legumes, supply N (Scott et al., 1987). Many studies in the North Central USA on legume interseeding in Cropping systems that improve soil conservation are needed for established corn stands report grain yield losses that mixed grain and forage enterprises in the upper Midwest. Our objective was to determine whether established kura clover (Trifolium are attributed to moisture stress (Kurtz et al., 1952; ambiguum M. Bieb.) stands could serve as a living mulch for no-till Pendleton et al., 1957), N deficiency (Scott et al., 1987 corn (Zea mays L.) production, followed by a return to clover without Triplett, 1962), and reduced corn populations associated replanting. Treatments included corn sown into established kura clowith wider row spacing (Schaller and Larson, 1955; ver that was: (1) killed and sidedressed with N, (2) killed, (3) band- Stringfield and Thatcher, 1951). Living mulch systems killed, leaving 15 cm of untreated kura clover between rows, (4) have also been evaluated where existing forage is mansuppressed and sidedressed with N, and (5) suppressed and (6) unaged with herbicides to allow corn production. Corn treated kura clover without corn. Corn whole-plant yield in 1996 yields in such systems were equal to or greater than ranged from 14.0 to 15.7 Mg ha Ϫ1 and was greatest in Treatments 2 those in conventional corn production, but these systems and 4 and least in Treatment 5. Corn whole-plant yield in 1997 ranged were evaluated in the northeastern states, where seafrom 9.5 to16.9 Mg ha Ϫ1 and was greatest in Treatments 1 and 2 and least in Treatment 5. Grain yields in 1996 were not different among sonal precipitation is higher than in the Midwest (Entreatments, while in 1997 yields ranged from 7.2 to 11.1 Mg ha Ϫ1 and ache and Ilnicki, 1990; Mayer and Hartwig, 1986). Simiwere greatest in Treatments 1 and 2 and least in Treatment 5. Clover lar results in the North Central states have not been yield in 1997 following 1996 corn production was greatest in the achieved consistently, primarily because of the continuntreated control, but there was no clover yield difference in 1998 ued problem of competition for water, and to a lesser following either 1996 or 1997 corn production. Kura clover can be extent, light and nutrients (Eberlein et al., 1992; Echtenmanaged as a living mulch in corn with little or no corn whole-plant kamp and Moomaw, 1989).or grain yield reduction and clover will recover to full production Kura clover is a long-lived, perennial, rhizomatous within 12 mo without replanting.

Field experiments were conducted in 1996 and 1997 to determine the effect of the rate and time of glyphosate application on weed emergence, survival, biomass, and Glycine max yield in reduced-tillage (RT) and no-tillage (NT) glyphosate-resistant G. max planted in rows spaced 18 (narrow-row) and 76 cm (wide-row). Glyphosate was applied at 0.42, 0.63, and 0.84 kg ae ha−1 at V2, V4, R1, and R4 growth stages. On separate plots, 0.84 kg ha−1 glyphosate was applied at each growth stage with hand weeding. A weed-free check was maintained with preemergence imazethapyr plus metolachlor supplemented with hand weeding, and a nontreated check was included. Weed population density before glyphosate application ranged from 239 to 606 plants m−2 in RT and 33 to 500 plants m−2 in NT systems. Setaria faberi and Chenopodium album were the predominant species. Weed control efficacy and crop yield were influenced more by application time than by glyphosate rate. Glyphosate applied at V2, V4, and R1 gave season-long control of weeds in 18-cm rows. In 76-cm rows, glyphosate applied at V2, V4, and R1 gave almost complete control of weeds, but broadleaf weeds emerged after application at V2. The critical time of weed removal, the time beyond which weed competition reduced G. max yield by 3% or more compared to the weed-free check, was at R1 and V4 in 18-cm RT G. max in 1996 and 1997, respectively, and at V2 in 76-cm RT G. max in both years. The predicted critical time of weed removal in 18- and 76-cm NT G. max was R1 and V4, respectively, in 1996 and R1 in 1997. This research showed that there was variation in the onset of the critical time of weed removal between tillage systems, as well as within tillage systems across years. The results indicate a single glyphosate application can prevent yield loss in narrow-row, glyphosate-resistant G. max under favorable conditions, but application timing becomes more critical in wide rows because the critical period of weed removal occurs earlier. Late-emerging weeds may warrant a second glyphosate application in wide-row G. max.

