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Genetic Test Predicts THC:CBD Ratio in Cannabis Plants

plants growing

Growing a crop that is too “hot” is something that keeps hemp farmers awake at night. But a new test should take the guesswork out of whether a plant will contain too much THC.

The CBD industry recently welcomed news that a team of researchers at the University of Minnesota has developed a genetics test that can predict the levels of CBD or THC in a plant.

This will come as particularly good news to people who grow hemp for a living and have had to bear the brunt of the risk of growing the “wrong kind” of cannabis.

Of course, all farmers have to contend with a high degree of uncertainty as a part of their jobs – but hemp farmers have an added layer of stress when it comes to their crop. Not only do they have to deal with all the regular threats of bad weather and pests, but they have the threat of a “hot” hemp crop looming.  

A “hot” crop simply means that the levels of THC have risen above the legally allowed limit of 0.3% (the limit set by the 2018 Farm Bill). THC levels peak at the plant’s maturity, and even with careful monitoring and testing, farmers can be caught off guard.

It can be a tricky business ensuring that the CBD has had time to develop to optimal levels without the THC increasing beyond legal limits. And the repercussions are pretty devastating for farmers who don’t harvest the plant early enough. After an enormous investment of time and money, many farmers have found themselves with a crop of hemp that has too much THC and simply has to be destroyed. 

(Hemp has many uses aside from cannabinoid extraction, but the law doesn’t allow farmers to re-purpose hot hemp for, say, textiles or bioplastic.)

The University of Minnesota-led research team hopes to change this.

The researchers also argue that a definition based on THC alone doesn’t match the biology of the plant. Instead, they propose using the ratio of THC to CBD to separate THC-type plants from CBD-type plants.

“We validated a simple genetic test that can predict whether a plant will produce mostly the CBD or THC molecule, using a variety of Cannabis sativa plants,” said George Weiblen, who is a professor in the College of Biological Sciences and the Science Director & Curator of Plants at the Bell Museum. The research team, led by the Weiblen Lab, published their findings in the American Journal of Botany

As part of their research, the team analyzed three different varieties of Cannabis plants: 

  • Industrial hemp
  • Wild, or feral cannabis (known as “ditch weed”)
  • Marijuana samples from the National Institute on Drug Abuse. 

They then compared genetic markers with the ratio of THC versus CBD and were able to verify that a plant’s genetics could predict the ratio. 

But they didn’t stop there. 

The article also takes on what has become the standard nomenclature of the cannabis industry – namely, the primary distinction between “hemp” and marijuana.” This distinction, unsurprisingly, makes little botanical sense: 

“The presence of more than one of the three cannabinoid classes in feral, industrial, and clinical populations renders the dichotomy between “hemp” and “marijuana” meaningless from a botanical perspective.”

The authors root this dichotomy in colonial history, stating that the terminology “perpetuates culturally biased and pejorative assumptions about Cannabis sativa that have hindered scientific investigation for nearly a century.” We assume they are referring to the “reefer madness” type paranoia that the word “marijuana” seems to trigger.

They recommend, instead, that “a decolonized definition recognizing THC‐type, CBD‐type, intermediate‐type, and CBG‐type plants would be more accurate botanically and perhaps more practical as the use and regulation of C. sativa continues to expand and diversify.”

So reasonable! Alas, whether the argument has any chance at convincing the authors of the next Farm Bill, their research should hopefully present a way forward for people who are farming the plant and extracting the cannabinoids.

“We hope this new test can assist in new seed certification for the hemp industry,” Weiblen said.

“For hemp to take off in Minnesota and elsewhere, there must be ways to assure growers they won’t have to destroy their crops at the end of the season.”

A modest proposal indeed.

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