Up here in the NE we use either Cornell or Idexx for labs. Idexx is about 15 minutes away.
Johnes, pronounced "yo-knees" aka Mycobacterium Paratuberculosis attacks the villi of the small intestine, they are finger like things that protrude to increase surface area for absorption. The bacteria break this down and they can't absorb all their nutrients, therefore causing slow weight loss and chronic diarrhea. It is very similar to Chrones in humans. One of the difficulties is that the bacteria isn't always shedding and you can get some false negatives when testing. Many cows in dairy herds are culled due to low production before they test positive to Johnes. There is no cure.
BVD, you need to make sure she isn't a PI cow, that's persistently infected. If a cow gets BVD at a certain period in her gestation she will produce a PI calf. Because of tha exposure at a critical time, the calf's immune system never recognizes the disease as forgein and therefore never makes antibodies against it. They will shed the virus out of every orifice for the rest of their life infecting other cows. Fortunately you can test it and cull them. We had a holstein heifer that was a PI calf, came from a reputable breeder too. One of the things they stressed back in school is testing animals before bringing them in your herd, I went to school for dairy and this was especially important in those types of facilities. A disease though a large herd can be very expensive.
Hopefully it's a nutritional problem, but rule out these diseases with some tests.
Acidosis is from two high of a concentrate to forage ratio. Too much starch and sugar will leed to the bacteria multiplying like mad. The by product of fermentation is acids. This lowers the pH of the rumen causing acidosis. This can then lead to laminitis, aka sore hooves. When the pH gets too low the bacteria start dying, when they die the create toxins. The bodies response to toxins is histamines, histamines cause blood vessels to swell. In the hoof there isn't room for the swelling and you get sore hooves.
Coccidiosis can come from birds crapping in the water, Ionophores (Rumensin) also act as a coccidiostat.
The rumen also needs to adjust to a change in feed, it takes 2-3 weeks for the bugs to change to the new feed. Different bugs break down different things. You shouldn't abruptly change, you need to mix the two different feeds to transition them for best results. If you have a cow that starts getting the squirts when switching to pasture, then put her back on to the old diet, then put her back on the grass diet she isn't getting that adjustment time.
