stress that prevents it from getting up to nurse very well. As a result, he says, “We’re starting at a defi cit in a set of calves. Not all of them are carrying the same level of protection.” Another problem involves the variability in the colos-
trum of each antigen, which stimulates the cow’s body to produce antibodies. Spire says, “If a cow has been exposed to red nose (infectious bovine rhinotracheitis or IBR) or BVD, or to a pinkeye antigen or blackleg, that’s what goes into the colostrum.” Those protections dissipate but at different lengths
of time after a calf has nursed. Spire says IBR and BVD antigens persist longer than those of other viruses that are very short in duration. The antigen for blackleg, which is most often caused by the bacterium Clos- tridium chauvoei, may only last 2 months. So one by one, from the calves’ birth to about 6 months of age, they are losing protection from each disease that they
tscra.org
received from the colostrum. At this point in time, says Spire, the calves have to
achieve “active immunity” — they have to be exposed to pathogens so they can develop their own antibodies. This is most commonly done via vaccination. “It’s very common to take spring-born calves, around
2 months old, and vaccinate them for various diseases,” he says. “The problem with that, since we have a mixed population of protected and unprotected calves, is the calves that are unprotected can respond pretty well to the vaccines, but the calves that are protected still have colostral antibodies there.” Those colostral antibodies block the vaccine so that
it does not work as well as it should, so by the time the calves reach 4 to 6 months of age, the rancher again has a dilemma. “We still have a very vulnerable set of calves there, even though we think we’ve done the right thing and vaccinated them,” Spire says.
August 2013 The Cattleman 81
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