Tillage method and glyphosate-resistant alfalfa termination timing affect soil properties and subsequent corn yield

Application of glyphosate [N-(phosphonomethyl) glycine]-containing herbicides and tillage, alone or in combination, has been the standard for removing declining alfalfa (Medicago sativa L.) stands. With glyphosate no longer an option to control glyphosate-resistant alfalfa, different termination strategies are needed. Field studies across four site-years in Utah evaluated the effect of tillage type and timing (fall conventional till, spring conventional till, fall strip-till, spring strip-till, and no-till) and herbicide timing (fall, spring, in-crop, and no herbicide) of 2,4-D (2,4-dichlorophenoxyacetic acid) and dicamba (3,6-dichloro-2-methoxybenzoic acid) on penetration resistance, alfalfa regrowth, corn (Zea mays L.) emergence rate, and silage corn yield. Across tillage treatments, fall, spring, and in-crop herbicide timings compared with no herbicide reduced alfalfa stem count and biomass by at least 74 and 92%, respectively. Emergence rate was greatest under fall and spring conventional till or spring strip-till compared with fall strip-till or no-till. Silage corn yield was greatest and similar with fall or spring herbicide applications for all tillage systems and conventional tillage with an in-crop herbicide application (19-27 Mg ha(-1)), followed by in-crop herbicide application for conservation tillage systems and fall and spring conventional till without herbicide application (14-20 Mg ha(-1)), and lastly when only conservation tillage was used to terminate alfalfa (5-15 Mg ha(-1)). Silage corn yield can be optimized when glyphosate-resistant alfalfa is terminated with herbicides prior to planting, regardless of tillage type or timing. Termination of glyphosate-resistant alfalfa by herbicides after corn emergence, depending on tillage, reduces silage corn yield 9-19%.