Growing hemp for cannabidiol is a burgeoning industry, thanks to the compound’s use in treating everything from pain, anxiety and depression, to easing cancer-related symptoms.

But when hemp contains more than the legal limit of THC (0.3%), the psychoactive ingredient in cannabis that gets people high, the plants can test “hot.” When plants exceed that amount, farmers can lose their entire crop.

Many websites and news articles have claimed that environmental or biological stresses, such as flooding or disease, cause an increase in THC production. But very little research exists to show that’s true.

Now, a new Cornell study, published July 28 in the journal Global Change Biology-Bioenergy, finds no evidence that stress on hemp plants increases THC concentrations or ratios of CBD to THC.

“One of our goals in our research and in fulfilling our Extension mission is to reduce the risks to growers as much as possible,” says Larry Smart, senior author of the study and professor in the horticulture section of the School of Integrative Plant Science in the College of Agriculture and Life Sciences. “With this research, growers should feel some comfort that stresses do not seem to have a strong effect on changing the ratio of CBD to THC.”

He says that the study further proves that genetics, rather than environment, determine THC content and CBD-to-THC ratios in hemp.

In-field studies

Lead author Jacob Toth, a graduate student in Smart’s lab, created a series of plots in Geneva, N.Y., that included control plots and five stress treatments applied to three genetically unrelated high-CBD hemp cultivars.

Stress treatments included flood conditions; exposure to a plant growth regulator called ethephon, used to promote fruit ripening; powdery mildew; herbicide; and physical wounding.

They then tested THC and CBD content over a four-week period when the flowers matured. One issue for farmers is that CBD and THC levels are linked, and both rise in the flowers at harvesttime, creating a precarious calculation to reap the highest possible CBD levels and value of the crop, without losing everything by surpassing THC thresholds.

“What we found over the weeks that we were sampling, the amounts of CBD and THC went up proportionately in all of these different cultivars for all of these different stresses,” Toth says.

By week four, at harvesttime, they found that nearly every plant — except those treated with herbicide, which were nearly dead — produced the expected ratio of CBD to THC, with high levels of CBD corresponding to levels of THC above the 0.3% THC threshold.

In previous work, Toth developed a molecular marker to identify genes that produce CBD in hemp and THC in cannabis. He was able to show that some hemp varieties included plants with different genetic arrangements — some with genes for mainly CBD production, others for mainly THC and some with a combination of both. In this study, all the plants were predominately CBD producers.

More research and breeding is needed to select appropriate genetics that lead to high CBD but low THC, and regulatory testing may be needed earlier, before harvest and before plants reach high THC levels, Toth says. Also, growers should make sure they get high-quality CBD-producing seeds, and not varieties tainted with THC-producing genes, Smart says.

The research from Smart’s lab has assisted USDA in developing hemp regulations that focus more on genetics rather than environmental stresses that lead to noncompliant THC levels.

The lab’s research indicates that USDA’s decision to raise the THC limit for what is considered a “negligent crop” from 0.5% to 1% THC in January will dramatically lower the legal risks for growers. Too many negligent violations can ban a grower from producing hemp for five years.

The study was funded by the New York State Department of Agriculture & Markets through a grant from Empire State Development Corp.

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