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Horticulture professor Larry Smart examines industrial hemp plants growing in a greenhouse at Cornell AgriTech in Geneva, New York.

Genetics, not field conditions, makes hemp ‘go hot’

By Krisy Gashler |

As the hemp industry grows, producers face the challenge of cultivating a crop that has received comparatively little scientific study, and that can become unusable – and illegal – if it develops too much of the psychoactive chemical THC.

In a new study, Cornell researchers have determined that a hemp plant’s propensity to “go hot” – become too high in THC – is determined by genetics, not as a stress response to growing conditions, contrary to popular belief.

Industrial hemp plants are shown in a greenhouse at Cornell AgriTech in Geneva, New York.

“Often that issue of going hot has been blamed on environment,” said Larry Smart, senior author of the study and professor in the horticulture section of the School of Integrative Plant Science.

“[People thought] there was something about how the farmer grew the plant, something about the soil, the weather got too hot, his field was droughted, something went wrong with the growing conditions,” Smart said. “But our evidence from this paper is that fields go hot because of genetics, not because of environmental conditions.”

Smart and his team conducted field trials at two sites, in Ithaca and Geneva, New York, studying the genetics and chemistry of 217 hemp plants. They found that differences in growing conditions between the sites had no significant influence on which chemicals the plants produced. But when they compared the CBD (cannabidiol) and THC levels of each of the plants against their genomes, they found very high correlation between their genetics and the chemicals they produced.

Hemp and cannabis are both of the family Cannabis sativa, but hemp plants produce low levels of THC (0.3% or less), whereas cannabis plants typically contain 5% to 20% THC. Hemp has high levels of the medically useful chemical CBD, while high-THC cannabis contains minimal CBD.

Jacob Toth, first author of the paper and a doctoral student in Smart’s lab, developed a molecular diagnostic to demonstrate that the hemp plants in the study fell into one of three genetic categories: plants with two THC-producing genes; plants with two CBD-producing genes; or plants with one gene each for CBD and THC.

To minimize the risk of plants going hot, hemp growers ideally want plants with two CBD-producing genes.

“The molecular assays developed in this paper provide useful tools in breeding hemp,” Toth said. “To keep THC levels low, ensuring a lack of THC-producing genes will be important for the development of future compliant cultivars. Molecular testing is also much quicker and less expensive than current methods, and it can be done on seedlings instead of mature plants.”

While conducting the research, the team also discovered that as many as two-thirds of the seeds they obtained of one hemp variety – which were all supposed to be low-THC hemp – produced THC above legal limits.

The researchers hope their work will help address this problem by providing breeders with easy-to-use genetic markers that can be utilized much earlier on seedlings and both sexes of plants. CBD and THC are produced by only females, but breeders may be using a male plant for cross pollination without knowing if it has genes for THC production, until it appears in their female offspring, Toth said.

The team also developed genetic markers to determine the sex of hemp plants prior to flowering, since the sexes of young plants are indistinguishable. “This technology is, at this point, too expensive for farmers to use on an entire field, but it will be very useful for breeders who want to separate males and females early on to better control cross-pollination,” Smart said.

Smart said future research in his lab will focus on breeding hemp cultivars – for CBD, grain and fiber – that are high-yield, legally compliant and adapted to New York’s growing conditions.

Also contributing were postdoctoral researcher Craig Carlson and doctoral student George Stack, from Smart’s lab; Rebecca Wilk, field coordinator for Cornell AgriTech; Don Viands, associate dean and director of academic programs and professor of plant breeding and genetics; Jamie Crawford, research support specialist in Viand’s lab; Christine Smart, professor of plant pathology and plant-microbe biology; Ali Cala, a graduate student in Christine Smart’s lab; Jocelyn K.C. Rose, professor of plant biology; and Glenn Philippe, a postdoctoral researcher in Rose’s lab.

The research was funded primarily by New York State Department of Agriculture and Markets, through a grant from Empire State Development Corporation.

Krisy Gashler is a freelance writer for the College of Agriculture and Life Sciences.

Cornell researchers have determined that a hemp plant’s propensity to “go hot” – become too high in THC – is determined by genetics, not as a stress response to growing conditions.

Hot Hemp Can Damage Your Business. Here’s How to Prevent It.

0.3% — it’s an absurdly low percentage that can make or break your entire hemp operation.

But that’s the limit set by Federal and state laws for the delta-9 tetrahydrocannabinol (THC) levels that can be in your hemp product.

What is hot hemp and why does it matter?

Anything above a 0.3% of THC level in hemp is considered “hot” — and illegal at the federal level. Cannabis with a THC concentration below 0.3% is classified as hemp, while plants above 0.3% THC are considered marijuana. This makes it wildly important to understand the sampling process and how to prevent hemp crops from overproducing THC.

In October 2019, the interim hemp sample collection guidelines were released. They outline precise requirements for collecting and analyzing THC levels. Among them:

  • Hemp producers must employ a USDA-designated official to collect samples within 15 days before harvest. Plants cannot be harvested until the lots have been sampled.
  • The sampling agency must have unrestricted access during business hours to all hemp and other cannabis plants.
  • Growing lots must be pre-defined, documented and reported to the Farm Service Agency prior to sampling.
  • Collection must follow a precise process, including sample volume requirements and explicit handling procedures.
  • Samples must be sent to DEA-licensed labs for compliance testing.

The sampling process is strict — and so are the consequences.

Samples returned with THC levels above federally-regulated levels can quickly become problematic for hemp producers. If samples are found above 0.3% THC, producers are typically required to forgo their harvest and destroy their crops. This can result in tens of thousands of dollars in lost upfront investment and potentially hundreds of thousands of dollars in lost future earnings.

What really causes hot hemp?

Many hemp producers attribute hot hemp to poor soil conditions, overly hot weather, droughted fields or improper growing conditions. But new research by Smart indicates that hemp crops likely go hot because of genetic factors — not necessarily environmental ones.

So what can be done? Since THC is based on unpredictable genetics, testing can be done on seedlings to weed out plants likely to overproduce THC in their mature form.

Producers can also prevent crops of hot plants by cloning ones that sample under 0.3% THC. Cloning involves taking a cutting from a “mother” plant and letting it grow its own root system. Because the genes are identical, clones will generate a plant with the same THC levels as the mother, saving time and money and ensuring a genetically consistent crop.

Futureproof your business.

Ensuring your hemp crop stays below 0.3% THC is a critical step in creating a sustainable and successful hemp business. Producers must not only ensure compliance with federal, state and local regulations — they must also ensure their products are properly produced for retail and safe for consumers. That’s a lot to juggle, but our team at KLER sees a smarter way through the sampling process via an all-in-one hemp and CBD management solution:

  • Track crops by acre with GPS plots or by individual plants. Generate unique IDs to track location movements, plant and harvest dates, strain types, and more.
  • View harvest efficiencies by nutrients used, cultivar, lots, test results and other data.
  • Review testing results for compliance and gain visibility with seed and clone testing comparisons.
  • Manage inventory and harvest expense tracking effortlessly.
  • Access and analyze important data with powerful analytics and multi-level filtering tools.
  • Streamline HR tasks by tracking employee costs and optimizing employee schedules to optimize productivity.
  • Optimize logistical and operational efficiency with tools to ease inventory management and shipping compliance.

Interested in learning more? Check out the KLER platform and see how it can help you streamline your hemp and CBD operations!

Hot Hemp Can Damage Your Business. Here’s How to Prevent It. 0.3% — it’s an absurdly low percentage that can make or break your entire hemp operation. But that’s the limit set by Federal and