1. Center for Agrarian and Environmental Sciences; Federal Univ. of Maranhao; Chapadinha MA 65500 Brazil
2. USDA-ARS; Pasture Systems and Watershed Management Research Unit; Curtin Rd. University Park PA 16802
3. Dep. of Crop Soil and Environmental Sciences, 115 Plant Sciences Building; Univ. of Arkansas; Fayetteville AR 72701
4. Dep. of Agriculture; Univ. of Maryland Eastern Shore; Princess Anne MD 21853