How to Breed a High CBD Strain

The Science of Breeding

In part 2 of our series on the science of breeding, we explain how to breed a high CBD version of your favorite strain. If you haven't watch part 1 about the genetic of THC and CBD yet, be sure to check that out as well by clicking here. Delta Leaf DNA testing to identify plants containing the genes for CBD and/or THC enzymes can be used to speed up your breeding projects, screening your F1 generation, verifying a batch of clones and/or maintaining compliance on industrial hemp farms.

How To Do An F1 Cross

Suppose you have an awesome strain, and you want to make a high CBD version. Here’s the traditional way of doing it.

First, you want to make sure your strain doesn’t already produce CBD. You also need to find a male plant from a strain that makes lots of CBD. You can get males by growing a few plants from seeds. Next, you breed these two strains in what’s known as an F1 cross. Let’s draw a Punnett square to diagram how different versions of a single gene are exchanged during an F1 cross. It looks like this: ​

We start with a high THC female plant (has two copies of the Bt allele), and a male plant from our high CBD strain (that has two copies of the Bd allele). You can also switch who is the male or female in any of the following crosses, because we aren't looking at any traits that are linked to to the sex chromosomes. When we cross the two strains mentioned above, then all of the resulting seeds will get one copy of the Bt allele from the mother plant, and one Bd allele from the father plant.

How To Do An F2 Cross

Once we’ve got some F1 plants growing, we can cross them with each other to make the F2 generation. When we draw our Punnett square for the F2 cross, we now see three possible outcomes in the resulting batch of seeds. Roughly 25% of the plants will have two copies of the Bt allele (Bt/Bt), while another 25% will have two copies of the Bd allele (Bd/Bd). The remaining 50%, of half the batch of F2 seeds, should contain one copy of each allele (Bt/Bd).

If you want a high CBD strain, but don’t want to change the terpene profile or growth characteristics very much, then you can take your F2 generation high CBD plants and cross them back to the F1 generation. This process is called “back-crossing” and it makes each new generation more and more similar to the original strains’ terpene profile and growth characteristics. You just have to keep selecting for the high CBD versions with each round of new plants.

Identifying High CBD and High THC Plants

At Delta Leaf we can identify whether a plant contains the Bt allele, the Bd allele, or both. This saves time and money by identifying the plants you want immediately after germination, instead of waiting months to observe the phenotype. We can also identify whether a plant carries the male, or “Y”, chromosome. Sample collection kits are available here on our website.

Below is a final summary graphic showing the entire process of breeding a high CBD strain. Thanks for reading and good luck with your breeding!