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... inmemor est demum nec frugum munere dignus, qui potuit...

non frugum munere dignus: 'frugum' returns in verse 134, where it says, that the bos worked on the 'fruges' (by ploughing the earth, I reckon); frux, -gis must mean then here some kind of corn, I guess. The bos has no brain and he is not worthy of his task of ploughing the ground either, when it comes to it. I translate 'munus' here with 'task, service'. This is my suggestion-translation for this clause.