Pre- and post-applied herbicides options for alfalfa interseeded with corn silage

Authors: Osterholz, William - Will; DIAS, JOSE LUIS - University Of Wisconsin; Grabber, John; RENZ, MARK - University Of Wisconsin

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2020
Publication Date: 9/17/2020
Citation: Osterholz, W.R., Dias, J.L.C.S., Grabber, J.H., Renz, M.J. 2020. Pre- and post-applied herbicides options for alfalfa interseeded with corn silage. Weed Technology. 35(2):263-270. https://doi.org/10.1017/wet.2020.104.
DOI: https://doi.org/10.1017/wet.2020.104

Interpretive Summary: Interseeding alfalfa into silage corn is an innovative method for alfalfa establishment that has been shown to provide agronomic, economic, and environmental benefits compared to conventional solo-seeded corn-alfalfa rotations. Since weed management is a potential challenge to effective adoption of this forage production system, the objective of this study was to determine the best herbicide options for weed control in the novel interseeded system. Glyphosate is an appealing herbicide option but requires that glyphosate tolerant crop varieties are planted. Additionally, herbicide programs that depend on a single site of action favor development and spread of herbicide resistant weeds. Our results show that several of the herbicides applied before or after weed emergence provided high levels of weed control and were safe for use in corn and alfalfa. Conversely, other herbicides were either ineffective for weed control or caused damage to alfalfa seedlings. Glyphosate provided consistently excellent weed control, but importantly we also identified several non-glyphosate options with similar efficacy. A recommended herbicide program consisted of a combination of acetochlor applied pre-emergence and bromoxynil applied post emergence. The glyphosate alternatives provide the option to use crop varieties that are not glyphosate-resistant, and encompass multiple sites of action that could prove useful to control herbicide resistant weeds. Finally, our findings also suggest that the use of herbicides when weed pressure is low to moderate may not be economical in this system as significant weed suppression by interseeded alfalfa itself was observed and little benefit to crop yields were provided by herbicides.

Technical Abstract: Weed management in alfalfa interseeded with silage corn is a potential challenge to adoption of this forage production system. Although a glyphosate-based herbicide program could be a simple and effective approach, concerns with weed resistance limit this strategy. Additionally, it has often been observed that conventional non-RR alfalfa varieties are better suited for establishment in corn. Therefore, field experiments were conducted to compare the efficacy and selectivity of several preemergence (PRE), postemergence (POST) and PRE followed by POST herbicide programs to a glyphosate (0.84 ae kg ha-1) only strategy. Experiment 1 treatments included PRE applications of acetochlor, mesotrione, S-metalochlor, metribuzin, and flumetsulam. Experiment 2 treatments included POST applications at two timings (early vs. late) of bentazon, bromoxynil, 2,4-DB, and mesotrione. A third experiment compared applications of acetochlor PRE, bromoxynil POST, and the combination of acetochlor PRE followed by bromoxynil POST. Results from experiment 1 indicated that both rates of acetochlor and metribuzin, and the 1.1 kg ha-1 rate of S-metalochlor were the most effective and selective PRE herbicides 4 weeks after treatment (WAT), but each resulted in greater weed cover than glyphosate by 8 WAT. In experiment 2, several POST herbicides provided similar effectiveness and selectivity as glyphosate including: early applications of bromoxynil (0.14 kg ha-1) and 2,4-DB (0.84 or 1.68 kg ha-1), as well as late applications of bromoxynil (0.42 kg ha-1), 2,4-DB (0.84 kg ha-1) and mesotrione (0.05 or 0.11 kg ha-1). In experiment 3, a split application of acetochlor (1.26 kg ha-1) followed by bromoxynil (0.28 kg ha-1) was found to be effective and safe for use in this interseeded system. Additionally, alfalfa provided significant weed suppression in silage corn, suggesting interseeded legumes could provide in-season weed control during corn silage production. As little effect of herbicide treatments on yields of either crop or alfalfa stand count was observed, weed management efforts to control moderate weed populationsressure may not be economically advantageous in the interseeded system.