Spring nitrogen and plant growth regulator effects on seed yield of orchardgrass

Orchardgrass (Dactylis glomerata L.) is an important forage seed crop, but unlike other cool-season grasses, seed yields have not increased over time. Seed yield increases in orchardgrass may be possible with plant growth regulators (PGRs) such as trinexapac-ethyl (TE) and chlormequat chloride (CCC). Field trials were conducted at Hyslop Experimental Farm near Corvallis, Oregon, over three crop years (2017–2019) to examine the effects of spring nitrogen (N) and PGRs on seed production characteristics in orchardgrass. Spring N treatments included 0, 112, 157, and 202 kg N ha⁻¹ and PGR applications were timed using the BBCH (Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie) scale. Four PGR treatments included an untreated control, 210 g TE ha⁻¹ at BBCH 32, 210 g TE ha⁻¹ at BBCH 51, and 105 g TE ha⁻¹ + 1500 g CCC ha⁻¹ at BBCH 32. An interaction of spring N and PGR increased seed yields in 2 years, while spring N and PGR increased seed yield independently in the other year. The combination of TE and CCC PGRs did not increase seed yield over TE alone. Seed yield increases from spring N were due to an increase in seed number m⁻², while increases in seed yield attributable to PGRs were the result of increased seed number m⁻² and harvest index. This study suggests that the combination of 112 kg ha⁻¹ spring N and 210 g ha⁻¹ TE PGR is the best practice to maximize seed yield in orchardgrass.