(CNN) On Wednesday, officials will begin rounding up 1,000 wild horses from federal land in Northern California and putting them up for sale and adoption.

Pregnant and younger horses will likely be adopted, federal officials say, but older horses will be sent to another corral where they could be sold for as little as $1 each to ranchers, horse trainers and other buyers -- including those who might ship them to slaughterhouses.

And this has animal advocacy groups concerned.

Why are these horses being rounded up?

The horses live on Devil's Garden Plateau, a protected territory inside Modoc National Forest near the Oregon border. It's home to the largest herd of wild horses in the country managed by the US Forest Service.

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