The retina is a light-sensitive tissue at the back of the eye and is crucial for vision. As RP progresses, the field of vision narrows, a condition known as tunnel vision until only central vision (the ability to see straight ahead) remains. Many people with Usher syndrome also have severe balance problems.
Calves can be ready for weaning by 3 to 4 weeks of age; however, in Penn State studies calves weaned at 3 weeks required extra attention. The increased time and labor required to get these calves eating starter suggest that waiting until 4 weeks of age would be prudent.
There are some cattle that will naturally wean their calves, but in my experience, the safest is to wean them at around 6 - 9 months. This will allow the cow to regain her strength. It takes a lot out of a cow to be supplying a very significant source of energy and protein to a calf that no longer needs it.