@peteski - I'm sure you're right about the music wire being overkill. But they worked well. And this reiterates the conclusion of this thread: there are maybe dozens of materials that would work fine for pasture fence posts
Anyhow, I sunk the first section of fencing on Friday night, then went out of town camping for the weekend. I spaced these posts at 20' apart, but after a weekend driving through Napa-Sonoma countryside, and observing fence after fence, I quickly realized that pasture fencing has post-spacing typically more like 7-10'. So I'm just going to add extra posts to make for 10' spacing.