Colostrums is mother Calf specific, meaning the first milk from the mother has benifits only for her calf.

Colostrum will benefit any newborn of the same species and can even sometimes benefit other species. It is of most benefit against the diseases that have been encountered by the mother within her herd.

To cut losses, don't buy from a sale barn. Sale barns are a major source of disease, becase often animals that are sent there are not completely healthy.

Buy directly from a farmer who doesn't want to raise the calves. Get all your calves from the same source if possible. Mixing animals from different herds is a great way to spread to all the calves the diseases any of them may have been exposed to.

If you can get the colostrum from the same source then this is better still.

Antibodies from the colostrum which is what you need, is absorbed through the stomach lining and that is permeable only for about 6 hours after birth. This is according to my vet. I would strive for 6-8 hours when we had the dairy. After that it is only nutrition. If a calf has missed the colostrum that first critical time frame, I wouldn't bother with it (imo). Raising baby dairy calves is very risky unless you buy from the dairy direct which is the only thing I would do! period. Then only after they are three days old during which time the dairy should be feeding colostrum to it.

I never used nuflor, but I would use something day one. Also that first day, feed reduced amount of dairy replacer. Day two increase the strength until you get the feed up to normal by day three. I mean strength, not volume. Keep on hand electrolytes for scours and an antibiotic for secondary infections. Try not to stress the calf for a few days, moving it, etc.

Good luck. I have raised 100's both from a local farmer and our own when we dairied. If I ever had a 50% loss, I would have been very, very disappointed.

The best advice is stay away from the sale barns!

Good advice, above, from gldno1 & sharon.

You don't say what the 50% of your calves succumbed to, but from experience, I'll hazard a guess that they scoured and died?
You don't want Sourguard, you want CalfGuard - it's a modified-live rota/coronavirus vaccine that you administer orally.

I don't recommend treating with any antibiotic as a 'preventative', but do recommend providing a dose of Vit.A - and Vit. E/Selenium solution, if selenium deficiency is a problem in your area. All of these are intimately involved in normal function of the immune system.

A good, high-quality calf starter ration, containing Deccox to help control cryptosporidiosis, should be made available to the calves from the time you get them, and each time you pass by, you oughta cram a handful of calf starter in their mouths, until they get the idea, and begin eating it readily on their own.

The cattle in feedlots are fed rations that are pretty much "death to calves" so they are already in trouble when you get them. Granted you can still raise these calves, but they will grow slower. Are you buying directly from the feedlot itself? If so maybe make an arrangement with them and say you will pay more for the calf once it has reached a month old, but if it dies then, no pay. Even if you give the calves colostrum when you get it, if it hasn't had it in time, it could still look good for a couple weeks and then just die! Best of luck,
S.