Yup, that's mastitis alright! Clean and milk that teat last, into a separate container, and try not to touch the other teats after touching it, or let any milk from it get on the other teats or on the cow's hooves.
You may want to get an intramammary antibiotic to clear it up. Today and Pirsue are the brands we use where I work. Oftentimes 1 treatment will clear it up. For a really persistent infection, or a cow who tends to be chronic, you can administer both at once, then follow up with just Pirsue on the second day if she is still showing signs of infection. If there is a feed store near you that stocks these products, check to see if they will sell you just 1 or 2 tubes rather than a whole box. The folks at our store are nice this way.
If you choose not to treat her, she may clear up on her own just with thorough milking. You can feed the milk from the infected quarter to your dogs or cats. It probably won't hurt you to drink it, although the bacteria in it may give it an "off" flavor or make it spoil quicker. I wouldn't recommend drinking it!
If the clumping increases, there is blood in the milk, or if the cow seems listless, feverish or in distress, you should definitely treat her and perhaps consider adding a dose of injectible antibiotic (agri-cillin or LA200). Severe mastitis can go systemic and prove fatal, although that is relatively rare.
I would definitely recommended using dry cow treatment on a cow that has had mastitis, when it's time to dry her off. The higher dose of antibiotics that can be used in a dry cow may help get rid of any lingering subclinical infection.
Regarding feed: If you are buying from a mill, they likely have a feedmaster you can consult, who can formulate a custom blend for you based on conditions in your area. I have a special 16 percent protein ration mixed up that I can feed to cattle, horses, sheep, goats and llamas. It's a mix of corn, oats, and pellets with molasses and minerals. They seem to like it!
