For the experiment to be a success, the selected animals had to be twins so it could be ascertained exactly which one produced the most milk fat on a given forage diet.
施行「牛胃開窗」手術的牛，必需是成對的，而且都是雙胞胎，生理條件相同，這樣才能在分別餵食不同飼料後，比較出那種飼料可在牛體中產生更多的乳脂肪，使乳牛增加產乳量。