I live in north Texas near Wichita Falls, but our conditions that time of year aren't much different than yours.
Are these cows calving out in open pastures? If they are, I'm surprised you have respiratory problems in calves that young whether or not the cows were vaccinated. I'd guess you don't vaccinate your cows though. Most people in our part of the world don't. If you are calving cows out in lots, that is where your problem is coming from.
First thing - put out some medicated mineral. That will help immediately as a preventative. Your feed stores should carry something with antibiotic in it.
You need to at least vaccinate your calves for respiratory diseases and preferably cows also. However, some vaccines can cause miscarriages if given to pregnant cows or calves nursing pregnant cows. The safest vaccine that we use is out of the Cattlemaster series. You can give it to any animal at any time. Not quite as good as some other vaccines (i.e. Vista Once), but we don't have to worry about miscarriages.
If you are like us, its impossible to vaccinate babies immediately at birth. So, you could vaccinate cows with something like Cattlemaster right now. That would improve the immunity they pass on to baby. There are better vaccines (i.e. Vista Once), but they can cause some miscarriages if given to pregnant cows that weren't already vaccinated before they were bred.
At birth or shortly thereafter, you can give the calves whatever respiratory vaccine you want. I'd recommend Vista Once, but there are lots of other options. If you don't vaccinate until after your cows are already bred, you have to be careful what you give the calves because they can pass it on to mother during nursing and cause some miscarriages. Best thing is to vaccinate cows and calves before you rebreed cows.
There are other brands on these vaccines, but these are just the ones I'm more familiar with.
