A split application approach to nitrogen and growth regulator management for perennial ryegrass seed production

Experiments were conducted during 2009 and 2010 in first year perennial ryegrass (Lolium perenne L.) fields under limiting or adequate spring N. Three N application methods (single, two-split, and three-split), five growth regulators (split and single applications of prohexadione Ca or trinexapac ethyl) and two spring N rates (56 or 100 kg N ha^-1) were evaluated in a split-split plot design. Under adequate spring N, all N application methods resulted in similar seed yields but when spring N was limiting, a single application of N resulted in up to 14% greater yield vs. the three-split. The three-split N application improved relative chlorophyll index (RCI) 6 to 20% late in the harvest season and reduced stem rust (Puccinia graminis Pers. subsp. graminicola) area under the disease progress curve (AUDPC) by 18 to 39% vs. the single N application. Other effects of three-split N applications observed in 2010 were 8 to 19% less biomass, 2 to 3% greater harvest index (HI), 4% greater seedling vigor, and 25% less lodging. The split prohexadione Ca treatment gave the most consistent results and when vegetative growth was greatest it resulted in a 36% increase in seed yield, a 5.5% increase in HI, 14 cm shorter plants, 70% less lodging, 30% greater RCI, 13% lower stem rust incidence, and 3% lower stem rust severity. Development of a model to optimize rates and timings for split N and growth regulator applications based on growth stage, soil N availability, and growing degree days (GDD) could improve efficacy of split applications.