Synthetic auxin herbicides do not injure intermediate wheatgrass or affect grain yield

Intermediate wheatgrass (IWG) is a cool-season perennial grass developed as a dual-purpose grain and forage crop. One barrier to adopting this crop is a lack of information on the effects of herbicides on IWG for grain production. An experiment was conducted to evaluate herbicide effects on IWG grain yield, crop injury, and weed control over 2 yr (2019-2021) at sites in Wisconsin, Minnesota, New York, and North Dakota, USA. This evaluation included Group 4 broadleaf herbicides registered in wheat: 2,4-D amine, clopyralid, MCPA, and a mixture of clopyralid + MCPA. Each herbicide or mixture was applied at 1X and 2X the labeled wheat application rate to newly planted and established (1 to 5 yr old) IWG stands in the fall or spring. Applications were made during IWG tillering or jointing stages in the fall or during the jointing stage in the spring. Across site years, application timing, herbicide, and application rate showed no effect on IWG grain yield or plant injury. Broadleaf weed control ranged from 71 to 92% across herbicide treatments relative to the nontreated check at the WI site while weed control at the MN site was variable among treatments. At the NY site, herbicides were equally effective for broadleaf weed suppression, whereas weed pressure was very low at the ND site and treatments did not affect weed cover. The results show that newly planted and established stands of IWG are tolerant to the synthetic auxin herbicides 2,4-D amine, clopyralid, and MCPA when applied during tillering or jointing in the fall or during jointing in the spring. Synthetic auxins represent a potentially useful tool for weed control in IWG cropping systems, especially for problematic broadleaf weed species.