Albert Barnes Commentary Psalms 78:71

"From following the ewes that have their young he brought him, To be the shepherd of Jacob his people, and Israel his inheritance." — Psalms 78:71 (ASV)

From following the ewes great with young - Margin, as in Hebrew, "From after." This means that he followed them; that is, he attended them or watched over them as a shepherd.

The single word rendered “the ewes great with young” - עול ‛ ûl - is a participle from עלוה ‛ âlâhô, meaning to ascend, to go up, and then to bring up or nourish. The exact idea here is undoubtedly that of bringing up or nursing them, and the word should have been translated this way. Luther, in fact, renders it so.

The significance is that God brought him from being a shepherd to be the ruler of his people - a transition still expressed in the language of a shepherd's life.

To feed Jacob his people - This means, rather, to be a shepherd to them: to fulfill for them the duties of a shepherd, which include governing, providing for, and defending them. See the notes at Psalms 23:1-2.