The report showed the production of marijuana, called garda in Afghanistan, rose 8 percent in 2012 from the year before even though the amount of farmland used to cultivate it fell by 17 percent.
The report issued by the United Nations' Office on Drugs and Crime and the Afghan Ministry of Counter-Narcotics said the national average was 120 pounds of marijuana per acre cultivated, an increase of 21 percent from 2011. The production fell slightly short of the 128 pounds per acre Afghan growers reported in 2009.
The report estimated 54 percent of the country's illicit crop was grown in the southern provinces.
The report said one reason for the reduction in acreage cultivated was "a strictly enforced ban by provincial authorities" that was imposed because "cannabis fields seemed to have been used by insurgent groups as hiding places."
The report cited a "dramatic decline" in the southern province of Uruzgan, where the number of acres cultivated for marijuana plummeted from 2,500 acres in 2011 to 250 last year.
The report notes farmers are attracted to the illicit crop because they can rake in more than $2,500 an acre -- considerably more than they can make growing another illegal crop, opium.
"This again indicates that a farmer decides on planting licit or illicit crops in a holistic way, including as much family needs, food security, access to markets, access to non-farm income, as there is risk," Jean-Luc Lemahieu, the U.N. agency's regional representative, said. "Only an 'opium farmer' or a 'cannabis farmer' or even a 'wheat farmer' simply does not exist."
He advocated that the country's agricultural police should take into account measures being taken against illegal crops.