Karen A. Renner

Post-dispersal weed seed predation in Michigan crop fields as a function of agricultural landscape structure.

Weed seed predation by invertebrates and vertebrates was compared between a simple (large crop fields embedded in a matrix of widely scattered woodlots and hedgerows) and a complex (small crop fields embedded in a matrix of numerous hedgerows and woodlots) agricultural landscape in southern Michigan. The structural differences between landscapes were evaluated by analysis of aerial photographs and digital land-use data. Seed predation experiments were conducted in four conventional tillage corn (Zea mays L.) fields within each landscape type. Trials included four common agricultural weed species, i.e., crabgrass (Digitaria sanguinalis), giant foxtail (Setaria faberii), pigweed (Amaranthus retroflexus), and velvetleaf (Abutilon theophrasti). Treatments to exclude vertebrates, invertebrates + vertebrates and no exclusion were established at 27 m from hedgerows. Fields in the complex landscape were 75% smaller, had 63% more wooded perimeter, and 81% more wide hedgerow perimeter than fields in the simple landscape. Fields in the simple landscape were surrounded mainly by herbaceous roadside and crops, whereas the complex landscape had fields surrounded primarily by wide hedgerows. In both the landscape types there was considerable post-dispersal weed seed removal with a tendency towards higher removal rates in the complex landscape. Although there were no differences in the rate of seed removal among the four weed species, seed predation showed a high degree of variability within and among fields.

Weed seedbank and community shifts in a long-term cropping systems experiment

Abstract Characterizing the long-term effect of agricultural management systems on weed communities will aid in developing sustainable weed management practices. Weed seedbanks and aboveground biomass were measured within a corn–soybean–wheat crop sequence from 1990 through 2002 at Hickory Corners, MI. Four management systems were compared: conventional (CONV; full rates of N fertilizer and herbicides, moldboard tillage), no till (NT; same as CONV with no primary tillage), reduced input (RI; reduced rates of N fertilizer and herbicides, moldboard tillage, mechanical weed control, wheat underseeded with red clover), and organic (ORG; same as RI but no synthetic inputs). Multivariate ordinations of weed seedbanks showed a divergence of the CONV and NT systems from the RI and ORG systems. The CONV and NT seedbanks were dominated by grass species (mainly fall panicum and large crabgrass), whereas the RI and ORG systems were dominated by common lambsquarters and common chickweed. Within a single growing season, weed seedbanks in the RI and ORG systems were positively correlated with weed biomass whereas seedbanks in the CONV and NT system had little predictive value. Weed biomass from 1990 through 2002 showed a strong association of grass weed species with the corn phase of the CONV and NT system and common lambsquarters and redroot pigweed with the corn and soybean phases of the RI and ORG systems. Weed biomass diversity measures were negatively correlated with soybean yields in RI and ORG and wheat yields in NT, RI, and ORG. It is not clear whether crops were less competitive in the NT, RI, and ORG treatments, allowing new weed species to enter the plots, or whether less effective weed management in the NT, RI, and ORG treatments resulted in increased species richness, causing reduced crop yields. Mechanistic studies are needed to elucidate the relationship between weed community diversity and crop performance. Nomenclature: Common chickweed, Stellaria media (L.) Vill. STEME; common lambsquarters, Chenopodium album L. CHEAL; fall panicum, Panicum dichotomiflorum Michx. PANDI; large crabgrass, Digitaria sanguinalis L. DIGSA; redroot pigweed, Amaranthus retroflexus L., AMARE; corn, Zea mays L. ‘Pioneer 3573’; red clover, Trifolium pratense L. ‘Michigan Mammoth Red’; soybean, Glycine max (L.) Merr. ‘Pioneer 9172’; wheat, Triticum aestivum L. ‘Pioneer 2552’.

