Question: "What was the significance of the commands against mixing different things in Deuteronomy 22:9-11?"
Answer: Deuteronomy 22:9–11 reads, “Do not plant two kinds of seed in your vineyard; if you do, not only the crops you plant but also the fruit of the vineyard will be defiled. Do not plow with an ox and a donkey yoked together. Do not wear clothes of wool and linen woven together.” These commands sound strange to us and cause many people to wonder why God would issue these kinds of laws.
These commands are found in the larger context of Deuteronomy 22:9—23:18 that includes many commands related to the seventh commandment of not committing adultery. The broader concept is one of not mixing together what was to stay separated. The underlying emphasis of these specific commands is spiritual.
Each of the three commands is about two different kinds: 1) two kinds of seed, 2) two kinds of animal, and 3) two kinds of yarn. The different seeds were not to be planted together; the different animals were not to plow together; and the different yarns were not to be interwoven.
There may be some practical reasons for the commands to keep these things separate. First, the command against mixing two kinds of seed sounds very familiar to Jesus’ parable regarding the wheat and the tares (Matthew 13:24–30). In the parable, an enemy plants the weeds (likely darnel, a wheat-like weed) in order to ruin the crop. The only way to spare the wheat was to wait until the harvest and separate the wheat from the weeds. A farmer would not intentionally mix these two types of seed.
Barring the yoking of an ox and donkey is more difficult to understand. Some interpreters understand the idea of “yoking” as a reference to crossbreeding the two animals (this is the emphasis in Leviticus 19:19). The command could also have been a means of preventing animal cruelty: an ox is much stronger than a donkey, and yoking the two together will quickly deplete a donkey’s strength. It is also important to note that the ox is a clean animal, while the donkey is unclean.
The prohibition against wearing clothing mixed of wool and linen is also difficult to understand. Some suggest that the focus is on dressing in clothing distinct from the pagan cultures that surrounded the Israelites.
Both spiritual and practical considerations were involved in these commands, as is often the case in the Mosaic Law. Even when the practical reasons are not fully clear, it is possible to find a spiritual purpose. God desires purity in His people, and there is to be no “mixture” between good and evil. “Do not be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness? What harmony is there between Christ and Belial? Or what does a believer have in common with an unbeliever?” (2 Corinthians 6:14–15).