Tonto National Forest officials say workers plan to begin installing 4 miles of steel fencing along and near the lower Salt River east of metro Phoenix Monday.
They say the fence is needed to prevent wild horses from crossing the Bush Highway and to keep livestock from mingling with the horse herd.
Officials said the fence work is the second phase of a project that saw workers on Friday complete installation of 10 miles of barbed wire fencing. That work began in November.
U.S. Forest Service and state Department of Agriculture officials said previous fences didn't prevent horses from getting onto the highway, resulting in the deaths of 20 or more horses each year.
The project includes installing 35 gates.