Diagnosing herbicide-resistant weeds as a first step in resistance manageme
nt and monitoring their nature, distribution, and abundance demands efficie
nt and effective screening tests. This review summarizes and recommends app
ropriate seed sampling techniques, protocols for screening weeds for resist
ance to herbicides of different sites of action, interpretation of results,
and information given to the grower. Elements common to all screening proc
edures are reviewed. Choosing appropriate discriminating doses to distingui
sh between resistant and susceptible weed biotypes is the most important fa
ctor in achieving accurate and consistent results. Interpretation of result
s is also critical because resistant weeds may comprise a small portion of
the population in suspected accessions or biotypes.