Weed Suppression in Dual-Purpose Intermediate Wheatgrass (Thinopyrum intermedium) Cropping Systems.

Intermediate wheatgrass is a cool-season perennial species that has been the focus of extensive plant breeding efforts to increase grain yield and improve several other agronomic traits. These efforts have contributed to increasing market demand for intermediate wheatgrass grain (tradename Kernza®). Managing intermediate wheatgrass as a dual-purpose grain and forage crop could provide farmers with an additional revenue stream and reduce economic risks for early adopters. However, little is known about how dual-purpose management impacts many agronomic aspects of the cropping system. The objective of our research was to determine how managing intermediate wheatgrass for both grain and forage affects weed community composition and suppression. We hypothesized that forage harvest would reduce the competitive ability of intermediate wheatgrass with weeds. An experiment was initiated in the fall of 2015 at the University of Wisconsin-Madison Arlington Agricultural Research Station using Cycle 4 intermediate wheatgrass developed at The Land Institute in Salinas, KS. The experimental design was a randomized complete block with three replications of four forage harvest timings (spring and summer, summer only, summer and fall, and no harvest), two nitrogen application rates (90 and 135 kg N ha^-1), two plant growth regulator application rates (0.22 and 0.66 kg ai ha^-1 trinexapac-ethyl, Palisade® EC), and weedy and weed-free control treatments. During stand establishment in 2016, field pennycress (Thlaspi arvense) and shepherd’s-purse (Capsella bursa-pastoris) were the most abundant species in the weed community. In 2017, dandelion (Taraxacum officinale), smooth hawksbeard (Crepis capillaris), field pennycress (Thlaspi arvense), and white clover (Trifolium repens) were the most abundant weed species across all treatments. Early-season (May 4) weed shoot dry biomass was greater for the spring and summer (37±3 g m^-2) and summer and fall (38±9 g m^-2) forage harvest treatments than the summer only harvest treatment (20±7 g m^-2). Weed shoot dry biomass was lowest in the no forage harvest treatment (7±2 g m^-2). Preliminary analysis of weed shoot dry biomass at the time of grain harvest (July 31) indicated that weed suppression was highest in the no forage harvest treatment. These results suggest that there may be important weed management tradeoffs to consider when producing intermediate wheatgrass for both grain and forage.