Nick says that he grew up with Mariano, and that they played together as boys. Apparently, Mariano's mother scrubbed floors and worked in the Barkley home.
When Mariano asks his men, and later Nick, to look at the herd and see whether they look sick, the exact same shot is used three times of a few cows with two mounted hands riding in the background. Previously in the episode, Nick states that all the herd is comprised of calves; however, in this shot, they are full-grown cows.
When the anthrax vaccine is administered to the bull, it's given after he's shown signs of infection. However, when Pasteur's vaccine first came out in the 1800's, it was given as a preventive measure by injecting a small dosage of microbes into the livestock before they were exposed to the infection, and thereby giving their systems time to build resistance. In reality, the way it's administered here would have zero effectiveness and the bull would have died.
This episode acts as though immunizations are brand new, untried things. The smallpox immunization was developed in 1790, almost 100 years prior to this episode! British physician Edward Jenner tested the possibility of using the cowpox vaccine as an immunization for smallpox in humans for the first time. By the 1830's, a generation had been inoculated, and there was a marked decline in smallpox cases within the United States. Most soldiers were given inoculations or vaccinations, and with all the boys having been in the army, they couldn't have not known.