Genesis 30:37-39

37But Jacob took fresh-cut branches from poplar, almond, and plane trees. He made white streaks by peeling them, making the white inner wood in the branches visible.

38Then he set up the peeled branches in all the watering troughs where the flocks came to drink. He set up the branches in front of the flocks when they were in heat and came to drink.

39When the sheep mated in front of the branches, they gave birth to young that were streaked or speckled or spotted.