Let Me Google That For You.
The movie you are thinking of is
The Rare Breed (1966). [
1]
While you remembered the setting as Montana, the journey in the film actually takes the characters to
Texas. The story follows a headstrong English widow, Martha Price (
Maureen O'Hara), and her daughter, Hilary (
Juliet Mills), who travel to America to fulfill her late husband's dream. They bring along a prized, hornless Hereford bull named
Vindicator to prove the breed can survive the harsh frontier elements and successfully crossbreed with the local Texas Longhorns to improve the quality of the beef. Along the way, they enlist the help of a cynical, reluctant cowboy named Sam "Bulldog" Burnett, played by
James Stewart