Mormons believe there are some things that are official canonized doctrine. These come through revelation and must follow a process: The prophet receives revelation. He presents it to the apostles, who must approve it. Then it is presented to the membership and recorded as an official doctrine. Anything that does not follow that process is not doctrine. Mormons are taught that if there is no official doctrine, they may choose for themselves. This holds true even for prophets. You'll note in the Bible that prophets often said things that reflected the worldview of the time, including scientific issues, that we now know aren't accurate. They weren't speaking as prophets in those cases, but as products of their time and place.

Every word spoken by a prophet is not doctrine. Brigham Young was a man of many opinions and his opinions were impacted by the commonly held views of his time period. It is difficult for non-Mormons to recognize what is doctrine and what is not–but visiting authentic sources can help. Remember, that to a Mormon, the most recent teaching is the one God expects us to hold onto. This is why Peter, in the New Testament, had a vision that put an end to the prohibition against teaching the gospel to non-Jews, whom they considered "unclean." It had been the practice, but Peter learned that was to change. If God couldn't move teachings forward to the next highest step, we'd all be living the Law of Moses or building arks.