A dzo is a cross between an ox and a yak - a very strong beast of burden. So the message is: don't take the burden of a dzo and place it upon an ox, which is a weaker animal. This rustic metaphor refers to issues of ability and responsibility. Each of us is endowed with certain talents, whether we were born with them or earned them in this life. We also have our responsibilities, some of which we may not be inclined to fulfill. If this text were originally written in America, it would probably say here: Don't pass the buck. Recognize what your role is and what you are here to contribute. What are your special abilities and responsibilities? At times these will be enjoyable and rewarding; at other times they may be grunt work. But having identified the grunt work, don't shift it onto other people's shoulders. They may not be as capable as you are for the task.
Excerpted from: The Seven-Point Mind Training(first published as A Passage from Solitude : Training the Mind in a Life Embracing the World), by B. Alan Wallace. Copyright 1992 by Snow Lion Publications, Ithaca, New York 14851.
This site provides an on-line database of commentaries on the Tibetan Buddhist meditation practices of lojong (Mind Training) and tonglen.