Okay. How strong?
Dr. Dan Putnam, a forage expert at UC-Davis, has extensively researched this question. His work reminds those willing to dig deeper than the topical media reports that the matter at hand is far more complicated than it seems. Putnam explores rates of contamination based on alfalfa crop distance, types of pollinators, and adjacent systems of production (i.e., seed-to-seed, hay-to-seed, and hay-to-hay). In a 2008 study evaluating the chances of a Roundup Ready alfalfa seed crop contaminating a non-Roundup Ready hay crop (the seed-to-hay scenario), Putnam found that when the crops are a modest 160 feet apart the rate of successful gene flow from GM seed crop to non-GM hay crop was a mere 0.25 percent. (Hay-to hay, rather than seed-to-hay, is the most common situation—but the chances of contamination in that scenario appear to be even lower.)
Even if one-fourth of 1 percent seems too much, Putnam notes that the figure is an overstatement. In his study he purposefully allowed the non-GM hay crop to go to seed—something that must happen in order for pollinators (bees or leafcutters) to cross-pollinate from the GM seed crop. In the real agricultural world, however, a farmer growing alfalfa hay would almost never allow this to happen, thereby radically reducing the chance of contamination. Writing in The Progressive Farmer, agriculture reporter Chris Clayton (who, I must add, is one of the fairest—if lesser known— agricultural writers around) notes, "Hay is often cut multiple times each year before flowering occurs." So the GM seed pollen, should it wander into a neighboring field, would have nothing to grab onto.
There's more. Let's say that the non-GM hay did flower and produce seeds. Two more unlikely events would also have to happen in order for successful contamination to occur. 1. There would have to be simultaneous flowering between seed crop and hay crop in order for cross pollination between GM and non-GM to happen. And 2: If that rare coincidence took place, the seeds in the hay field contaminated with GM pollen would have to fall and germinate on-site rather than being carried afield by a puff of wind. There is, Putnam recently told a meeting of concerned farmers at the recent World Agriculture Expo, "a pretty low level of risk." (PDF)
Purists will argue that a "low level of risk is not enough." But seeking a zero-tolerance policy when it comes to contamination denies the reality of floral life. Pollen moves. To seek an absolute guarantee against contamination of any crop would be like driving a car while insisting that the manufacturer promise you'll never be harmed. Even so, with a contamination possibility that's less than 1 percent, we are not looking at a scenario in which GM alfalfa is going to overtake its organic counterpart.