"Milking the cow" is a specific phrase, so there is no one perfect synonym. The best choice depends on your context.
Literal Meanings
If you are talking about actually milking a cow, consider:
- Lactating
- Extracting milk
- Collecting milk
- Dairying
Figurative Meanings
If you mean taking advantage of a situation, try:
- Exploiting
- Taking advantage of
- Capitalizing on
- Profiteering
- Squeezing
If you mean getting something out of someone in an unfair way, options include using, manipulating, pressuring, or extorting.
Providing the context of your sentence can help narrow down the most accurate synonym.