1. a Department of Agronomy and Horticulture , University of Nebraska—Lincoln , Lincoln , NE , 68583-0914 , U.S.A.
2. b Department of Agronomy and Horticulture , University of Nebraska—Lincoln , Lincoln , NE , 68583-0914 , U.S.A.
3. c Department of Biometry , University of Nebraska—Lincoln , Lincoln , NE , 68583-0712 , U.S.A.