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PostCrossing the Border with Your Cows; "Butter Smuggling" (John Heelan, UK, 04/12/19 6:17 am)
Farmers near the border between the Irish Republic and the North used to drive their milking herds across the border to take advantage of the higher prices for dairy products in the North.
Likewise, "Butter smuggling" between Holland and other countries was a well-known ploy adopted as a result of the disastrous EU Common Agricultural Policy (CAP) from which I suffered personally in the 1970s.
JE comments: John Heelan went into detail about his dairyman days in this post from 2009. He mentioned the sad fact that in the 1970s, many UK dairy farmers ended up bankrupt, or (worse) dead from suicide.