A cow could be treated with several antibiotics and each one would be permitted to be in milk at a level of 1ppm without additional consumer safety testing. Effects of the different antibiotics could be additive and this is not taken into account. Michael Hansen of Consumers Reports testified and brought attention to Congressional committees that 52 drugs are known to be used as antibiotics to treat mastitis. According to Consumer's Union, FDA had approved only 30 of those antibiotics. Milk is routinely tested for the presence of six different antibiotics.
Farmers are aware of the antibiotics being tested.
When something doesn't work for the dairy industry the FDA often helps private industry by changing the standard. This is usually done at the expense of the consumer. It seems that FDA recently approved a powerful liquid antibiotic for lactating dairy cows. Before approval, FDA allowed residues of 30 parts per billion of this antibiotic to show up in milk samples. Unfortunately, milk samples tested out at 300 parts per billion.