"Sierra Sue" is a Gene Autry western with a most unusual plot. Apparently, there's an infestation of 'Devil-Weed' and it's deadly to cattle if they eat it. I read up on it and there actually IS something called 'Devil-Weed', though it's more commonly called 'Jimsonweed' and indeed kills cattle. This is indeed an unusual plot but where it goes next isn't exactly logical!
The valley is infested with Devil-Weed and attempts to control it by burning the grass haven't helped. But when a government agent (Gene Autry) suggests spraying the range to eradicate the pest, Mr. Larrabee insists it won't work and wants to keep burning...even though it's ineffective. Now this is a problem with the plot...that Larrabee was so against the spraying but you never really understood why...nor why his opinion at the end changes so much! Also illogical is Larrabee's daughter...and she's a pretty typical inexplicably angry woman that you too often see in B-westerns.
So despite not being especially logical, is there much to enjoy about the film? Well, the scenery around Mammoth Lakes is gorgeous...even prettier than Lone Pine (which is just south of Mammoth Lakes), a very popular site for old westerns. And, the plot is original. But the story still doesn't make a ton of sense and Frog (Smiley Burnett) is a bit annoying in this one as well. Clearly not one of Autry's better films.
By the way, perhaps this plot involving airplanes was created because Autry was himself a pilot and serves in the Army Air Corps during WWII as a flight instructor.