Nitrogen fertility rates affecting weed population dynamics in a perennial ryegrass stand in western Oregon

Few projects have explored the long-term effects of N rates on multiple weed populations in the Pacific Northwest. The objectives of this research were to evaluate the effects of N application timing and annual N rates on weed population dynamics in a perennial ryegrass (Lolium perenne L.) stand in western Oregon. Factors in this project included N application timing [spring–heavy (April, May, June, and October), fall–heavy (May, September, October, and November), early spring and late fall (April, June, October, and November), and traditional spring and fall (May, June, September, and October)] and annual N rates (75, 150, and 300 kg N ha⁻¹ annually). An untreated control was also included in this project. The initial findings suggest that after 18 mo without herbicides, the highest annual N rate had the highest annual bluegrass populations but the lowest moss and broadleaf weed populations. The spring–heavy fertilization program at the low annual rate resulted in higher broadleaf weed populations than all other N application timing × N rate combinations. Data will continue to be collected from this experiment to determine the effects of N application timing and annual N rates on weed population dynamics.