And it came to pass, whensoever the stronger cattle did conceive,.... Whose limbs were well compact, and were strong and healthy:that Jacob laid the rods before the eyes of the cattle in the gutters,that they might conceive among the rods; and bring forth party coloured ones, and such as were robust and strong like themselves; and this was another device of Jacob's to get the best of the flock.
Aben Ezra thinks this refers to the two seasons of the year, when the flocks conceived; the one was in Nisan, in the spring, and such as were brought on that conception were strong, and therefore Jacob chose to lay the rods in the gutters at that time, that he might have the best cattle; and so the Targum of Jonathan calls these here the forward ones, as it does those in Ge 30:42 we render feeble, the latter ones; which, according to Aben Ezra, conceived in Tisri or September, and what they brought were weak and feeble.
(Based on the laws of genetics, Jacob's breeding of the best of the flock would produce higher quality offspring. Ed.