Plant growth regulator and spring nitrogen effects on grass seed crops in western Canada

Abstract Plant growth regulators (PGRs) are used in herbage seed crops to mitigate lodging risk and protect seed yield, though their effects on aging perennial grass stands remain understudied. This study evaluated the effect of PGRs on the agronomic characteristics of herbage seed crops of creeping red fescue ( Festuca rubra L.), meadow brome ( Bromus riparius Rehmann), and timothy ( Phleum pratense L.) in their third and fourth years after establishment in western Canada during 2021 and 2022. A split‐plot experimental design comprised PGR treatments of trinexapac ethyl (TE), chlormequat chloride (CC), and ethephon (ETH) as main plots with paired subplots of spring nitrogen (N) top‐dressed at 40 kg N ha −1 and an unfertilized control. TE, CC, and ETH were applied at 0.20, 1.12, and 0.60 kg ha −1 active ingredient, respectively, at the Biologische Bundesanstalt, Bundessortenamt and Chemical industry (BBCH) 31‐32 crop development stage. TE reduced internode length, tiller height, and lodging severity in meadow brome and timothy, and increased culm diameter in creeping red fescue and timothy. However, no PGRs enhanced seed yield, seed weight, and seed density. Spring top‐dressed N increased biomass and seed yield in timothy and meadow brome, and seed density in all grasses, with no interaction between PGRs and supplemental N. Despite modifying plant morphological attributes and reducing lodging risk, PGRs did not consistently improve seed yield under dry to average soil moisture conditions. Future studies should examine weather variables and plant physiological thresholds that contribute to lodging risks to better inform the use of PGRs.