Feeding Preferences of Weed Seed Predators and Effect on Weed Emergence

Abstract We determined feeding preferences of invertebrate seed predators and the effect of seed predation on weed emergence. Feeding choice studies were completed with three species of common ground beetles: (Amara aenea DeGeer, Anisodactylus sanctaecrucis F., and Harpalus pensylvanicus DeGeer) (Coleoptera: Carabidae) and the northern field cricket (Gryllus pennsylvanicus DeGeer) (Orthoptera: Gryllidae). Anisodactylus sanctaecrucis, H. pensylvanicus, and the female and male G. pennsylvanicus consumed more redroot pigweed seeds compared with giant foxtail seeds; A. aenea seed consumption did not differ between these two weed species. All invertebrates consumed fewer velvetleaf seeds compared with redroot pigweed and giant foxtail seeds; however, when seed biomass was compared, A. aenea consumed similar biomass of velvetleaf, giant foxtail, and redroot pigweed, whereas A. sanctaecrucis and H. pensylvanicus consumed greater biomass of velvetleaf compared with giant foxtail seed. Seed burial depths of 0.5 or 1.0 cm reduced redroot pigweed and giant foxtail seed consumption by A. aenea and A. sanctaecrucis but not by the larger carabid beetle, H. pensylvanicus. In a greenhouse study, A. sanctaecrucis decreased total weed emergence by 15%, and G. pennsylvanicus females and males decreased weed emergence by 16 and 5%, respectively. Emergence of redroot pigweed, but not velvetleaf or giant foxtail, decreased when A. sanctaecrucis and the male G. pennsylvanicus were present, whereas the emergence of all three weed species decreased in the presence of the female G. pennsylvanicus. In field experiments, vertebrate access to velvetleaf seeds reduced emergence from 4 to 9% across field sites; invertebrate access reduced emergence 4 to 6%. Vertebrate access to giant foxtail seeds reduced emergence 3 to 7%, and invertebrate access reduced emergence 4 to 13%. These results suggest that predation of weed seeds by both vertebrates and invertebrates may reduce weed emergence and influence the weed community. Nomenclature: Giant foxtail, Setaria faberi Herrm. SETFA; redroot pigweed, Amaranthus retroflexus L. AMARE; velvetleaf, Abutilon theophrasti Medic. ABUTH

Effect of Glyphosate Application Timing and Row Spacing on Corn (Zea mays) and Soybean (Glycine max) Yields1

Corn and soybean were planted in narrow and wide row spacings to determine the effect of glyphosate application timing and row spacing on crop yield. Glyphosate was applied when average weed canopy height reached 5, 10, 15, 23, and 30 cm. Weeds present in these studies included velvetleaf, redroot pigweed, common ragweed, common lambsquarters, jimsonweed, barnyardgrass, fall panicum, giant foxtail, yellow foxtail, green foxtail, and eastern black nightshade. Under highly competitive growing conditions (below normal rainfall and high weed density), corn yield was first reduced when weeds reached 10 and 15 cm in height with corn planted in 38- and 76-cm rows, respectively. Under similar conditions, soybean yield was first reduced when weeds reached 15 and 23 cm with soybean planted in 19- and 38-cm rows, respectively. Yield losses occurred only in the untreated control when soybean was planted in 76-cm rows. When growing conditions were less competitive (adequate rainfall and lower weed density), yield losses occurred only when weeds reached 30 cm or more in corn and only in the untreated control in soybean. Corn and soybean yields were higher when planted in narrow rows in three of 4 yr but were more susceptible to early- season weed interference than corn and soybean in wide rows. Corn yield was affected more by weed interference than was soybean yield. The product of weed height by weed density, as the independent variable, resulted in the best linear fit for both corn and soybean yields. High weed densities increase the risk of yield loss and must be considered when determining the appropriate timing for total postemergence herbicide applications such as glyphosate. Sequential glyphosate applications in corn did not increase yield. Nomenclature: Glyphosate; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; eastern black nightshade, Solanum ptycanthum Dun. # SOLPT; fall panicum, Panicum dichotomilflorum Michx. # PANDI; giant foxtail, Setaria faberi Herrm. # SETFA; green foxtail, Setaria viridis (L.) Beauv. # SETVI; jimsonweed, Datura stramonium L. # DATST; redroot pigweed, Amaranthus retroflexus L. # AMARE; velvetleaf, Abutilon theophroasti Medik. # ABUTH; yellow foxtail, Setaria glauca (L.) Beauv. # SETLU; corn, Zea mays L. ‘DK 493RR’; soybean, Glycine max (L.) Merr. ‘Pioneer 92B71’. Additional index words: Application timing, competitive load, narrow-row corn, narrow-row soybean, weed competition, weed interference. Abbreviations: DAE, days after crop emergence; GDDAE, growing degree days after emergence; POST, postemergence; SAS, Statistical Analysis Systems.

