1. Dep. of Biosystems and Agricultural Engineering Oklahoma State Univ. Stillwater OK 74078 USA
2. USDA‐ARS Grazinglands Research Lab. El Reno OK 73036 USA
3. USDA‐ARS Southeast Area Stoneville MS 38776 USA
4. Dep. of Farm Structures and Irrigation Ege Univ. Izmir 35100 Turkey