Effect of plant growth regulators on physiological and agronomic characteristics of grass seed crops in western Canada

In grass seed crops, plant growth regulators (PGRs) reduce plant height and lodging severity, thereby promoting reproductive success. A study was conducted to determine the effect of PGRs applied with and without spring topdress nitrogen (N) on various physiological and agronomic characteristics of forage seed crops at Beaverlodge Research Farm in northwestern Alberta, Canada in 2021. A field experiment laid out in split-plot design comprised PGR treatment as main plots and spring N topdressing through urea at 40 kg ha-1 as subplots on three-year old stands of creeping red fescue (Festuca rubra L. ssp. rubra), timothy (Phleum pratense L.), and meadow bromegrass (Bromus riparius Rehmann). Three types of PGRs - trinexapac ethyl, chlormequat chloride, and ethephon - were applied at 0.200 kg, 1.116 kg and 0.6 kg a.i. ha-1, respectively, at early-heading stage (BBCH Scale 40-49) of crop development. The PGRs did not affect photosynthetic quantum yield (PQY), normalized difference vegetation index (NDVI), spike length, internode diameter, lodging, and seed yield of all forage grasses but significantly affected the biomass of timothy and meadow bromegrass. Trinexpac ethyl reduced plant height, internode length, and biomass in timothy and meadow bromegrass compared to other PGRs. While spring-applied N did not affect plant height, spike length, biomass, and seed yield of creeping red fescue and meadow bromegrass, N addition increased timothy seed yield. This study revealed that the use of PGRs did not have consistent positive effects on seed yield of grasses unless there is the likelihood of severe lodging. However, unusual high temperatures and intense drought in spring and summer of 2021 might have masked the effect of PGRs and nitrogen addition on seed yields. Further studies under diverse lodging-prone environments are needed to determine if the use of PGRs is beneficial for enhancing grass seed crops.