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My great uncle was a 30 year Army man (Master Gunnery Sergeant) during and after WW I. The family said he wouldn't think twice about drinking out of a cow track.
You get thirsty enough, you'll drink it. Tired enough, you'll sleep anywhere. Plus, makes great stories later to impress the whippersnappers.
I think that every Texan does this as a rite of passage. If you're a texan and haven't had a sip from a slew or a muddy hoof print, you're essentially a disgrace to Texas.