Response of direct-seeded guayule to preemergence herbicides

Direct-seeding guayule (Parthenium argentatum Gray) in west Texas has been successful; however, seedlings grow slowly and are extremely susceptible to weed competition. An effective preplant or preemergence herbicide is necessary for establishment. The objective of this study was to examine the effects of preemergence herbicides on guayule stand establishment by direct seeding. Experiments were initiated on a Dalby clay in 1991 and 1992 at the Texas Agricultural Experiment Station Guayule Research Site near Fort Stockton, Texas. Conditioned guayule seed (Mexican Bulk) was planted 10 mm deep on raised beds with a Gaspardo SV255 pneumatic planter at 100 seeds/m. The following herbicides were applied immediately after planting: dimethyl 2,3,5,6-tetrachloro-1,4-benzenedicarboxylate (DCPA) 4.5, 9.0, 11.0 kg ai/ha (hectare)], pendimethalin N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] (0.3, 0.6, 1.1 kg/ha) and prodiamine N³,N³-Di-n-propyl-2,4-dinitro-6-(trifluoromethyl)-m-phenylenediamine] (0.3, 0.6, 1.1 kg/ha). Herbicides were applied in a 0.5-m band on each bed by using a CO₂-powered backpack sprayer with a single-nozzle boom delivering 300 L/ha at 172 kPa pressure. The lowest rates of all herbicides did not significantly reduce guayule stand density in 1991 when compared to the control. DCPA at 9.0 kg/ha was adequately selective in both studies. Guayule stand reduction varied from only 2% with DCPA at 4.5 kg/ha in 1991, to 71% with 1.1 kg/ha of pendimethalin in 1992. DCPA (4.5 and 9.0 kg/ha) would be recommended for preemergence weed control, depending on soil type, when direct seeding guayule. Based on row spacing recommendations for transplants, at least three established seedlings/m would be required for an acceptable guayule stand. Seeding rate could be reduced and still allow for effective preemergence weed control.