Abstract
AbstractApplication 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%.
Highlights
Crop rotation from alfalfa (Medicago sativa L.) to corn (Zea mays L.) is common in dairy production areas of the United States
On the coarse-textured soil, tillage operations only reduced penetration resistance compared with no-till to the depth of 25 cm for spring conventional tillage, 5 cm for fall conventional tillage, 10 cm for spring strip-till, and at no depth for fall strip-till (Figure 1), which was not to the full depth of conventional till (45 cm) or strip-till (25 cm)
Non-glyphosate herbicides should be applied to terminate glyphosate-resistant alfalfa before corn planting to optimize corn emergence rates and corn silage yield for both conservation and conventional tillage systems. The fact that both fall and spring herbicide application timings sufficiently controlled glyphosate-resistant alfalfa and optimized corn silage yield indicates that growers could wait until spring to assess alfalfa stands for winterkill before deciding to terminate alfalfa
Summary
Crop rotation from alfalfa (Medicago sativa L.) to corn (Zea mays L.) is common in dairy production areas of the United States. A mixture of the herbicides 2,4-D (2,4dichlorophenoxyacetic acid) and dicamba (3,6-dichloro2-methoxybenzoic acid) has been suggested as an alternative to glyphosate for alfalfa stand removal (Buhler & Mercurio, 1988; Moomaw & Martin, 1976; Van Deynze et al, 2004) The labels for these products suggest that this herbicide combination will require different application timings than glyphosate due to restrictions on application timing and subsequent alfalfa harvest, feeding, tillage, and subsequent corn planting. A preharvest application of glyphosate can be made 36 h before alfalfa harvest or grazing to terminate the alfalfa stand, and tillage and corn planting can take place immediately (Monsanto, 2019) No such application can be made with 2,4-D or dicamba alone or in combination. The application must be delayed until alfalfa has reached 10–15 cm of post-harvest regrowth, tillage and planting must be delayed another 7–14 d to allow for sufficient herbicide translocation for alfalfa control and to minimize the risk of corn injury (BASF, 2010; Winfield, 2020)
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