Genesis 30:41-43

41Whenever the stronger females were in heat, Jacob would place the branches in the troughs in front of the animals so they would mate near the branches,

42but if the animals were weak, he would not place them there. So the weak animals went to Laban and the strong ones to Jacob.

43In this way the man grew exceedingly prosperous and came to own large flocks, and female and male servants, and camels and donkeys.