Are they bred? Or do you just not like MLV. Just wondering why the change? MLV is all I use, I know it's all debatable. Killed says booster 14 to 28 later so 2 rounds ID say most don't do. MLV booster annually so 1 round unless you booster the spirovac which I'd say most don't do.
I have a small herd so I figure the unused MLV will be wasted but I refrig unused killed and use the next year, it should still be good right?
That is not recommended.
First, did you use a sterile needle each time you recharged your syringe? If the answer is not, then you introduced microorganisms into the vaccine.
Second, even if you used a sterile needle each time you recharged your syringe, bacteria suspended in particulates settle on the seal and are pushed into the vaccine by the sterile needle.
Third, it is a law of physics, elements seek their state of lowest energy. Meaning that compounds breakdown. A year is pushing it. If you hold an attenuated or killed bacterin or a toxoid for a few weeks, you are not taking on too much risk but if you are holding for a year, the risk that the vaccine is compromised is great. Spend "A Few Dollars More" and drop the risk!