What's the Name of That Book??? discussion

I read this book in the early 2000s, and I vaguely remember that my edition had a red cover with a horse on it. The story follows a family traversing the American plains in covered wagons that has a mare and a foal. There's a young mother in the group, and at one point, there is a big drought, and they are forced to walk to limit water intake for the horses. They eventually run out of water and food, and the mother can't produce milk anymore for her baby, so they take the milk the mare has for the foal and feed it instead. I can't remember if the foal dies or not, just that everyone feels bad as it grows weaker. This part happens somewhere in the beginning or middle. That's pretty much all I can recall. I remember when I did get the book that it was already older, so maybe from the 70's or 80's?

Mustang: Wild Spirit of the West by Marguerite Henry. At least one of the editions has had a reddish cover.

pg. 16:

"My baby yelled at first, hour after hour. Then he whimpered. And in no time it seemed he got all shriveled looking, like a little old man. One day your Grandpa says, 'There ain't no other way. Nelly's got to be Baby Joe's nurser.' And he milked Nelly and we spooned her milk into the baby's mouth, a slow drop at a time."
[...]
"Your Grandpa was glad Baby Joe was saved. But a few days later a misery came over him when he told me he had to kill Nelly's colt. 'I got to do it,' he said, his eyes full of hurt. 'She ain't got enough milk for two.' Oh, Annie, it hurt him so. He'd rather of shot off his right arm. 'I'll just wait till morning,' he said, 'and then I'll do it. Painless as I can.'"

Thank you so much! I’ve been wracking my brain for this title forever!!

You're welcome!