Soil characteristics, latitude and climatic stresses have been found to have significant effects on THC concentrations, and there are seasonal and even diurnal variations (Small 1979; Pate 1998b). However, the range of THC concentrations developed by low-THC cultivars (those typically with £0.3% THC) under different circumstances on the whole is limited, for the most part generally not varying more than 0.2 percentage points when grown in a range of circumstances, and usually less (note information in Scheifle et al. 1999; Scheifle 2000, Scheifle and Dragla 2000). Practically, this has meant in Canadian experience that a few cultivars have been eliminated from further commercial cultivation because they sometimes exceed the 0.3% level (‘Fedora 19’ and ‘Futura,’ authorized in 2000, have now been removed because some test results in several years exceeded 0.3%; ‘Finola’ and ‘Uniko B’ are under probation because of elevated levels), but on the whole most of the permitted cultivars have maintained highly consistent development of quite low levels of THC.
The question of whether heteromorphic sex chromosomes are indeed present is most conveniently answered if such chromosomes were clearly visible in a karyotype. Cannabis was one of the first plant species to be karyotyped; however, this was in a period when karyotype preparation was primitive by modern standards (see History of Cytogenetics). Heteromorphic sex chromosomes were reported to occur in staminate individuals of dioecious "Kentucky" hemp, but were not found in pistillate individuals of the same variety. Dioecious "Kentucky" hemp was assumed to use an XY mechanism. Heterosomes were not observed in analyzed individuals of monoecious "Kentucky" hemp, nor in an unidentified German cultivar. These varieties were assumed to have sex chromosome composition XX. According to other researchers, no modern karyotype of Cannabis had been published as of 1996. Proponents of the XY system state that Y chromosome is slightly larger than the X, but difficult to differentiate cytologically.
The Gateway Hypothesis states that cannabis use increases the probability of trying "harder" drugs. The hypothesis has been hotly debated as it is regarded by some as the primary rationale for the United States prohibition on cannabis use. A Pew Research Center poll found that political opposition to marijuana use was significantly associated with concerns about health effects and whether legalization would increase marijuana use by children.
Former President Barack Obama defined industrial hemp as a distinct crop from marijuana in the 2014 U.S. Farm Bill, authorizing higher education institutions and state agricultural departments to give the green light to "regulate and conduct research and pilot programs." But with its classification as a Schedule 1 drug still in effect, the path toward cultivating industrial hemp hasn't been an easy one.
A chief argument that has been advanced in favor of developing hemp as a paper and pulp source has been that as a non-wood or tree-free fiber source, it can reduce harvesting of primary forests and the threat to associated biodiversity. It has been claimed that hemp produces three to four times as much useable fiber per hectare per annum as forests. However, Wong (1998) notes evidence that in the southern US hemp would produce only twice as much pulp as does a pine plantation (but see discussion below on suitability of hemp as a potential lumber substitute in areas lacking trees).
The woody core (hurds, sometimes called shives) of hemp makes remarkably good animal bedding (Fig. 28, 29). The hurds are sometimes molded into small pellets for bedding applications (Fig. 30). Such appears to be unsurpassed for horse bedding, and also make an excellent litter for cats and other pets (Fig. 31). The hurds can absorb up to five times their weight in moisture (typically 50% higher than wood shavings), do not produce dust (following initial dust removal), and are easily composted. Hemp bedding is especially suited to horses allergic to straw. In Europe, the animal bedding market is not considered important (Karus et al. 2000), but in North America there are insufficient hemp hurds available to meet market demand.
Since then, many different types of sex determination systems have been discovered, particularly in plants. Dioecy is relatively uncommon in the plant kingdom, and a very low percentage of dioecious plant species have been determined to use the XY system. In most cases where the XY system is found it is believed to have evolved recently and independently.
Although cannabis as a drug and industrial hemp both derive from the species Cannabis sativa and contain the psychoactive component tetrahydrocannabinol (THC), they are distinct strains with unique phytochemical compositions and uses. Hemp has lower concentrations of THC and higher concentrations of cannabidiol (CBD), which decreases or eliminates its psychoactive effects. The legality of industrial hemp varies widely between countries. Some governments regulate the concentration of THC and permit only hemp that is bred with an especially low THC content.