Hybridization of transgenic crops with nearby conventional crops raises concerns on several fronts. Movement of pollen from a transgenic field to an organic field involves farmers in discussions about the distance required between fields to ensure purity of a crop, and about who must pay if unwanted genes move into a neighbor's crop. As "Identity Preservation" and segregation of GM from non-GM crops become factors in marketing products, it will be important to ensure that hybridization is not occurring in the field.

Experimental fields of transgenic canola, which has been shown to hybridize with canola from neighboring fields.

Many factors influence the potential for gene flow from crop to crop. Some crops are highly outcrossing. Corn pollen is carried by the wind to neighboring plants. Alfalfa pollen is carried by insects from one plant to another. Other species, such as wheat and barley, are highly self-pollinating instead of outcrossing. Because of the differences among crops species, every case must be evaluated individually for potential to contribute to gene flow from transgenic to conventional crops.

A report by the European Environment Agency assessing the potential for gene flow between transgenic and conventional plants of six major crops--oilseed rape, sugar beet, potatoes, maize, wheat, and barley--is available at http://reports.eea.eu.int/environmental_issue_report_2002_28/en.

A Finnish study (Ritala et al., 2002) of the potential for gene flow from transgenic barley, a mostly self-pollinating crop, concluded that cross-pollination occurred at a low rate up to 50 m away from the transgenic field. Cultivation of transgenic barley in Finland probably would have very low risk of gene flow because of the low out-crossing rate and because the severe winters would kill most seeds that might be accidentally left behind in the fields after harvest, the report concluded.

In crop species that are outcrossers, many environmental factors influence the maximum pollination distance. These include the size of the pollen grains, the humidity in the air, and the wind speed. Light pollen can travel farther than heavy pollen. In windy areas, pollen may travel farther than in areas with still air. Where humidity is high, the pollen grains will not dry out as fast and may retain their ability to pollinate longer than in areas with dry air.

Researchers have tested the distance traveled by pollen in many experiments, with widely varying results. Luna et al. (2001) calculated a theoretical maximum distance for the spread of corn pollen of 32 kilometers under the conditions in their area of Mexico, but observed actual cross-pollinations up to only 200 meters (600 feet), only slightly more than the industry standard isolation distance of 185 meters. Jones and Brooks (1950), working in Oklahoma, showed that corn pollen can fertilize an ear up to 500 meters (about 1500 feet or 1/3 mile) from the source field. In a study done at the University of Maine (http://www.agbioforum.org/Default/vol4no2ar2jemison.htm), one percent cross-pollination was found at a distance of 30 m downwind from the source field. Recommended isolation distances to avoid cross-pollination between different cultivars of sweet corn are given at http://www.orst.edu/Dept/NWREC/corn-pr.html#isolation.

The table below provides some recent measurements for the distance traveled by corn pollen. These measurements were taken as part of a study of the effect of transgenic corn pollen on Monarch butterfly larvae. At other locations and with other crops, distances would almost certainly differ.

distance	pollen grains per square centimeter	pollen grains per square inch	source
in the field	0 to 506	0 to 3,264	Hansen-Jesse and Obrycki, 2000
	0 to 1,600	0 to 10,320	Pleasants et al., 2001
	65 to 425	419 to 2,741	Sears et al., 2001
at the edge of the field	0 to 1,100	0 to 7,095	Pleasants et al., 2001
	158 to 266	1,019 to 1,716	Sears and Stanley-Horn, 2000)
0.2 meters away	0 to 427	0 to 2,754	Hansen-Jesse and Obrycki, 2000
0.5 meters away	260	1,677	Zangerl et al., 2001
1 meter away	0 to 222	0 to 1,432	Hansen-Jesse and Obrycki, 2000
	0 to 1,300	0 to 8,385	Pleasants et al., 2001
	170	1,097	Zangerl et al., 2001
2 meters away	0 to 400	0 to 361	Pleasants et al., 2001
	192	1,238	Zangerl et al., 2001
3 meters away	0 to 56	0 to 361	Hansen-Jesse and Obrycki, 2000
5 meters away	0 to 11	0 to 71	Hansen-Jesse and Obrycki, 2000
	0 to 200	0 to 1,290	Pleasants et al., 2001
	34 to 175	219 to 1,129	Sears and Stanley-Horn, 2000)
10 meters away	0 to 4	0 to 25	Hansen-Jesse and Obrycki, 2000

In Canada, farmers have planted three different kinds of GM canola, each resistant to a different herbicide. Canola plants and seeds have been found that are resistant to all three kinds of herbicide (Hall et al., 2000), indicating that cross-pollination has occurred among the GM varieties. An extended discussion of this phenomenon is available in the report "Gene stacking in herbicide tolerant oilseed rape: lessons from the North American experience" (http://www.english-nature.org.uk/pubs/publication/PDF/Enrr443.pdf), prepared by English Nature to address concerns about whether similar gene flow would occur in Britain if GM canola varieties are eventually approved for commercial production there.

Many agencies publish recommended minimum separation distances for a variety of crops. A table developed by the Seeds of Texas Seed Exchange (http://csf.colorado.edu/perma/stse/table.htm) contains recommendations for small gardeners and also the USDA recommendations designed for large acreages. These distances have been developed to maintain a level of purity that has been acceptable to the agricultural community in the past. They do not guarantee complete protection from gene flow. More research may be needed to determine the separation distances needed for crops under a standard of zero tolerance or very low tolerance for the accidental presence of transgenic material in organic or non-GM products.

When there is a danger of gene flow to nearby fields, it is possible to prevent contamination of nearby crops by planting tall barrier plants to physically block the flow of pollen. It is also possible to plant a border of "trap" plants around the vulnerable field. These trap plants capture the majority of the undesireable pollen as it enters the field. The trap plants along the edges of the field are harvested separately from the main body of the field and are disposed of, so that unwanted genes do not contaminate the product that is sold. Both of these methods were originally developed to keep different conventional varieties from crossing with each other, but the methods can be applied to the problem of keeping GM and non-GM crops separate.

If GM pollen pollinates plants in a neighboring field, then the issue of genetic trespass may arise. Many segments of the agricultural community have an interest in maintaining the purity of their products.

• Organic foods must be free of GM DNA and protein, so a crop of corn or soybeans that has been pollinated by a neighboring GM crop is unfit for sale as organic produce. The consequent financial loss to the farmer may be substantial.
• People who buy organic produce are worried that they may be exposed to some unwanted GM DNA and protein if gene flow from GM crops cannot be prevented.
• Producers of the specialty GM products that are proposed for future release may want to protect their niche GM products from contamination by other kinds of GM products.
• Biotech companies that have patented their GM products may find it difficult to profit from their patents or to pursue claims against alleged patent violators if GM materials become widely distributed in conventional crops.

What level of GM presence, if any, should be allowed in products that are sold as organic or conventional? Can the technology for detecting low levels of GM material keep pace with the decisions on what level of GM material will be allowed? Should GM farmers and companies bear responsibility for preventing gene flow, or should conventional and organic farmers pay to protect their products from gene flow? Should GM versions of outcrossing plants be banned as too risky, while GM versions of self-pollinating plants are permitted?

These issues have already prompted several lawsuits and they will continue to be a factor in the development and use of trangenic plants for years to come.