Christy L. Sprague

Temperature effects on germination of nine Amaranthus species

Abstract Germination of weed seed and time of emergence are greatly affected by temperature. The effects of temperature on seed germination of tumble pigweed, prostrate pigweed, smooth pigweed, Palmer amaranth, Powell amaranth, spiny amaranth, redroot pigweed, common waterhemp, and tall waterhemp were examined under constant and alternating temperature regimens at 5, 10, 15, 20, 25, 30, and 35 C. Averaged over all temperatures, alternating temperature regimens increased total germination of all species, except Powell amaranth, which germinated similarly under both constant and alternating temperatures. In addition, Powell amaranth seed exhibited the highest total germination across all temperatures compared with the other amaranth species. Prostrate pigweed seed demonstrated the lowest total germination. Optimal temperatures for maximum germination were greater than 20 C for all species, except prostrate pigweed. The alternating temperature regimen centering at 30 C was used to compare the germination rates of the nine species. Palmer amaranth and smooth pigweed attained complete germination on the first day. The rate of germination for these species was much more rapid than the other Amaranthus spp., which took 3 to 8 d to reach 50% germination. Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; Palmer amaranth, Amaranthus palmeri S.Wats. AMAPA; Powell amaranth, Amaranthus powelli S.Wats. AMAPO; prostrate pigweed, Amaranthus blitoides S.Wats. AMABL; smooth pigweed, Amaranthus hybridus L. AMACH; spiny amaranth, Amaranthus spinosus L. AMASP; redroot pigweed, Amaranthus retroflexus L. AMARE; tall waterhemp, Amaranthus tuberculatus Sauer AMATU; tumble pigweed, Amaranthus albus L. AMAAL.

Common waterhemp (Amaranthus rudis) interference in corn.

Abstract Knowing the interference potential of common waterhemp in corn could be beneficial in planning waterhemp management strategies. In 2000, 2001, and 2002, field studies were conducted to examine both early- and late-season common waterhemp interference in corn. Early-season interference was determined by removing common waterhemp at the VE (vegetative emergence), V4 (four visible leaf collars), V6, V8, V10, V12, and V14 growth stages of corn for the entire season, and late-season interference was determined by allowing common waterhemp to emerge and compete from the VE, V4, V6, V8, V10, V12, and V14 corn growth stages. The interference potential of common waterhemp varied between the year 2000 and the combined years of 2001–2002. This is probably due to differences in precipitation in May and June in these two environments (297 mm in 2000 compared with 198 mm in 2001–2002). An excess of 590 g m−2 of dry matter and 13,000 and 1,200 seeds per female plant were produced when common waterhemp emerged at V4 and V6 corn, respectively, the 2 yr that corn was drought stressed. When corn was not moisture stressed, common waterhemp that emerged at V4 and V6 corn produced less than 220 g m−2 and less than 500 seeds per female plant. Season-long common waterhemp interference reduced corn yield 74% in 2 yr of the study and 11% in the third. Early-season common waterhemp interference began at V6 corn, with a 4 and 23% yield loss in 2000 and 2001–2002, respectively. Common waterhemp interference from late-season emergence reduced corn yield when emergence occurred before the V8 corn growth stage. Taking into account early- and late-season common waterhemp interference. the critical common waterhemp–free period was around the V6 corn stage to optimize corn yield. Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA; corn, Zea mays L.

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 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’

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.

Late-season common waterhemp (amaranthus rudis) interference in narrow- And wide-row soybean

Field studies were conducted in 2000, 2001, and 2002 at Urbana, IL, to examine the interference potential of common waterhemp that emerged at soybean growth stages VE, V2-V3, V4-V5, R1-R2, and R3-R4 in 19- and 76-cm row soybean. Soybean row width and common waterhemp emergence timing significantly influenced common waterhemp density, biomass, seed production, mortality, and soybean yield loss. Common waterhemp density declined as emergence timings were at later soybean growth stages. This decline happened at earlier growth stages in narrow-row soybean. Significant reductions in common waterhemp biomass and seed production occurred at the V2-V3 and V4-V5 emergence timings for the narrow- and wide-row soybean, respectively. Common waterhemp seed production was more than 23,000 seeds per plant at the VE emergence timing for both soybean row widths. Survival of common waterhemp that emerged after the V4-V5 soybean growth stage was less than 20% in both row widths. Common waterhemp interference reduced soybean seed yield at the VE, V2-V3, and the V4-V5 emergence timings. Row width affected the magnitude of yield reductions at these interference timings, with reductions being less in narrow-row soybean. This research suggests that control measures need to be implemented to common waterhemp plants that emerge before V4-V5 soybean to protect soybean yield and reduce common waterhemp seed production. Nomenclature: Common waterhemp, Amaranthus rudis Sauer #3 AMATA; soybean, Glycine max (L.) Merr. ‘Asgrow 3701RR’. Additional index words: Interference, integrated weed management, late-season competition.