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 Fertilizer Nitrogen on Weed Emergence and Growth

Abstract The timing of nitrogen (N) fertilizer application may influence germination, emergence, and competitiveness of weeds. Research was conducted to determine the influence of total inorganic soil N (Nit) on the germination, emergence, and growth of five weed species. In a greenhouse experiment, seed of five weed species were exposed to four levels of N, and seed germination was measured. In the field, urea ammonium nitrate (UAN 28%) was applied at multiple rates at three spring timings, and Nit, weed emergence, and growth were measured for 21 to 35 d after application (DAA). Germination of the four dicotyledonous and single grass species was not stimulated by 450 ppmw of N compared with the untreated control. In the field, Nit of 112 or 168 kg N ha−1, measured at 7 and 21 DAA, was always greater than Nit in the untreated control. The duration of the available N pulse in the upper 8 cm of soil was dependent on N application rate and timing. At 8 to 16 cm of soil depth, Nit was greater when 168 kg N ha−1 was applied compared with no N at 21 and 35 DAA in 2004. Emergence of common lambsquarters increased as N application rate increased for each application date in 2003, but not in 2004. Emergence of ladysthumb increased with N application rate for the April 15, 2003, date; emergence of giant foxtail increased with N application rate for the April 6, 2004, date. Weed biomass was always greater when 168 kg N ha−1 was applied compared with no N, and at four of six N application dates, when 112 kg N ha−1 was applied. This research shows that spring N fertilizer applications increase Nit and weed growth, but the influence of N on weed emergence is dependent on the weed species, seed source, and environmental conditions. Nomenclature: Common lambsquarters, Chenopodium album L. CHEAL; ladysthumb, Polygonum persicaria L. POLPE.

Chemical and Physical Defense of Weed Seeds in Relation to Soil Seedbank Persistence

Abstract Effective weed seedbank management requires mechanistic understanding of ecological determinants of seed persistence in the soil seedbank. Chemical and physical defense of common lambsquarters, field pennycress, giant foxtail, kochia, velvetleaf, and yellow foxtail seeds were quantified in relation to short- and long-term seedbank persistence. Seed content of ortho-dihydroxyphenols (o-DHP), a class of putative seed defense compounds, varied more than threefold between the least protected species (common lambsquarters, 9.2 µg g seed−1) and the most protected species (kochia, 34.1 µg g seed−1). Seed o-DHP was inversely related (r  =  −0.77, P < 0.001) to seed half-life in the soil and to short-term seed persistence in burial assays (r  =  −0.82, P < 0.05). The relative importance of chemical seed protection in comparison to physical seed protection, as represented by the ratio of seed o-DHP concentration to seed coat thickness, decreased linearly with increasing short-term seed persistence (r  =  −0.96, P < 0.01) and nonlinearly with increasing long-term seed persistence in the soil seedbank (y  =  0.16 + 0.21/(0.0432 + x), R2  =  0.99, P < 0.001). Mechanical damage to the seed coat, via piercing, slicing, or grinding treatments, increased short-term mortality during burial for all six species. Mortality of pierced seeds was negatively associated (r  =  −0.35, P < 0.05) with seed phenol concentration and positively associated with seed half-life (r  =  0.42, P < 0.01) and seed coat thickness (r  =  0.36, P < 0.05). Seed phenolics, as a class, supported the results for o-DHPs. Overall, these findings suggest a potential weakness, with respect to seedbank management, in the way weed seed defenses are constructed. Weed species with transient seedbanks appear to invest more in chemical defense than those species with highly persistent seedbanks. As a result, seeds in the latter category are relatively more dependent upon physical seed protection for persistence in the soil seedbank, and more vulnerable to management tactics that reduce the physical integrity of the weed seed coat. Nomenclature: Common lambsquarters, Chenopodium album L.; field pennycress, Thlaspi arvense L.; giant foxtail, Setaria faberi Herrm.; kochia, Kochia scoparia (L.) Schrad.; velvetleaf, Abutilon theophrasti Medik.; yellow foxtail, Setaria glauca (L.) Beauv.

Effect of soybean row width and population on weeds, crop yield, and economic return

Field studies were conducted in 2004 and 2005 to determine the effect of soybean row width and population on weeds, canopy closure, crop yield, and economic return in glyphosate-resistant soybean. Soybean leaf area index (LAI) was greater in 19- and 38-cm, compared with 76-cm rows from 8 to 12 wk after planting in the low, moderate, and high soybean populations. Canopy closure was delayed by 2 wk in the moderate population in 76-cm rows compared with the high population in 19-cm rows. Fewer weeds emerged in 19-cm, compared with 76-cm rows following glyphosate application, and increasing the soybean population within a row width did not influence late-season weed emergence. Weed biomass in the weedy control was greater in the very low soybean population compared with the high soybean population within each row width; however, weed biomass in the weedy control was similar in the high and moderate soybean populations. Soybean yield in the weed-free and 10-cm glyphosate treatment did not differ, and yield was greater in 19-cm rows planted at moderate or high, compared with low populations. There was no difference in weed-free soybean yield at low, moderate, and high populations within 38- and 76-cm rows. Gross margins were usually greater in 19- and 38-cm, compared with 76-cm rows. The gross margin for soybean planted in 19-cm rows was usually greater at moderate or high soybean populations compared with lower populations. In 76-cm rows, the gross margin was greatest at the low and moderate soybean populations. When rainfall or other factors limited soybean yield, increasing the soybean population from approximately 300,000 plants/ha to 445,000 plants/ha in 19-, 38-, and 76-cm rows did not result in quicker canopy closure, reduced weed emergence, or greater soybean yield and gross margins. Nomenclature: Glyphosate potassium salt, glyphosate-resistant soybean, Glycine max (L.) Merr., ‘AG2701’

