Selecting for plants and animals with useful traits

If you have 2 plants, take wheat for example. 
If there's one species who can produce lots of grain but takes a long time to grow, and there's a different species who produces very little grain but grows quickly, many farmers will try and get both of the good traits.

If they want fast-growing wheat with lots of grain, then they'll breed them together and hope that they get a species with both of the useful traits.

The same goes for animals. E.g, cows that are very big but don't produce much meat and a cow that's small but produces lots of meat in proportion.