I had noticed this before, but it sure looks like one continuous shot as he falls into the fiver until he goes past his horse, and it really doesn't look like enough time for him to grab it and get it back on his head again. So I'm still debating whether it came off or just started to, or if it's just a freak reflection, but I agree it sure does look like quite a bit of forehead gleaming in the sun in that shot.
I think a lot of the time he didn't wear the toupee when he was wearing the hat. On the Bonanza Season 2, Volume 2 that just came out, there were photos from the episode "The Fugitive" that show PR sitting at the table with his costar, apparently rehearsing the scene. You go from one photo with him with the hat on to one with the hat off, and he does not have the toupee on, which is quite an eye-opener. You sure can't tell from the photo with the hat on that he wasn't wearing it because the side hair and sideburns look just like they normally do. I made a comment one day about a 'rug' to my 16-year-old son, who didn't know what it referred to or even what a toupee was, so I showed him the two photos, and he was quite shocked to see his first glimpse of Adam Cartwright without his hair. I also showed him the clip from when PR guest-starred on "Cannon" and his character actually has his toupee ripped off his head by the detective. Gotta love a man who's willing to 'bare' himself that way on national television. Was just reading a portion of Lorne Greene's biography by his daughter, and she confirmed that her dad would not appear anywhere without the toupee, even would not let family members see him at home without it. I don't mean that as a criticism of LG in any way, but I think it just shows how much PR actually did hate the thing. And, sigh, I know I'm shallow, but as much as I have come to think that PR was very sexy as Trapper, I have to admit I prefer my Adam with hair on top of his head and a clean-shaven face, and I'm glad they 'made' him wear the toupee for the role.