A comprehensive, Wisconsin state-wide assessment of waterhemp response to a diverse group of herbicide sites of action has not been conducted. Our objective was to characterize the response of a state-wide collection of waterhemp accessions to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean in Wisconsin. Greenhouse experiments were conducted with more than 80 accessions from 27 counties. POST treatments included 2,4-D, atrazine, dicamba, fomesafen, glufosinate, glyphosate, imazethapyr, and mesotrione at 1× and 3× label rates. PRE treatments included atrazine, fomesafen, mesotrione, metribuzin, and S-metolachlor at 0.5×, 1×, and 3× label rates. Ninety-eight percent and 88% of the accessions exhibited ≥50% plant survival after exposure to imazethapyr and glyphosate POST 3× rate, respectively. Seventeen percent, 16%, and 3% of the accessions exhibited ≥50% plant survival after exposure to 2,4-D, atrazine, and dicamba, respectively, applied POST at the 1× rate. Survival of all accessions was ≤25% after exposure to 2,4-D or dicamba applied POST at the 3× rate, or glufosinate, fomesafen, and mesotrione applied POST at either rate evaluated. No plant of any accession survived exposure to glufosinate at either rate. Forty-five percent and 3% of the accessions exhibited <90% plant density reduction after exposure to atrazine applied PRE at the 3× rate and fomesafen PRE at the 1× rate, respectively. Plant density reduction of all accessions was ≥96% after exposure to fomesafen applied PRE at the 3× rate, or metribuzin, S-metolachlor, and mesotrione applied PRE at the 1× rate. Our results suggest that waterhemp resistance to imazethapyr and glyphosate applied POST is widespread in Wisconsin, whereas resistance to 2,4-D, atrazine, and dicamba applied POST is present to a lower extent. One accession (A75, Fond du Lac County) exhibited multiple resistance to imazethapyr, atrazine, glyphosate, and 2,4-D when applied POST. Overall, atrazine applied PRE was ineffective for waterhemp control in Wisconsin. Proactive resistance management and the use of effective PRE and POST herbicides are fundamental for waterhemp management in Wisconsin.

Establishment of alfalfa by interseeding it with corn planted for silage can enhance crop productivity but weed management is a challenge to adoption of the practice. Although a simple and effective approach to weed management would be to apply a glyphosate-based herbicide, concerns about herbicide resistance and limitations in available alfalfa varieties exist. Field experiments were conducted to compare the efficacy and selectivity of PRE, POST, and PRE followed by POST herbicide programs to a glyphosate-only strategy when interseeding alfalfa with corn. Experiment 1 compared PRE applications of acetochlor, mesotrione, S-metalochlor, metribuzin, and flumetsulam. Results indicate that acetochlor and metribuzin, and S-metalochlor used at a rate of 1.1 kg ai ha−1 were the most effective and selective PRE herbicides 4 wk after treatment (WAT), but each resulted in greater overall weed cover than glyphosate by 8 WAT. Experiment 2 evaluated applications of bentazon, bromoxynil, 2,4-DB, and mesotrione at early and late POST times. Several herbicides used POST exhibited similar effectiveness and selectivity as glyphosate, including early applications of bromoxynil (0.14 kg ai ha−1) and 2,4-DB (0.84 or 1.68 kg ai ha−1), as well as late applications of bromoxynil (0.42 kg ai ha−1), 2,4-DB (0.84 kg ai ha−1), and mesotrione (0.05 or 0.11 kg ai ha−1). A third experiment compared applications of acetochlor PRE, bromoxynil POST, and a combination of acetochlor PRE with bromoxynil POST. All treatments were effective and safe for use in this interseeded system, although interseeded alfalfa provided 65% to 70% weed suppression in corn planted for silage without any herbicide. Herbicide treatments had no observable impacts on corn and alfalfa yields, so weed management was likely of limited economic importance. However, weed competitiveness can vary based on several different factors including weed species, density, and site-specific factors, and so further investigations under different environments and conditions are needed.

Interseeding alfalfa (Medicago sativa L.) into corn (Zea mays L.) silage can increase forage production, but the effects on soil nitrate pools remain unknown. We compared soil nitrate pools during and after interseeded alfalfa establishment in corn with pools during production of solo‐seeded corn followed by spring‐seeded alfalfa. Nitrogen (N) fertilizer was applied at three rates as a preplant broadcast or a 50:50 split between preplant broadcast and banded sidedress applications. During corn growth, soil nitrate levels from 0 to 30 cm were increased by higher N rates but were usually similar for both production systems. However, growth of interseeded alfalfa after corn harvest resulted in substantially lower residual fall and spring soil nitrate levels to a 90 cm depth, particularly under the highest N rate. Additionally, interseeded alfalfa influenced spring nitrate stratification by reducing nitrate in deeper horizons and increasing nitrate in the surface horizon. Interseeded alfalfa can thereby reduce the risk of nitrate leaching.

The continued dispersal of Palmer amaranth can impose detrimental impacts to cropping systems in Wisconsin. Our objective was to characterize the response of a recently introduced Palmer amaranth accession in southern WI to postemergence (POST) and preemergence (PRE) herbicides commonly used in corn and soybean. Greenhouse experiments were conducted with the WI putative herbicide-resistant accession (BRO), and two additional control accessions from Nebraska, a glyphosate-resistant (KEI2) and a glyphosate-susceptible (KEI3). Postemergence treatments were 2,4-D, atrazine, dicamba, glufosinate, glyphosate, imazethapyr, lactofen, and mesotrione at 1× and 3× label rates. Preemergence treatments were atrazine, mesotrione, metribuzin, S-metolachlor, and sulfentrazone at 0.5×, 1×, and 3× label rates. Plant survival of each accession was ≥ 63% after exposure to imazethapyr POST 3× rate. Survival of BRO and KEI2 was 44% (±13) and 50% (±13), respectively, after exposure to atrazine POST 3× rate. Survival of BRO was 69% (±12) after exposure to glyphosate POST 1× rate, whereas survival of KEI2 was 44% (±13) after exposure to glyphosate POST 3× rate. After exposure to 2,4-D POST 1× rate, KEI2 and KEI3 survival was 38% (±13) and 50% (±13), respectively. Survival of all accessions was ≤ 31% after exposure to 2,4-D POST 3× rate, or dicamba, glufosinate, lactofen, and mesotrione POST at either rate. Plant density reduction of KEI2 was 77% (±13) after exposure to atrazine PRE 1× rate, whereas density reduction of BRO was 56% (±13) after exposure to atrazine PRE 3× rate. Plant density reduction of all accessions was ≥ 94% after exposure to mesotrione PRE 1× and 3× rates, or metribuzin, S-metolachlor, and sulfentrazone PRE at either rate. Our results suggest that each accession is resistant (≥ 50% survival) to imazethapyr POST, that BRO and KEI2 are resistant to atrazine and glyphosate POST, and KEI2 and KEI3 are resistant to 2,4-D POST. The recently introduced BRO accession exhibited multiple resistance to imazethapyr, atrazine, and glyphosate POST. In addition, atrazine PRE was ineffective for BRO control, suggesting that diversified resistance management will be critical for its effective management.