John Cardina

Crop rotation and tillage system effects on weed seedbanks

Abstract We characterized the size and species composition of the weed seedbank after 35 yr of continuous crop rotation and tillage system treatments at two locations in Ohio. Spring seedbanks were monitored during 1997, 1998, and 1999 in continuous corn (CCC), corn–soybean (CS), and corn–oats–hay (COH) rotations in moldboard plow (MP), chisel plow (CP), and no-tillage (NT) plots where the same herbicide was used for a given crop each growing season. There were 47 species at Wooster and 45 species at Hoytville, with 37 species occurring at both locations in all 3 yr. Crop rotation was a more important determinant of seed density than was tillage system. Seed density was highest in NT and generally declined as tillage intensity increased. Seeds accumulated near the surface (0 to 5 cm) in NT but were uniformly distributed with depth in other tillage systems. At both locations there was a significant interaction between tillage and rotation for estimates of the total seed density. Seed density was highest in NT-CCC, with 26,850 seeds m−2 at Wooster and 8,680 seeds m−2 at Hoytville. At Wooster total seed density in CCC plots was 45 and 60% lower than in COH plots for CP and MP. In NT the total seed density was 40% greater in CCC than in COH. At Hoytville total seed density in CCC plots was 72% lower than in COH plots that were CP or MP, whereas seed density was 45% higher in CCC than in COH plots that were in an NT system. There were more significant differences in seedbank density for any given species for crop rotation than for tillage treatments. Seed densities of three broadleaves (shepherds-purse, Pennsylvania smartweed, and corn speedwell) at Wooster and four broadleaves (yellow woodsorrel, redroot pigweed, Pennsylvania smartweed, and spotted spurge) at Hoytville were more abundant in COH (140 to 630 seeds m−2) than in CS (10 to 270 seeds m−2) or CCC (< 1 to 60 seeds m−2), regardless of the tillage system. At both locations Pennsylvania smartweed seeds were more abundant in COH (260 and 630 seeds m−2) than in other rotations (10 to 20 seeds m−2). Relative importance (RI) values, based on relative density and relative frequency of each species, were lower in CS than in CCC for common lambsquarters and five other weeds at Wooster; RI of giant foxtail was 80% lower in COH than in CCC at Hoytville. The data show how species composition and abundance change in response to crop and soil management. The results can help to determine how complex plant communities are “assembled” from a pool of species by specific constraints or filters. Nomenclature: Redroot pigweed, Amaranthus retroflexus L. AMARE; shepherds-purse, Capsella bursa-pastoris (L.) Medicus CAPSA; common lambsquarters, Chenopodium album L. CHEAL; spotted spurge, Euphorbia maculata L. EPHMA; yellow woodsorrel, Oxalis stricta L. OXAST; Pennsylvania smartweed, Polygonum pensylvanicum L. POLPY; giant foxtail, Setaria faberi L. SETFA; corn speedwell, Veronica arvensis L. VERAR; oats, Avena sativa L.; soybean, Glycine max (L.) Merr.; corn, Zea mays L.

Weed seedbank community composition in a 35-yr-old tillage and rotation experiment

) were greatest in the no-tillage system, where the soil was disturbed only by the coulter units of the planter. Differences in weed seedbank community composition among tillage and rotation treatments were examined using two multivariate analyses. Using a multiresponse permutation procedure and canonical discriminant analysis, results suggest that the weed seed community in a corn‐oat‐hay rotational system differs in structure and composition from communities associated with continuous corn and corn‐soybean systems. Additionally, germinable weed seed communities in no-tillage differed in composition from those in conventional and minimum tillage. Crop sequence and tillage system influenced weed species density and diversity and therefore community structure. Manipulation of these factors could help reduce the negative impact of weeds on crop production.

