Uva uvam vivendo varia fit.
This Latin phrase from Lonesome Dove is neither nonsense nor bad Latin. It means "A grape becomes mottled by being a grape," with the sense: it is the nature of a grape to become "mottled" as it ripens. In the context of the novel, this means, "Character is destiny" -- the truth of which is illustrated time and again in the novel. The repetition of the 'v' strengthens the point. It is lovely Latin.
vivendo here is a gerund of vivo, in the ablative (instrumental). uvam is a cognate or internal accusative (a play on vitam vivendo), a not uncommon feature of ancient Latin. Most often in Classical Latin the gerund and its object are rendered into the same case and gender, but the usage here is also well attested. The grammatical formation of the gerund and its object are too involved to discuss here, but for a Latinist it would present no problem. I have seen elsewhere the claim that vivendo is a "false gerund." That's absurd on the face of it (the gerund form of vivo is actually quite common, e.g. modus vivendi), unless the author of the post meant that vivendo was simply thrown in without any regard for its grammar or meaning. In either case, the claim is unfounded. The effort to derive this superb Latin sentence as a corruption of a line from Juvenal is likewise misguided and is likely responsible for the "false gerund" claim. At best, Juvenal's line is a red herring. Clearly, an accomplished Latinist, perhaps an acquaintance of the author if not McMurtry himself, is responsible for Uva uvam vivendo varia fit.
Just as a grape or a man can become nothing other than what each is, pigs remain true to themselves. "We don't rent pigs. . . Goats and donkeys are neither bought nor sold," only cattle and horses whose character man has fundamentally changed.