The effectiveness of 5-bromo-3-sec-butyl-6-methyluracil (bromacil) for the selective removal of orchardgrass (Dactylis glomerata L.) from Kentucky bluegrass (Poa pratensis L.) turf and some physiological responses of the grasses to bromacil were evaluated in field studies. Bromacil reduced photosynthesis to a greater extent in orchardgrass than Kentucky bluegrass turf. Foliage carbohydrates were depressed initially in bluegrass followed by recovery in 2 weeks to 1 1/2 months following bromacil application. Orchardgrass, however, did not recover. Application of 0.56 kg/ha bromacil each summer and fall for 2 years gave the best control of orchardgrass. During the second year, increase in foliage crude protein was observed in treated bluegrass. Bluegrass foliage yields were equal to or greater than the controls. Orchardgrass yields were drastically reduced by bromacil. Effective seedling orchardgrass control was obtained with minimal bluegrass injury at 0.14 to 0.28 kg/ha of bromacil. Bromacil treated areas were effectively fall-seeded with bluegrass approximately 8 weeks after a 0.56 kg/ha application, Rooting of bluegrass sod was not inhibited at the rates used for selective control of orchardgrass in bluegrass turf. In controlled environmental growth chamber studies, bromacil had no effect on germination of Merion Kentucky bluegrass or Virginia Common orchardgrass, but reduced the growth of emerging shoots of orchardgrass more than bluegrass. Fresh weight gain, root growth, and transpiration were reduced in orchardgrass plants at 0.125 ppmw bromacil whereas 1.0 ppmw gave reductions in bluegrass. Photosynthesis was inhibited initially in both grasses, however, bluegrass recovered in 6 days. Foliage carbohydrate content was greater and was affected less by bromacil treatment in bluegrass than orchardgrass. Root absorption and translocation of 2-¹⁴C-bromacil to the shoot was directly related to transpiration in both grasses. Bromacil was absorbed and translocated acropetally in sheath and foliar treatments in both grasses. Higher metabolic conversion of 2-¹⁴C-bromacil occurred in bluegrass as compared to orchardgrass with 1.0 ppmw treatment. The major metabolites were 5-bromo-3-(2-hydroxy-1-methylpropyl)-6-methyluracil and an unknown. Trace amounts of 3-sec-butyl-6-methyluracil and 5-bromo-3-sec-butyl-6- hydroxymethyluracil were also detected. Bluegrass tolerance to bromacil involves high carbohydrate levels in tissues, hydroxylation of bromacil, and rapid recovery of photosynthesis. / Ph. D.