The use of POST herbicides has been limited in sweet corn because of the narrow spectrum of weed control or potential crop injury. Field experiments were conducted to evaluate the 4-hydroxyphenyl pyruvate dioxygenase (HPPD)-inhibiting herbicides mesotrione, tembotrione, and topramezone applied POST in sweet corn at three locations. Efficacy of mesotrione, tembotrione, and topramezone applied alone or mixed with atrazine was compared to other labeled POST herbicides following PRE S-metolachlor. Giant foxtail control was greater with tembotrione or topramezone than mesotrione alone or mixed with atrazine. Common lambsquarters, velvetleaf, and common ragweed were controlled 98% or greater with the HPPD-inhibiting herbicides when mixed with atrazine. Tolerance of six sweet corn hybrids was determined in the field when treated with 1× and 2× rates of these herbicides mixed with atrazine. Tolerance of six sweet corn hybrids to these herbicides was determined in the greenhouse when treated with 0.5, 1, 2, 4, 8, and 16 times the labeled rate. Differential hybrid tolerance to each herbicide was observed in both the field and greenhouse evaluations. Tembotrione killed ‘Merit’ in both evaluations. Excluding Merit, hybrids generally had good tolerance to tembotrione and topramezone in the field, but had differential tolerance to mesotrione. With the exception of Merit, hybrids generally had greater tolerance to tembotrione than topramezone and less tolerance to mesotrione in the greenhouse. These HPPD-inhibiting herbicides provide POST weed control, but the potential for sweet corn injury varies among the herbicides and hybrids and warrants further characterization.

and Moody, 1976). Also, when legumes are used as a living mulch, they can supply N to the main crop (Scott Previous research has shown that with adequate suppression, kura et al., 1987). clover (Trifolium ambiguum M. Bieb.) can be managed as a living mulch in corn (Zea mays L.); however, significant yield loss was Corn production in living mulch and interseeded sysobserved in some environments. This study evaluated two herbicide-tems typically results in yield loss in the North-Central resistant corn hybrids at three levels of kura clover living mulch USA from competition for moisture and N (Kurtz et suppression over multiple environments. In 1999 and 2000 near Arlingal., 1952; Pendleton et al., 1957). Fisher and Burrill ton and in 2000 near Lancaster, WI, glyphosate [N-(phosphonometh-(1993) and Zemenchik et al. (2000) also note that cool yl)glycine]-resistant corn (Roundup Ready corn, RRC) and glufosispring temperatures could reduce corn yields because nate [2-amino-4-(hydroxymethylphosphinyl) butanoic acid]-resistant of delayed planting or by giving a cool-season clover a corn (Liberty Link corn, LLC) hybrids were planted where kura clover competitive advantage over corn, a warm-season grass. had been (i) killed for monocrop corn, (ii) strongly suppressed with The key to the successful use of living mulches for glyphosate and dicamba (3,6-dichloro-2-methoxybenzoic acid), or (iii) corn production is controlling competition from the lightly suppressed with only glyphosate. Suppressed kura clover also had a 25-cm clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) plus mulch crop. However, if the mulch suppression is excesdicamba-killed band into which corn was planted. Subsequent post-sive, it will not recover. Zemenchik et al. (2000) showed emergence applications of glyphosate or glufosinate herbicide were that corn could be grown in a kura clover living mulch made for each hybrid. Corn whole-plant yield ranged from 17.3 to when adequately suppressed by herbicide, without re-19.9 Mg ha Ϫ1 , and grain yield ranged from 10.8 to 12.3 Mg ha Ϫ1 .duced corn whole-plant or grain yields. In this system, Yield of whole-plant and grain across both corn hybrids did not differ kura clover will recover to full production within 12 mo between monocrop corn and corn in strongly suppressed kura clover. of corn harvest. However, corn performance in the living Whole-plant yield of monocrop corn was 8 to 11% greater and grain

Knowledge of weed community structure in vegetable crops of the north central region (NCR) is poor. To characterize weed species composition present at harvest (hereafter called residual weeds) in processing sweet corn, 175 fields were surveyed in Illinois, Minnesota, and Wisconsin from 2005 to 2007. Weed density was enumerated by species in thirty 1-m2quadrats placed randomly along a 300- to 500-m loop through the field, and additional species observed outside quadrats were also recorded. Based on weed community composition, population density, and mean plant size, overall weed interference level was rated. A total of 56 residual weed species were observed and no single species dominated the community of NCR processing sweet corn. Several of the most abundant species, such as common lambsquarters and velvetleaf, have been problems for many years, while other species, like wild-proso millet, have become problematic in only the last 20 yr. Compared to a survey of weeds in sweet corn more than 40 yr ago, greater use of herbicides is associated with reductions in weed density by approximately an order of magnitude; however, 57% of fields appeared to suffer yield loss due to weeds. Sweet corn harvest in the NCR ranges from July into early October. Earlier harvests were characterized by some of the highest weed densities, while late-emerging weeds such as eastern black nightshade occurred in fields harvested after August. Fall panicum, giant foxtail, wild-proso millet, common lambsquarters, and velvetleaf were the most abundant species across the NCR, yet each state had some unique dominant weeds.