Effect of Planting Date, Residual Herbicide, and Postemergence Application Timing on Weed Control and Grain Yield in Glyphosate-Tolerant Corn (Zea mays)1

Studies were conducted in 1998 and 1999 in Ohio to determine the effect of postemergence (POST) application timing of glyphosate on weed control and grain yield in glyphosate-tolerant corn, and how this was influenced by corn planting date and the use of soil-applied herbicides. Glyphosate was applied based on giant foxtail height. Two applications of glyphosate provided better weed control than a single application, especially when applied to weeds 10 cm or less in early-planted corn. Yield was reduced occasionally with a single application on 5- or 10-cm weeds, because of weed re-infestation. Failure to control weeds before they reached a height of 15 to 30 cm also resulted in occasional yield loss. Application of atrazine or acetochlor plus atrazine prior to glyphosate did not consistently increase weed control or yield. Results suggested that glyphosate should be applied before weeds reach 15 cm in height to avoid corn grain yield loss. Nomenclature: Glyphosate; giant foxtail, Setaria faberi Herrm. #3 SETFA; corn, Zea mays L. Additional index words: Herbicide-resistant crops, weed interference. Abbreviations: CEC, cation exchange capacity; POST, postemergence; PRE, preemergence; RS, respray; WAP, week after planting.

Environmental factors affecting seed persistence of annual weeds across the U.S. corn belt

Abstract Weed seedbanks have been studied intensively at local scales, but to date, there have been no regional-scale studies of weed seedbank persistence. Empirical and modeling studies indicate that reducing weed seedbank persistence can play an important role in integrated weed management. Annual seedbank persistence of 13 summer annual weed species was studied from 2001 through 2003 at eight locations in the north central United States and one location in the northwestern United States. Effects of seed depth placement, tillage, and abiotic environmental factors on seedbank persistence were examined through regression and multivariate ordinations. All species examined showed a negative relationship between hydrothermal time and seedbank persistence. Seedbank persistence was very similar between the two years of the study for common lambsquarters, giant foxtail, and velvetleaf when data were pooled over location, depth, and tillage. Seedbank persistence of common lambsquarters, giant foxtail, and velvetleaf from October 2001 through 2002 and October 2002 through 2003 was, respectively, 52.3% and 60.0%, 21.3% and 21.8%, and 57.5% and 57.2%. These results demonstrate that robust estimates of seedbank persistence are possible when many observations are averaged over numerous locations. Future studies are needed to develop methods of reducing seedbank persistence, especially for weed species with particularly long-lived seeds. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medik. ABUTH.

Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn: Results of a 2-yr multistate study

Field studies were conducted at 35 sites throughout the north-central United States in 1998 and 1999 to determine the effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn. Glyphosate was applied at various timings based on the height of the most dominant weed species. Weed control and corn grain yields were considerably more variable when glyphosate was applied only once. The most effective and consistent season-long annual grass and broadleaf weed control occurred when a single glyphosate application was delayed until weeds were 15 cm or taller. Two glyphosate applications provided more consistent weed control when weeds were 10 cm tall or less and higher corn grain yields when weeds were 5 cm tall or less, compared with a single application. Weed control averaged at least 94 and 97% across all sites in 1998 and 1999, respectively, with two glyphosate applications but was occasionally less than 70% because of late emergence of annual grass and Amaranthus spp. or reduced control of Ipomoea spp. With a single application of glyphosate, corn grain yield was most often reduced when the application was delayed until weeds were 23 cm or taller. Averaged across all sites in 1998 and 1999, corn grain yields from a single glyphosate application at the 5-, 10-, 15-, 23-, and 30-cm timings were 93, 94, 93, 91, and 79% of the weed-free control, respectively. There was a significant effect of herbicide treatment on corn grain yield in 23 of the 35 sites when weed reinfestation was prevented with a second glyphosate application. When weed reinfestation was prevented, corn grain yield at the 5-, 10-, and 15-cm application timings was 101, 97, and 93% of the weed-free control, respectively, averaged across all sites. Results of this study suggested that the optimum timing for initial glyphosate application to avoid corn grain yield loss was when weeds were less than 10 cm in height, no more than 23 d after corn planting, and when corn growth was not more advanced than the V4 stage. Nomenclature: Glyphosate; Amaranthus spp. #3 AMASS; Ipomoea spp. # IPOSS; corn, Zea mays L. ‘Roundup Ready®’ # SETFA. Additional index words: Herbicide-resistant crops, weed interference. Abbreviation: POST, postemergence.

