The bright red blood is oxygenated fresh blood. As the lining of the uterus becomes older (the underlying layers) the blood along with tissue and other cells/fluids contains less oxygen so they...
The blood is more brown tinged toward the end of the period due to the amount of "old" blood being in the mix. The brown is brought on by dead red blood cells and bilirubin content.
Cows DO NOT menstruate. This is solely a human reproductive physiology characteristic, not a bovine reproductive physiology characteristic. Cows have an Estrous cycling period, NOT Menstrual period....