1. Dep. of Crop and Soil Sciences North Carolina State Univ. Campus Box 7620, 101 Derieux Place Raleigh NC 27695 USA
2. Dep. of Crop and Soil Environmental Science Virginia Polytechnic Institute and State Univ. Blacksburg VA 24061‐0131 USA
3. The Nature Conservancy Arlington VA 22203‐1606 USA
4. Dep. of Dairy Science Virginia Polytechnic Institute and State Univ. Blacksburg VA 24061‐0131 USA