A recent study from the United States has found that the yields of transgenic Bt corn are not higher than those of non-GMO corn. The research, conducted by Dr. Dominic Resig, a professor and agricultural extension specialist at North Carolina State University, was published in a report provided to mainstream U.S. agricultural media.
Bt Corn Performance Disappointing
Dr. Resig’s report highlights the poor performance of transgenic Bt corn in the United States, noting that its yields do not surpass those of non-GMO corn. The Bt corn was originally developed to kill the corn borer pest, but Dr. Resig points out that the pest has developed resistance to the Bt toxin (Cry toxin) in the corn.
Since 2016, we have recorded resistance to all Cry-toxin-containing hybrids by the corn borer – for eight years now, Dr. Resig says. In 2016, we observed unexpected damage to Bt corn and recommended that growers spray Bt cotton with chemical insecticides.
Dr. Resig adds that corn varieties containing VIP toxin, another transgenic insecticide, are still effective against the pest, although GMWatch reported signs of resistance to these toxins in a 2021 paper. He warns that widespread resistance is only a matter of time, and Bt corn will eventually become ineffective.
Non-GMO Corn Yields on Par with Bt Corn
Contrary to the exaggerated claims of GMO supporters, Dr. Resig also notes that transgenic Bt corn does not yield higher than non-GMO corn. He points to evidence showing that timely planting of non-Bt refuge hybrids can match the yields of Bt hybrids.
We have much evidence to show that timely planted non-Bt refuge hybrids can match the yields of Bt hybrids. We have included them in official variety trials (OVTs), and you can see their performance, Dr. Resig says. We also collaborated with a graduate student on a large-scale farm project. Over two years, we conducted large-scale tests of Bt and non-Bt hybrids at about 60 locations across the state using growers’ equipment. We found, as expected, similar levels of corn borer damage, and Bt and non-Bt hybrids with similar genetic backgrounds had similar yields.
Dr. Resig concludes, Growers should choose the corn hybrid that yields the best for their farm, whether Bt or non-Bt.
Bagged Refuges Accelerate Resistance
For years, the strategy known as refuge-in-a-bag has been considered a way to delay or avoid resistance to Bt toxins. This strategy, also known as seed mixing, involves mixing a certain proportion of non-Bt corn seeds with Bt corn seeds before selling them to farmers. The idea is that non-Bt corn plants will maintain a portion of the susceptible pest population, which will mate with resistant pests, producing offspring sensitive to Bt toxins.
However, Dr. Resig argues that refuge-in-a-bag actually accelerates the development of resistance. We have recently shown that resistance can develop within a generation when corn borers feed on refuge corn, he says.
Dr. Resig also notes that to slow the development of resistance rather than completely stop it, North Carolina growers must plant at least 20% of their cornfields with non-Bt hybrids when planting Bt corn. He adds that non-Bt seeds are cheaper, and growers can save costs by purchasing non-Bt seeds.
Vote Integration Analysis Misleading
GMO advocates, such as the Genetic Literacy Project, which is funded by the GMO industry, often cite vote integration analyses to claim that genetically modified corn has increased yields over twenty years. However, many of the field data used in the vote integration analyses come from the early 2000s, a period when pests had not yet developed resistance to Bt toxins.
The authors of the vote integration analyses focused on comparing genetically modified corn with its closest non-GMO relatives (near isogenic lines). While this approach can tell us about the impact of genetic modification on the yield of a particular variety, it does not show how the yield of genetically modified corn compares to that of non-GMO varieties actually planted by farmers. Non-GMO isogenic lines of genetically modified varieties are not the best-performing varieties; they are just the ones genetic engineers find easiest to genetically modify. Farmers told GMWatch that isogenic varieties planted with Bt varieties were outdated low-yielding varieties and that they would never plant these varieties – making genetically modified varieties appear better.
A 2013 study by Heinemann et al. provided a more realistic picture of the yield of genetically modified corn compared to non-GMO corn, based on yield data reported to the Food and Agriculture Organization of the United Nations (FAO). The study found that the combination of non-GMO seeds and management practices adopted in Western Europe increased corn yields faster than the genetically modified-led approach chosen in the United States, and used fewer pesticides. An updated analysis of FAO data by The New York Times in 2016 confirmed Heinemann et al.’s findings.
No Reason to Plant Genetically Modified Bt Corn
Considering the problem of pest resistance, lack of yield advantage, and higher prices, there seems to be no reason to plant genetically modified Bt corn any longer, and there are good reasons to choose non-GMO corn. Dr. Chris DiFonzo, an entomology professor at Michigan State University, has compiled a chart showing which Bt corn varieties may have populations of corn borers with resistance. Spoiler: there are many. This should be food for thought for any GMO enthusiast.
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