Common sunflower resistance to acetolactate synthase–inhibiting herbicides

Abstract In 1996 a common sunflower population near Howard, SD, was suspected to be cross-resistant to imazethapyr and chlorimuron. Whole-plant acetolactate synthase (ALS) assays confirmed ALS-inhibitor resistance in the Howard biotype. The I50 values (inhibition of 50% of the enzyme activity) indicated that the resistant population required 39 and 9 times more imazethapyr and chlorimuron, respectively, to obtain the same level of enzyme inhibition compared with the sensitive biotype. Herbicide dose response data supported the whole-plant enzyme assay data; control (> 90%) was not achieved with less than a four-times application rate of chlorimuron. Control with imazethapyr was not achieved even with a 16-times rate. Chlorimuron and imazethapyr controlled 70 and 95% of the population, respectively, when a four-times rate of each herbicide was applied separately. Differences in 14C-herbicide absorption were observed, suggesting that there may be physical or chemical differences in leaf surface composition between the resistant and sensitive biotypes. Although translocation of 14C-herbicide was less in the resistant biotype than in the sensitive biotype, the differences were not enough to explain chlorimuron and imazethapyr selectivity between the two biotypes. Overall results suggested that the differences in the common sunflower populations were attributed to an altered site of action on the ALS enzyme. Nomenclature: Chlorimuron; imazethapyr; common sunflower, Helianthus annuus L. HELAN.

Weed seed rain, soil seedbanks, and seedling recruitment in no-tillage crop rotations

Abstract Relationships among weed seed rain, soil seedbank, and seedling recruitment in no-tillage systems were studied from July 1993 to May 1996. Multiple regression analysis indicated that seedling recruitment of only six of the 25 weed species present was correlated with seed rain samples from the previous autumn, spring soil seedbank samples, or a combination of the two. However, seedling recruitment of the dominant annual grasses (yellow foxtail, giant foxtail, and fall panicum in Field 1—1994, Field 2—1995, and Field 3—1996, respectively) was related to seedbank populations or a combination of seedbank and seed rain densities. These grasses accounted for at least 32% of the emerged seedlings, 12 to 78% of the seedbank, and 16 to 77% of the seed rain. Seedling recruitment of large crabgrass and two broadleaf species, Virginia copperleaf and wild carrot, also were described by seedbank densities or a combination of seedbank and seed rain densities. However, both the broadleaf species were minor components of the cropping system, representing ≤ 4% of all seedlings. In each year, the sum of all weeds in the seedbank exceeded 1,300 seeds m−2 (0 to 10 cm deep). The fraction of the total seedbank that emerged each year ranged from 3 to 17%, but there was great variability among species. The aforementioned dominant grasses ranged in emergence from 3 to 25% of their seedbanks. Emergence of other species ranged from a low of 1% for common lambsquarters up to 41% for large crabgrass. Prediction of seedling recruitment from seed rain or seedbank densities was variable; however, combining both components improved the fit of regression describing seedling recruitment. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; fall panicum, Panicum dichotomiflorum Michx. PANDI; giant foxtail, Setaria faberi Herrm. SETFA; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; Virginia copperleaf, Acalypha virginica L. ACCVI; wild carrot, Daucus carota L. DAUCA; yellow foxtail, Setaria (Poir.) Roem & Schult SETLU.

Integration of cover crops with postemergence herbicides in no-till corn and soybean

Abstract The integration of cover crops with selected postemergence herbicides was evaluated on the basis of weed control and grain yields in no-till soybean and corn. Soybean was planted into wheat residue, whereas corn was planted into hairy vetch residue. Full, half, and quarter rates and sequential herbicide applications were made. The wheat cover crop did not increase weed suppression but increased soybean grain yields. Half rates of thifensulfuron plus quizalofop-P as single or split applications were as effective as full rates in reducing weed weight in soybean. Soybean grain yields were similar in the half- and full-rate treatments in 1994, but yield was highest in the full-rate treatment in 1995. The hairy vetch cover crop did not increase weed suppression but lowered corn stands and grain yields in 1995 and enhanced corn grain yields in 1996. Full, half, and quarter rates (1996 only) of nicosulfuron plus primisulfuron were equally effective in reducing weed weight. Corn grain yields were similar at all herbicide rates in 1995 but were inversely related to herbicide rate in 1996. Split herbicide applications did not improve weed suppression over single applications of the same herbicide rate in either crop. Results indicate that cover crops can improve crop productivity and reduced rates of environmentally benign herbicides can minimize the herbicide requirements in no-till corn and soybean. Nomenclature: Glyphosate; nicosulfuron; primisulfron; quizalofop-P; thifensulfuron; corn, Zea mays L.; hairy vetch, Vicia villosa Roth; soybean, Glycine max L.; wheat, Triticum aestivum L.

