Cannabidiol (CBD)-rich hemp and tetrahydrocannabinol (THC)-rich cannabis are nearly identical except for one key feature: their THC content. Besides this, it’d be quite difficult to differentiate between these plants if we were to find them in the wild, for they’re both Cannabis sativa, and they look and smell very much alike.
The legalization of hemp and CBD in 2018 came with a set of regulations. One of these highlights that hemp plants cannot exceed 0.3% THC. If a farmer grows plants containing even just 0.4% THC, this plant is no longer legal to be sold for hemp or CBD products.
Upon passing the 2018 Farm Bill, Congress needed a way to differentiate THC-rich cannabis from hemp as cannabis remains a Schedule I substance. A good way to do this was to limit the THC content to such a low number that it would be practically impossible for someone to get high from a hemp plant.
But where exactly did this number come from?
In 1976, Small and Cronquist  were studying cannabis and randomly chose the 0.3% THC limit to define hemp. Their study writes, “It will be noted that we arbitrarily adopt a concentration of 0.30% Δ9-THC (dry weight basis) in young, vigorous leaves of relatively mature plants as a guide to discriminating the two classes of plants.” 
It’s important to remember that the government saw a lot of potential for hemp outside of CBD. Hemp is an extremely versatile crop that can be used for many different things (e.g., food, clothing, textiles) and this was a giant pusher towards legalization. However, with that push remained concerns over the stigma surrounding cannabis as a whole.
All in all, the main purpose of the 0.3% THC regulation is for the sake of making sure farmers are growing hemp rather than a federally illegal crop.
But these regulations haven’t come without their flaws. Since it’s challenging to predict the exact amount of THC a hemp plant will generate until it’s ready for harvest, many farmers have found themselves technically growing illegal crops.
According to Hemp Industry Daily, one in five of all hemp plants grown within the United States surpasses the legal limit for THC. In turn, these lots must be destroyed. Last year, 511,442 acres of hemp was grown on U.S. soil. Using these numbers means that more than 100,000 acres were prevented from ever reaching the marketplace and, in turn, cultivated for no purpose at all.
This is a big problem considering hemp’s utility outside of consumption. Those plants could’ve easily been used for another purpose—such as rope, clothing, or even shelter—that wouldn’t put anyone at risk of getting high.
Due to this major loss, many farmers are proposing a new, 1% THC standard when it comes to their hemp acreage. This number remains low enough to not put people at risk and high enough to greatly diminish the risk of losing crops.
Reference Small E, Cronquist A. “A Practical and Natural Taxonomy for Cannabis.” Taxon, vol.25, no.4, 1976, pp.405-35. [journal impact factor = 3.823; timed cited = 85]