1. a Panhandle Research and Extension Center , 4502 Ave I, Scottsbluff , NE , 69361-4939 , U.S.A.
2. b Department of Agronomy , University of Nebraska , P.O. Box 830915, Lincoln , NE , 68583-0915 , U.S.A.
3. c Department of Biometry , University of Nebraska , P.O. Box 830712, Lincoln , NE , 68503 , U.S.A.