The Collaborative International Dictionary of English v.0.48:

Serfage \Serf"age\, Serfdom \Serf"dom\, n.
   The state or condition of a serf.
   [1913 Webster] Serfhood

WordNet (r) 3.0 (2006):

serfdom
    n 1: the state of a serf [syn: serfdom, serfhood,
         vassalage]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "serfdom":
   abjectness, absolutism, attendance, baseness, bond service,
   bondage, captivity, control, debt slavery, deprivation of freedom,
   disenfranchisement, disfranchisement, domination, employ,
   employment, enslavement, enthrallment, feudalism, feudality,
   helotism, helotry, indentureship, meanness, menialness,
   ministration, ministry, peonage, restraint, serfhood, servility,
   servitium, servitorship, servitude, slavery, slavishness,
   subjection, subjugation, submissiveness, subservience,
   subserviency, tendance, thrall, thralldom, tyranny, vassalage,
   villenage, yoke