Cover crop effects on wheat yield, nitrogen use efficiency, and soil health in a dryland wheat system

Abstract

Cover crops (CC) are increasingly adopted in dryland wheat systems, yet their agronomic and soil health impacts under semi-arid conditions with limited and variable precipitation remain poorly understood. This field trial at the Southern Agricultural Research Center in Huntley, Montana, evaluated six CC treatments and a fallow control, at four nitrogen (N) rates (0, 56, 112, and 168 kg N ha ⁻¹), on spring wheat grain yield, grain protein concentration, agronomic nitrogen use efficiency (ANUE), and soil health indicators across five growing seasons. Inter-annual weather variability was the dominant driver of all agronomic outcomes, substantially outweighing the influence of any management factor tested. Effective precipitation (EP) captured much of this variability; however, two years with nearly identical EP (2021 and 2024) produced notably different outcomes, indicating that precipitation timing or in-season distribution contributed to variation not fully explained by seasonal EP totals alone. Cover crop reduced wheat yield relative to fallow on average, although the magnitude depended on CC type and annual EP. N rate improved yield and protein with diminishing returns at higher rates, and ANUE was consistently lower under CC than under fallow when averaged across years and N rates. Grain protein responses were driven primarily by year and N rate, with wheat protein following pea averaging 7% higher than following fallow. Soil health indicators showed limited responsiveness to CC management over the study period, with soil depth explaining more variation than CC treatment or N rate. Neither CC type nor N rate produced consistent improvements across the soil health metrics evaluated, except for aggregate stability. These findings highlight a fundamental challenge for dryland producers: CC performance and N response are governed primarily by precipitation, a resource that is variable and difficult to predict. In environments where a single dry year can shift the agronomic and economic outcome of CC adoption, understanding how seasonal moisture mediates CC and N fertilization responses is essential. This study provides baseline evidence for that understanding in semi-arid Montana dryland systems.