Van Sittert, Sybrand J.; Drew, Tessa M.; Kotze, Johann L.; Strydom, Tom; Weyer, Camilla Theresa; Guthrie, Alan John(OpenJournals Publishing, 2013-09-13)
Howell, P.G.; Nurton, Jane P.; Nel, Daleen; Lourens, Carina W.; Guthrie, Alan John(Agricultural Research Council, ARC-OVI and the University of Pretoria, 2008-06)