Genesis 30:37-39

37Now Jacob took for himself rods of green poplar and of the almond and chestnut trees, peeled white strips in them, and exposed the white which was in the rods.

38And the rods which he had peeled, he set before the flocks in the gutters, in the watering troughs where the flocks came to drink, so that they should conceive when they came to drink.

39So the flocks conceived before the rods, and the flocks brought forth streaked, speckled, and spotted.