- Perennial ryegrass (Lolium perenne L.) is a widely grown cool season, bunch type, short lived perennial grass that is native to Europe, Asia, and North Africa, but has been adapted globally. Perennial ryegrass is quick to establish, has a long growing season, and produces high seed yield and forage or turf quality (high density, fine leaves, and dark green color). Mesotrione is a selective 4-hydroxyphyenyl pyruvate dioxygenase (HPPD) inhibiting herbicide used for weed control in perennial ryegrass seed production and turfgrass in the USA. HPPD inhibiting herbicides cause a reduction in carotenoids which protect chlorophyll from high energy light. The photodegradation of chlorophyll results in a bleaching of leaf tissue in susceptible plants. In seed production, mesotrione, commercialized in the USA as Callisto®, is applied postemergence in the spring to fall planted perennial ryegrass seed crops. Mesotrione for use in turfgrass, commercialized in the USA as Tenacity®, is applied pre or postemergence. The effects of a postemergence application of mesotrione on chlorophyll concentration, dry biomass, plant height, and seed yield and germination compared to an application of another HPPD inhibitor, pyrasulfotole-bromoxynil, on perennial ryegrass cultivars were evaluated. There were differences in perennial ryegrass cultivar response to a postemergence application of both HPPD inhibitors in chlorophyll concentration and to a postemergence application of pyrasulfotole-bromoxynil in dry biomass. No reduction in plant height was caused by the HPPD inhibitors. Seed yield of the perennial ryegrass cultivars was not negatively impacted by a postemergence application of mesotrione. However, only twelve perennial ryegrass cultivars were evaluated in this study; therefore, caution may be warranted when using mesotrione with other perennial ryegrass cultivars. The effects of mesotrione applied postemergence on perennial ryegrass cultivars at three different temperatures, 5, 25, and 35°C, on chlorophyll concentration, leaf color, and total chlorophyll were investigated. At each temperature, there was a response within some cultivars. The cultivar ‘Derby Xtreme’ exhibited the most sensitivity to mesotrione at 5°C and ‘SR 4660ST’ was the most sensitive at 35C. The cultivar ‘PR 8821’ was the most sensitive overall at all evaluation timings and temperatures. The cultivar exhibiting the most tolerance to mesotrione was ‘Esquire’. Assessing perennial ryegrass cultivar response to mesotrione at one temperature versus another temperature resulted in ‘Allstar 3’ exhibiting less sensitivity at 35 than at 5 or 25°C. The majority of cultivars exhibited more sensitivity at 25 than at either 5 or 35°C. Turfgrass managers should use caution when applying mesotrione postemergence to some perennial ryegrass cultivars if temporary bleaching of the turf is undesirable.