It is a big week for opening shots on horseback: First Lynn Redgrave digs her spurs into a thoroughbred on PBS, and over theopening credits of TNT’s Montana, Gena Rowlands guides a big brown horsie down a rocky slope. Rowlands hoists a rifle and, still loping along, shoots at a truck that’s trespassin’ on her spread.

Gena Rowlands as a cowgirl? Somewhere in heaven, her husband, the late John Cassavetes, is laughing that great, raspy laugh of his. In ”Montana,” Rowlands plays Bess Guthrie, who runs a vast ranch in present-day Montana with her ornery, scraggly, drunken, no-account husband, Hoyce. He’s played by a miscast Richard Crenna.

Ludicrous miscasting is, in fact, the chief pleasure to be drawn from ”Montana.” Crenna, who is ordinarily a fine, understated actor, goes unaccountably loopy here, limping around like Chester from ”Gunsmoke,” chawin’ tobaccy, and getting rip-snortin’ snokkered in the local saloon.

Crenna’s best scene comes early on, when he takes part in a local custom: lassoing neighborhood Indians as they ride by on motorcycles. ”Ropin’ Injuns,” Crenna calls it, and if this sounds cruel and condescending to Indians, I haven’t told you about the huge knife one Indian pulls out to exact revenge on Crenna. In ”Montana,” everyone gets to behave equally badly.

This is the first teleplay by Larry McMurtry since his glorious ”Lonesome Dove,” but not really: You see, ”Montana” was written at least a decade ago, and only now, in the wake of ”Dove,” has McMurty’s script been produced. ”Montana” doesn’t exactly have the spirit and vision of ”Lonesome Dove,” it’s more like an episode of ”Dallas,” if J.R. were an inebriated bum.

The plot is resolutely predictable: Crenna and Rowlands have to fight to deep their ranch from being taken over by some mean old oil drillers. McMurty has made his reputation refuting the clichés of the Western: here, he succumbs to them. C-