What happens if you cross hemp and marijuana?

 --A brief discussion by David P. West with special reference to work by Small and Beckwith.

 
The question I'm most frequently asked concerns the so-called "pollination effect." This issue devolves around the notion that marijuana growers will attempt to hide their plants in hemp fields and what would happen to these plants.

Cannabis compares (I have done so often) with maize (corn): in maize you have the "sweet corn" type that people eat, and "field corn" type that is fed to cattle and pigs and chickens. Breeders of the respective types take great pains to prevent their inter-pollination.*
The same is true between high and low THC types of Cannabis. High THC ("narcotic, "drug" or "marijuana") Cannabis and low THC ("high CBD," "fiber," "industrial," "hemp") Cannabis readily cross pollinate. The offspring tend toward characteristics intermediate between the parents. Nobody wants them. They're "good for nothing."

Actual research demonstrating this was published in 1979 in the now-classic work The Species Problem in Cannabis by the Canadian researcher, Dr. Ernest Small.** Here is an excerpt of that report which I have annotated for clarity for the non-specialist.

Cannabinoid Composition of F1 Hybrids between "Drug" and "Non-drug" Strains of Cannabis

E. Small and H. D. Beckwith. 1979. In E. Small, The Species Problem in Cannabis. Corpus. Toronto. p 121-127

Abstract Twenty-five sets of F1 hybrids [an F1 hybrid means the plants grown from the seed of a cross pollination between types] , mainly between "drug strains" of Cannabis (those in which the resin is chiefly of tetrahydrocannabinol[THC]) and "non-drug strains" (those in which the resin is composed chiefly of cannabidiol [CBD]) were examined. The majority of these were chemically intermediate between their respective parents, showing no dominance toward either parent.
[So, in the plants grown from the resulting seed (the "F1" generation), THC went down with respect to the "marijuana" parent and up with respect to the "hemp" parent. The converse was true of CBD.]

Discussion:
...It appears that generally crosses between drug strains and non-drug strains produce plants of intermediate potency.

...[In studies of drug strains...] The importance of protecting the [genetic] stock against contamination from pollen by non-drug strains is stressed by the fact that the amount of THC may be halved in hybrid plants.

-------------------------------------------------------------------

What these authors don't go on to discuss is what would happen next, in practical terms.

We know, then, that the first generation has been degraded by half. If that "mixed-blood" seed is grown hidden again among hemp plants that will again provide the pollen, then the next generation will again lose by half, gradually by this means (it's called a "backcross" by plant breeders) converging toward the hemp type. But it is unlikely it would get that far as the material would be undesirable after the first contamination.

This is why marijuana growers want to stay away from hemp.
*  
This discussion excepts those instances of plant genetic work where such crosses may be made intentionally in order to transfer specific genes, such as disease resistances, between varieties.

 

**

Small, Ernest, 1979 The species problem in cannabis
ISBN 0-919217-10-9
© Minister of Supply and Services Canada,

Published by
CORPUS
51 Bloor 5t. West,
Toronto, M5S 1S4