I would suggest doing it like this. It refers to Genesis 41, in which Joseph interprets Pharaoh's dream about seven fat cows that came out of the Nile, followed by seven thin cows which ate the fat ones. Joseph explained that it meant that the current seven years of plenty would be followed by seven years of famine, and therefore they should save food now while it's plentiful so as to be able to eat when the famine comes. So actually the message your boss is putting in his Christmas card has it the wrong way round: in the Bible, the "lean" cows/years come after the "fat" cows/years, not before.
http://biblehub.com/esv/genesis/41.htm

In Spanish it's quite common to refer to "vacas gordas" and "vacas flacas" in this sense, but in English we tend to refer to "lean years" rather than "lean cows".

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Note added at 1 hr (2014-12-12 09:56:43 GMT)
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I do hope the "vacas gordas" are coming, for everyone's sake, but I suspect that the Genesis version is right: we had the "vacas gordas" up to 2008 and we blew all the money (and more). :)

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Note added at 1 hr (2014-12-12 09:57:35 GMT)
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(But that's too gloomy to put in a Christmas card.)