The team also tested their milk and found high levels of THC - the main psychoactive compound in cannabis that produces the ‘high’ sensation.
Mood alterations
Levels were so high that in humans it could lead to drowsiness, impair working memory performance and cause mood alterations, the researchers said.
“We observed significant changes in respiratory and heart rate as well as a reduction of feed intake and milk yield,” a spokesman told The Telegraph.
“Other signs included yawning, salivation, prolapse and reddening of the nictitating membrane [a third eyelid present in some animals], tongue play and nasal secretion.
“Consumption of milk with Delta-9 THC concentrations as measured in the current study could result in intake levels above the acute reference dose, with especially high intakes in children.
“Higher intake levels are undesired, since adverse effects may occur. These exposure levels may especially affect the central nervous system, eg. increased sedation, impaired working memory performance, and mood alterations.”
As well as behavioural changes in cows, the researchers also noticed a significant slump in their heart rate and breathing rate in the hours following feeding with the hemp silage. The animals also ate less and did not produce as much milk.
Health consequences
The team concluded that the feeding of industrial hemp silage to dairy cows, even in small amounts, is associated with health consequences, which varied depending on the cannabinoid concentration of the silage.
Silage made from leaves, flowers and seeds had the biggest impact, while low-cannabinoid industrial hemp silage made from the whole hemp plant showed no effects on cow health and performance.
However, independent experts questioned whether it was realistic to think farmers would ever feed such highly psychoactive substances to their herds.
Jack Corless, a global dairy consultant from the nutrition and management consulting firm Progressive Dairy Solutions Inc, said: “It is certainly possible to produce a psychoactive effect on livestock by feeding them sufficient cannabinoids.
“But over the last three years in the US, farmers in Oregon have been feeding hemp residue to both dairy and beef cows, which has shown some positive response in intakes and milk production without any psychoactive effects on the animals, or, as far as I know, significant transfer of cannabinoids into the milk.
“In this study, these cows were fed silage made from leaves, flowers and seeds of a variety of cannabis sativa high in cannabinoids.
“As far as I know, there is no commercial production of such a crop for feeding dairy cows anywhere in the world. But I hope the cows enjoyed it while it lasted.”
Several companies now grow hemp to be used for animal feed. Last year, the state of Montana passed a law allowing hemp to be used as commercial feed for horses, pets and speciality pets, such as gerbils, hamsters, birds, fish, snakes and turtles.
Earlier this month, Germany’s health minister unveiled plans to legalise the possession of up to 30 grams of cannabis and allow the sale of the substance to adults for recreational purposes and cultivation of up to two or three plants per person.
The research was published in the journal Nature Food.
