Ignorance Leo, let 's put an example ( I use the dairy farmer as an example because I 'm familiar with it, many of my relatives are and have been dairy farmers for generations and I 've worked with some of them ), the dairy farmer makes it 's livelyhood out of milk, to him cows are not only a source of income but also are viewed as milk producing factories on four legs. In order to produce milk the animal has to get pregnant and deliver a calf ( otherwise it doesn 't produce milk ), but that "simple" process is not as easy at it seems and it cost money, every day the cow is not pregnant is costing him food, shelter, workmanship, property taxes ( not upon the cow but upon the land ) and so on; farmers may posses a lot of empiric knowdlege about cattle, you may add to that empiric knowledge a little scientific knowledge provided by us vets or by pharmaceutical companies however you seldomly see a dairy farmer who is a veterinarian.
The entire universe of a dairy farmer begins at the dairy farm door and ends at the same place, they don 't know and don 't care to know about waterways, ecosystems n 'all that stuff, all they want to know is about cows, sheep or goats producing milk to make his livelyhood so in a way we vets and pharmaceutical companies have part of blame because we only circusncribe our knowledge to provide the farmer with the elements he need in order for the animals to produce milk.
Dang ! you didn 't notice the cow was in heat so you can 't inseminate it right now ..... no problem, I grab a bottle of Lutalyse ( a synthetic prostaglandin ) take out a dose, inject it to the cow and in about 72 hours the cow will be in heat and ready for insemination. But what happens to the product once it 's done the job it has to do ? it 's excreted whole or as a methabolite in the cow 's urine, falls on the floor and nobody worries about it anymore.
One of my uncles has a 250 in-line cow dairy farm, that means he 's got at least twice as much animals, you know how many times I perform the procedure I described in a week ? at least 10 times a week on average, so it 's just a matter of multiplying, if that procedure takes place only in one dairy farm how many times it 's done in a region of dairy farms ? thousands of times.
But we have rain, it rains and the product that nobody worried about is washed from the dairy farm floor and it goes into the soil and if it rains more then it leaches from the soil into the waterways, do that on hundreds of dairy farms and you are talking about pounds of a very powerful medication leaching into the waterways. But we ignore that ( not willingly most of the times ) because our entire universe is circunscribed to the dairy farm we own or we work on.