1. Dep. of Agroecology; Aarhus Univ.; Research Centre Foulum; P.O. Box 50 DK-8830 Tjele Denmark
2. Faculty of Agriculture; Payame Noor Univ.; Golestan Blvd. Ahwaz Iran
3. Division of Agricultural and Environmental Sciences; Univ. of Nottingham; LE12 5RD Nottingham UK