Q, I'll be interested in what you learn. Meanwhile, here's a site that repeats the assertion, this time with an explanation of sorts. You're better qualified than I am to comment on its plausibility from the viewpoint of ranch work :

http://www.nationalcowboymuseum.org/g_corr_info.html

"Cowboys called motherless calves 'dogies.' The term came from the Spanish word "dogal," meaning a short rope used to keep a calf away from its mother while she was being milked."