift by measuring outcrossing in neighboring corn. This latter approach is probably more meaningful when it comes to assessing the impact of pollen drift from GMO corn ﬁelds.
Once released from the anthers into the atmosphere, pollen grains can travel as far as ½ mile with a 15 mph wind in a couple of minutes (Nielsen, 2003b). However, most of a corn ﬁeld’s pollen is deposited within a short distance of the ﬁeld. Past studies have shown that at a distance of 200 feet from a source of pollen, the concentration of pollen averaged only 1% compared with the pollen samples collected about 3 feet from the pollen source (Burris, 2002). The number of outcrosses is reduced in half at a distance of 12 feet from a pollen source, and at a distance of 40 to 50 feet, the number of outcrosses is reduced by 99%. Other research has indicated that cross-pollination between corn ﬁelds could be limited to 1% or less on a whole ﬁeld basis by a separation distance of 660 ft., and limited to 0.5% or less on a whole ﬁeld basis by a separation distance of 984 ft. However, cross-pollination could not be limited to 0.1% consistently even with isolation distances of 1640 ft.
Several studies have been performed evaluating the impact of pollen drift from GMO ﬁelds on neighboring non-GMO ﬁelds. A Colorado study (Byrne et al. 2003) tracked the drift of pollen from blue corn and GMO Roundup Ready corn into adjacent conventional corn. Corn with marker traits (blue kernels or Roundup herbicide tolerance) was planted adjacent to corn without those traits. Cross pollination was greatest at the closest sampling site—up to 46% outcrossing about 3 ft. from the edge of the test plots containing blue corn. Cross pollination dropped off rapidly with only 0.23% cross pollinated kernels near the blue corn plot at 150 ft. Only 0.75% of the corn showed cross-pollination with the Roundup Ready plot at 150 ft. The farthest distance any cross pollination was detected was 600 ft. These results suggest that 150 ft. may be a reasonable buffer between GMO and non-GMO corn to prevent signiﬁcant cross pollination due to pollen drifting from one ﬁeld to another.
Planting practices to minimize GMO pollen contamination
Isolation and Border Rows
One of the most effective methods for preventing pollen contamination is use of a separation or isolation distance to limit exposure of non-GMO corn ﬁelds from pollen of GMO ﬁelds. The potential for cross-pollination decreases as the distance between GMO and non-GMO corn ﬁelds increases. Several state seed certiﬁcation agencies that offer IP grain programs for corn programs require that non-GMO IP corn be planted at a distance of at least 660 ft. from any GMO corn. This isolation distance requirement may be modiﬁed by removing varying numbers of non-GMO border rows, the number of which is to be determined by the acreage of the non-GMO IP corn ﬁeld. The border rows ensure that the non-GMO ﬁeld is “ﬂooded” with non-GMO pollen which will dilute adventitious pollen from a GMO source.
- For corn ﬁelds over 20 acres in size, the isolation distance (of 660 ft.) may be modiﬁed by post pollination removal of 16 border rows if the actual isolation distance is less than 165 feet.
- For corn ﬁelds over 20 acres in size, the isolation distance may be modiﬁed by post pollination removal of 8 border rows if the isolation distance is between 165 and 660 feet.
These isolation and border row requirements are designed to produce corn grain that is not more than 0.5% contaminated with GMOs.
Planting Dates and Hybrid Maturity
Use of different planting dates and hybrid maturities can also be used to reduce the risk of cross-pollination between ﬁelds of GMO and non-GMO corn. For example, planting short season non-GMO corn hybrids followed by full season GMO hybrids later will reduce the chance for pollen from the GMO ﬁeld to fertilize the early planted, earlier maturity non-GMO hybrid in an adjacent ﬁeld. However, there are shortcomings with this approach. Differences in maturity between the early and late hybrid may not be large enough to ensure that the ﬂowering periods of each hybrid will not overlap, especially when certain climatic conditions may accelerate or delay ﬂowering. Moreover this strategy will only work if you control the adjacent ﬁelds or can closely coordinate your corn planting operations with those of your neighbors.
Prevailing Wind Direction
In Ohio, the importance and consistency of relative wind direction during pollen shed has not been established. However, in states to the west of Ohio, the south and west edges of non-GMO ﬁelds are often more vulnerable to pollen drift because the prevailing winds during the summer are from the southwest. Therefore, it may be beneﬁcial to follow recommendations regarding isolation distances and border row on these sides of non-GMO ﬁelds.
Genetic Pollen Blockers
Several seed companies producing non-GMO corn seed for organic growers have been marketing hybrids that contain the PuraMaize™ gene system, also known as the Ga1-s isolating mechanism. This is a naturally occurring gene in corn that impedes pollen originating from a plant that does not have the Ga1-s gene from being able to pollinate a plant that does have the Ga1-s gene. As a pollen recognition system, corn plants that contain the PuraMaize gene system will quickly accept pollen from other PuraMaize plants and essentially block pollen from foreign plants such as GMO or blue corn, allowing the pollen from PuraMaize plants to fertilize the developing kernels.
Other factors that can negatively impact non-GMO grain purity are volunteer corn plants resulting from no-till or minimum till continuous corn, purity level of the seed planted, planting errors, and drought or ﬂood conditions which stunt border rows and reduce desirable pollen production and ﬂow.
Planting operations to control pollen drift are only part of the process of producing an IP corn grain crop. Other major issues include harvesting, drying and storage, along with thorough record keeping. Seed certiﬁcation agencies offer IP programs for grain. These IP programs, which are similar to seed certiﬁcation, assist in preserving the genetic identity of a product, and verify speciﬁc traits through ﬁeld inspections, laboratory analysis, and record keeping (Brittan, 2006; Riddle, 2012).
- Abendroth, L.J., R.W. Elmore, M.J. Boyer, and S.K. Marlay. Corn Growth and Development. PMR 1009. Iowa State University Extension, Ames, Iowa.
- Brittan, K. 2006. Methods to Enable the Coexistence of Diverse Corn Production Systems. University of California Cooperative Extension Agricultural Biotechnology in California Series Publication 8192. Online at anrcatalog.ucanr.edu/pdf/8192.pdf [URL verified 3/14/16].
- Burris, J. 2002. Adventitious Pollen Intrusion into Hybrid Maize Seed Production Fields. American Seed Trade Assoc.
- Byrne, P.F., K.A. Terpstra, T.A. Dabbert, and R. Alexander. 2003. “Estimating Pollen-Mediated Flow in Corn Under Colorado Conditions.” In Annual Meetings Abstracts [CD-ROM]. ASA, CSSA, SSSA, Madison, WI.
- Gray, Mike. 2003. Pollen Drift and Refuge-Management Considerations for Transgenic Hybrids. Illinois Pest & Crop Bulletin, Univ. of Illinois. Online at bulletin.ipm.illinois.edu/pastpest/articles/200304e.html [URL verified 3/14/16]
- Nielsen, Bob. 2003a. Corn Segregation: A Necessary Evil in Today’s Biotech Age. Purdue Univ. Online at www.agry.purdue.edu/ext/corn/news/articles.03/GMO_Segregation-0423.html [URL verified 3/14/16].
- Nielsen, Bob. 2003b. Tassel Emergence & Pollen Shed. Purdue Univ. Online at kingcorn.org/news/timeless/Tassels.html [URL verified 3/14/16]
- Riddle, J. 2012. GMO Contamination Prevention—What Does it Take? Univ. of Minnesota SW Research and Outreach Center. Online at extension.umn.edu/garden/master-gardener/volunteers/teaching-tools/docs/minimizing_gmo_contamination.pdf [URL verified 3/14/16].