Horseweed is a common pest in vineyards of the San Joaquin Valley (SJV) ofCalifornia. Interest in controlling this weed has increased with the recentdiscovery of a glyphosate-resistant (GR) biotype that has been observed tobe more vigorous than a glyphosate-susceptible (GS) biotype in the SJV.However, the impact that either biotype may have on grapevine growth has notbeen assessed. Therefore, two glasshouse experiments were conducted tocharacterize the competitiveness of GR and GS horseweed biotypes from theSJV with young grapevines. ‘Syrah’ grapevines grafted to Freedom rootstockswere planted in 8-L plastic pots, alone, or with a single GR or GShorseweed. Additional GR and GS horseweeds were also planted separately inindividual pots, and all plants were grown for 14 and 16 wk in 2006 and2007, respectively. Grapevines grown with either biotype of the weedproduced fewer leaves and amassed approximately 20% less dry mass (DM) thanvines grown alone. The GR biotype reduced grapevine stem DM and length by30%, but the GS biotype did not. The GR biotype accumulated more than twicethe DM as the GS biotype, whether in competition with grapevine or not.Grapevines reduced the total leaf number of both horseweed biotypes byalmost 50% and aboveground DM of GR and GS biotypes by 50 and 75%,respectively. These preliminary findings indicate that competition fromhorseweed can substantially reduce the growth of young grapevines and thatthe GR biotype may be more competitive than the GS biotype.