The major cause of the salty flavor in milk is ___________.

The major cause of salty flavor in milk is indeed linked to the presence of mastitis. Mastitis is an inflammation of the mammary gland typically caused by bacterial infection, and it can lead to changes in the composition of milk, including alterations in salt levels and the presence of other metabolites. When mastitis occurs, the body responds with increased white blood cells and changes in the blood supply to the udder, resulting in changes to the milk's flavor profile, including a salty taste.

In the context of milk quality, mastitis is a significant concern because not only does it affect the flavor, but it can also impact the overall quality and safety of the milk produced. The other options, while they may influence milk quality, do not directly account for the specific salty flavor as clearly as mastitis does. High salt content can occur in milk for various reasons, but it is often not the predominant issue unless deliberately added or as a result of other factors. Feed contamination and bacterial growth can certainly affect milk quality, but they are less specifically tied to the salty flavor compared to the physiological changes induced by mastitis.