A Strange Harvest (1980) is a documentary about unexplained cattle mutilations that were widespread in the American West in the 1970s and 1980s. (From a Wikipedia article on cattle mutilation: "A 1979 FBI report indicated that, according to investigations by the New Mexico State Police, there had been an estimated 8,000 mutilations in Colorado, causing approximately $1,000,000 damage.") Many instances of cattle mutilations can be attributed to predation, but others appear to be the result of unexplained human activity.
This well-made documentary, which runs about 1.5 hours, is a masterpiece of low-budget creepiness. The synthesizer soundtrack is ominous and weird, and the cinematography is hallucinatory. I don't buy the argument that space aliens are responsible, but it's still a compelling movie.