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Harvest Hemp Early for Maximum CBD, Scientists Say

By Alexander Beadle

Published: Jun 27, 2022   
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Hemp cultivators should harvest their plants before they reach full seed maturity if they want to maximize the amount of CBD that can be extracted, a new study published in Royal Society Open Science suggests.

According to new research from scientists at the University of York and Khon Kaen University, CBD levels and overall cannabinoid levels reach a maximum in the popular Finola variety of hemp around nine weeks after seeding, before beginning to wane. However, the best CBD-to-THC ratios for extraction were achieved at the bud stage, just eight weeks after seeding.

The results demonstrate the importance of studying metabolite evolution in hemp plants over time, the researchers say, especially given the increasing demand for CBD that is being fueled by a multi-billion dollar CBD product industry.

Harvest early to maximize CBD production

For this study, the researchers planted 160 seeds of the Finola variety which yielded a total of 90 female hemp plants that could be studied. These plants were then harvested during one of four potential harvesting windows: the vegetative stage, four weeks after seeding, the bud stage, eight weeks after seeding, the full-flowering stage, nine weeks after seeding, or the seed maturity stage, eleven weeks after seeding.

During harvesting, the plants were split out into their leaves, stems, and tops, with each batch of material from the plant prepared separately for testing. Testing was done using a Sohxlet extraction technique to extract the crude wax product, which was weighed before being analyzed using a high temperature gas chromatography (HTGC) method.

Comparing the yields for each part of the plant at each step of the harvest cycle, the researchers found that the crude yield of extract was highest in the stems at the first harvesting stage, but peaked for the leaves and tops by the fourth harvest.

Compositional analysis revealed that the greatest number of cannabinoids were found in the extracts from plant tops, while the extract from the leaves was largely made up of wax esters and steroid compounds. In these plant tops, the cannabinoid content increased over time to reach a peak by the third harvest, which decreased slightly by the final harvesting window. The highest quantity of CBD was also found in the third harvest.

“Initially, as hemp grows, the levels of cannabinoids are low until the plant reaches flowering,” study author Andrew Hunt, told Newsweek. “Cannabinoid content then decreases until seed maturity is reached.”

“CBD distillate sells for $3,000 per kg, while CBD isolate sells for $1,000 per kg,” Hunt added, illustrating the significant demand for this cannabinoid.

Early harvesting can limit the development of THC

Peak CBD levels aren’t the only important compositional factor to study in the maturation of hemp; equally important are the plant’s THC levels.

“In the USA, the legal THC concentration limit cannot exceed 0.3 percent on a dry weight basis in any parts of the plant, seeds or extracts,” Hunt explained.

A plant that produces high amounts of both CBD and THC will need extensive post-harvest processing to remove that THC and create a sellable CBD isolate, distillate, or other product. But having a plant that naturally contains a high amount of CBD and low levels of THC will make the isolation and purification of CBD much easier.

The researchers found that, although total CBD amounts peaked by the third harvest in this study, the most ideal CBD:THC ratio was seen in the second harvest window as the THC levels were significantly lower here than during the third harvest.

“The greatest proportion of cannabidiol was extracted from the tops at full flowering, however a significant increase (63 percent) in the banned psychoactive tetrahydrocannabinol (THC) was observed at this harvest stage as compared to the budding stage of development, where the plant initial begins flowering,” Hunt said.

“Harvesting the tops after budding would be preferable due to the high cannabidiol content and low amounts of THC.”

The researchers say that this study highlights the importance of studying the variation in cannabinoids and other plant compounds in the hemp plant as it matures, in order to identify different harvest times that could maximize the plant’s potential industrial value.


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