Yellow Nutsedge (Cyperus esculentus) Control and Tuber Production with Glyphosate and ALS-Inhibiting Herbicides1

Greenhouse and field research evaluated yellow nutsedge growth, vegetative control, and tuber production after application of glyphosate, various acetolactase synthase (ALS)–inhibiting herbicides, and tank mixtures thereof. Yellow nutsedge was controlled by the herbicides halosulfuron at 35 g ai/ha, chlorimuron at 12 g ai/ha, and imazethapyr–imazapyr at 62 g ai/ha (> 70% control); imazethapyr at 70 g ai/ha, glyphosate at 840 g ae/ha, cloransulam at 17.5 g ai/ha, and rimsulfuron at 17.5 g ai/ha (40 to 70% control); and imazamox at 45 g ai/ha (< 40% control). Compared with the untreated control, tuber fresh weight in the field was reduced 45 to 91%, and tuber density was reduced 33 to 90% by all herbicide treatments 42 wk after treatment (WAT) except imazamox and rimsulfuron. Tuber sprouting was reduced to 19% in plots treated with halosulfuron and pyrithiobac compared with untreated yellow nutsedge 42 WAT. Chlorimuron and imazethapyr–imazapyr controlled yellow nutsedge at least 90%, prevented panicle formation, and reduced tuber density and fresh weight by 90% or more 14 WAT in the greenhouse. The addition of glyphosate to cloransulam or imazethapyr increased yellow nutsedge control and reduced tuber density and fresh weight when compared with either ALS-inhibiting herbicide or glyphosate applied alone. Tuber density data indicated that there were 8 tubers for every gram of tubers harvested. Yellow nutsedge height was 15 to 20 cm 4 to 5 wk after tillage, using growth analysis data. Long-term yellow nutsedge management may be aided with treatments that reduce tuber production. Nomenclature: Chlorimuron; cloransulam; glyphosate; halosulfuron; imazamox; imazapyr; imazethapyr; pyrithiobac; rimsulfuron; yellow nutsedge, Cyperus esculentus L. #3 CYPES. Additional index words: Acetolactase synthase inhibitor, plant height, postemergence, shoot production, tubers. Abbreviations: ALS, acetolactate synthase; COC, crop oil concentrate; DAS, diammonium sulfate ((NH4)2SO4); MSO, methylated seed oil; NIS, nonionic surfactant; WAT, weeks after treatment.

Effect of Glyphosate Application Timing and Row Spacing on Weed Growth in Corn (Zea mays) and Soybean (Glycine max) 1

Corn and soybean were planted in narrow- and wide-row spacings to study the effects of glyphosate application timing and row spacing on light interception and subsequent weed growth. Corn planted in narrow rows (38 cm) had greater light interception than corn planted in wide rows (76 cm) from 35 to 55 d after crop emergence. Soybean planted in narrow rows (both 19 and 38 cm) had greater light interception throughout the growing season than soybean in 76-cm rows. At maximum canopy closure, narrow-row soybean (both 19 and 38 cm) intercepted more light than narrow-row corn. Biomass of weeds emerging after glyphosate application was greater when soybean was planted in 76-cm than in 19- or 38-cm rows. However, weed biomass was generally similar in both row spacings of corn. Sequential glyphosate applications reduced weed biomass in corn each year compared with a single glyphosate application at the 5-cm weed height. Sequential glyphosate applications that followed initial glyphosate application to 10- or 15-cm-tall weeds did not reduce weed biomass compared with a single application. Nomenclature: Glyphosate; corn, Zea mays L. ‘DK 493RR’; soybean, Glycine max (L.) Merr. ‘Pioneer 92B71’. Additional index words: Light intensity, narrow-row corn, narrow-row soybean, shading, weed control, weed interference. Abbreviations: DAE, days after crop emergence; POST, postemergence.