John Gill Commentary Genesis 30:42

"but when the flock were feeble, he put them not in: so the feebler were Laban`s, and the stronger Jacob`s." — Genesis 30:42 (ASV)

And when the cattle were feeble, he put them not in
Or "when covered", as Menachem, that is, with wool, and so not so desirous of copulation with the males, nor so fit and strong for generation; and therefore he put not in the rods into the gutters, partly that he might have none feeble in his flock, and partly that he might not spoil Laban of his whole flock, strong and weak: so the feebler were Laban's, and the stronger Jacob's ;
not only his flocks became more numerous than Laban's, but were a better quality.