<ref name="Velthuis_&_Wiebes_1977">{{cite book |first1=H. H. W. |last1=Velthuis |first2=J. T. |last2=Wiebes |year=1977 |title=Proceedings of the Eighth International Congress of the IUSSI, Wageningen, The Netherlands, September 5-10, 1977 |url= |location=Wageningen |publisher=Centre for Agricultural Publishing and Documentation |pages=325 pp. |isbn=}}</ref>