You are thinking of somatotropin, a hormone banned in Canada. For price comparison, we also have to look at milk without somatotropin. Canadian milk does not have the hormone, unlike American milk.
We must also keep the climate in mind. Canada is a northern country. In addition, U.S. farms are significantly larger, and in some states, we are talking about industries. Canadian farms are family farms with an average of 78 cows per farm.