Hemp goes 'hot' due to genetics, not environmental stress

Growing hemp for CBD (cannabidiol) is a burgeoning industry, but when hemp contains more than the legal limit of THC, the plants can test "hot." State and federal regulations classify hemp as containing 0.3% or less THC; when plants exceed that amount, farmers can lose their entire crop.

"One of our goals in our research and in fulfilling our extension mission is to reduce the risks to growers as much as possible," said 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."

In the study, lead author Jacob Toth, a graduate student in Smart's lab, created a series of plots in Geneva, New York, 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.

"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 said.

By week four, at harvest time, 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.

The study further proves that genetics, rather than environment, determine the THC content and CBD to THC ratios in hemp, Smart said.

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 said.