Certified Crop Advisors’ Perceptions of Giant Ragweed (Ambrosia trifida) Distribution, Herbicide Resistance, and Management in the Corn Belt

Abstract Giant ragweed has been increasing as a major weed of row crops in the last 30 yr, but quantitative data regarding its pattern and mechanisms of spread in crop fields are lacking. To address this gap, we conducted a Web-based survey of certified crop advisors in the U.S. Corn Belt and Ontario, Canada. Participants were asked questions regarding giant ragweed and crop production practices for the county of their choice. Responses were mapped and correlation analyses were conducted among the responses to determine factors associated with giant ragweed populations. Respondents rated giant ragweed as the most or one of the most difficult weeds to manage in 45% of 421 U.S. counties responding, and 57% of responding counties reported giant ragweed populations with herbicide resistance to acetolactate synthase inhibitors, glyphosate, or both herbicides. Results suggest that giant ragweed is increasing in crop fields outward from the east-central U.S. Corn Belt in most directions. Crop production practices associated with giant ragweed populations included minimum tillage, continuous soybean, and multiple-application herbicide programs; ecological factors included giant ragweed presence in noncrop edge habitats, early and prolonged emergence, and presence of the seed-burying common earthworm in crop fields. Managing giant ragweed in noncrop areas could reduce giant ragweed migration from noncrop habitats into crop fields and slow its spread. Where giant ragweed is already established in crop fields, including a more diverse combination of crop species, tillage practices, and herbicide sites of action will be critical to reduce populations, disrupt emergence patterns, and select against herbicide-resistant giant ragweed genotypes. Incorporation of a cereal grain into the crop rotation may help suppress early giant ragweed emergence and provide chemical or mechanical control options for late-emerging giant ragweed. Nomenclature: Glyphosate; giant ragweed; Ambrosia trifida L. AMBTR; common earthworm; Lumbricus terrestris L.; corn; Zea mays L.; soybean, Glycine max (L.) Merr.

Comparison of survey methods for an invasive plant at the subwatershed level

Invasive plant survey methods that are practical and economical are needed to describe established colonies and detect nascent invaders. We compared results from random and roadside surveys of Alliaria petiolata (Bieb.) Cavara & Grande across a 5730-ha subwatershed. The random survey included 150 1-ha plots; the roadside survey examined 0.1-mile increments (10-m deep) along paved roads (totaling 1104 0.16-ha plots). In the random survey, agriculture was the dominant land use (49% of sampled area), and most A. petiolata patches were in wooded, shaded riparian, and waste areas (34%, 34%, and 29% of patches, respectively). In the roadside survey, right-of-way land use was dominant (38% of sampled area), and most A. petiolata patches were in right-of-way, wooded, and shaded riparian areas (53%, 22%, and 19% of patches, respectively). According to generalized linear model analysis, survey methods did not differ in the overall probability of finding A. petiolata (P=0.17 and 0.11 for random and roadside surveys, respectively). Shaded riparian, wooded, and mixed-species right-of-way land uses were the dominant habitat for A. petiolata in both surveys, but only the random survey indicated waste areas as significant habitat. Alliaria petiolata occurred mostly as small patches in roadsides, but as large patches in random plots, suggesting faster spread in the roadside. Results indicated that disturbed lands along roadsides were important for invasion and spread of A. petiolata; therefore, the roadside survey was a useful, practical method for detecting nascent invasions and management planning. The random sampling lacked a land use bias, and provided data that could be generalized across the subwatershed; however, this method required at least four times more person hours to complete than the roadside survey for a similar amount of area. Although roadside sampling did not provide a completely reliable assessment of target plant populations within the landscape, it may provide an adequate approximation, depending on the specific goals of the survey. Concurrent surveys would provide the most complete information.