From manure (verb) + -age. <span class="searchmatch">manurage</span> (uncountable) (obsolete) Cultivation of land. 1602, William Warner, “The Third Booke. Chapter XIIII.”, in Albions...
Compare <span class="searchmatch">manurage</span> and Old French manouvrement. manurement (uncountable) (obsolete) cultivation 1651, Henry Wotton, A Philosophical Survey of Education:...