Tandem selection

Tandem selection is a method of artificial selection in which useful traits are selected for sequentially. For instance, one could select for both increased milk yield and increased milk fat content in cows via tandem selection by first selecting those with the best of one trait, say those that produce highest milk yield, and then when that trait is at a satisfactory level, by starting to select for those cows that produce milk with the greatest milk fat content instead. However, for cows to produce milk with greater fat content, yield may have to go down due to, perhaps, limits of the cows metabolism. So, while you are selecting for cows which have increased milk fat content, the yield of milk they are producing may also go down, thus reversing the selection process previously performed to increase it.