ABSTRACT
Summary The aim of grassland reseeding is to establish a high-quality sward of improved herbage cultivars. To obtain the maximum value from the improved sward, weed invasion should be prevented. Herbicides are the main tool available. There are many herbicides for selectively controlling dicotyledonous, but not graminaceous, weeds. The control of graminaceous weeds would be possible in swards of grass cultivars selected for tolerance to grass-killing herbicides. Two cycles of recurrent selection for glyphosate tolerance in Loliurn perenne L. were carried out by spraying large seedling populations with critical dosages in the greenhouse and intercrossing the survivors. It was found that two populations selected in this way were about 50% and 67% more tolerant than two control cultivars. To combine the paraquat tolerance of the L. perenne cultivar Causeway with the agronomic characters of cultivar Aberystwyth S 23, crosses were made between the two cultivars and selection for tolerance was practiced on seedlings of the F2, F3, and F4 generations. The F3 and especially the F4 were found to be even more tolerant than Causeway. These results illustrate the feasibility of breeding for herbicide tolerance in grasses. Herbicide-tolerant cultivars would facilitate the establishment and maintenance of weed-free pastures and seed crops, the maintenance of varietal purity, and the distinction of the cultivars from others in the species.