Effects of shading on common waterhemp (Amaranthus rudis) growth and development

Abstract Common waterhemp is a significant weed problem in Midwestern cropping systems partly because of its potential for multiple emergence events during the growing season. The effects of shade and time of emergence on this weed have not been characterized. In the field, common waterhemp vegetative and reproductive growth were evaluated under different irradiance levels at two emergence times. In full sunlight a common waterhemp plant emerging in late May produced 720 g of biomass and over one million seeds, and a plant emerging in late June produced 350 g of biomass and over 730,000 seeds. Plant biomass and seed production were lower as irradiance levels were decreased to 40, 68, and 99% shade. Mortality was high for common waterhemp grown in 99% shade; however, surviving plants produced some viable seed. Common waterhemp plants grown under reduced irradiance had higher leaf area ratios and lower relative growth rates. Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA.

Tillage, Cropping System, and Soil Depth Effects on Common Waterhemp (Amaranthus rudis) Seed-Bank Persistence

Abstract A field experiment was conducted in Urbana, IL, from 1997 to 2000 to evaluate the effect that crop, tillage, and soil depth have on common waterhemp seed-bank persistence. A heavy field infestation of common waterhemp (approximately 410 plants m−2) was allowed to set seed in 1996 and was not allowed to go to seed after 1996. In 1997, 1998, 1999, and 2000, the percentage of the original common waterhemp seed bank that remained was 39, 28, 10, and 0.004%, respectively, averaged over tillage treatments. Initially, germination and emergence of common waterhemp was greater in no-till systems. Consequently, the number of remaining seeds was greater in the till treatments compared with no-till in the top 0 to 6 cm of the soil profile. This reduction was in part explained by the higher germination and emergence of common waterhemp in the no-tillage treatments. Tillage increased the seed-bank persistence of common waterhemp in the top 0 to 2 cm of the soil profile in 1997 and the top 0 to 6 cm in 1998. Crop had no effect on common waterhemp emergence or seed-bank persistence. In 2001, > 10% of the seed germinated that was buried 6 to 20 cm deep compared with 3% for seed 0 to 2 cm deep. Nomenclature: Common waterhemp, Amaranthus rudis Sauer AMATA

Weed Control with Fall and Early-Preplant Herbicide Applications in No-Till Soybean1

Field studies were conducted during 1999 and 2000 to compare weed control after fall and early-preplant (EPP) herbicide applications in no-till soybean. Three residual treatments (chlorimuron plus metribuzin, chlorimuron plus sulfentrazone, and metribuzin) were applied at two rates and timings (fall and 30 d EPP) either alone or in combination with glyphosate and 2,4-D. The addition of glyphosate and 2,4-D to fall-applied residual herbicides significantly increased control of common chickweed, annual bluegrass, cressleaf groundsel, and shepherds-purse. The effect of application rate on weed control was species dependent. Fall-applied residual herbicides were comparable with EPP treatments with respect to winter annual weed control; however, at planting control of summer annual weed species with fall treatments was less consistent compared with EPP residual herbicides. Nomenclature: Chlorimuron; glyphosate; metribuzin; sulfentrazone; 2,4-D ester; annual bluegrass, Poa annua L. #3 POAAN; common chickweed, Stellaria media L. Vill. # STEME; cressleaf groundsel, Senecio glabellus Poir. # SENGL; shepherds-purse, Capsella bursa-pastoris (L.) Medicus # CAPBP; soybean, Glycine max (L.) Merr. Additional index words: AMATA, AMBEL, AMBTR, CHEAL, fall applied, LAMAM, LAMPU, no till, residual control, winter annuals. Abbreviation: EPP, early preplant.

Physiological basis for tolerance of corn hybrids to foramsulfuron

Abstract Foramsulfuron (AE F130360) is a sulfonylurea herbicide for postemergence control of grasses and some broadleaf weeds in corn. Greenhouse and laboratory experiments were conducted to determine the physiological basis for differential tolerance of corn hybrids to foramsulfuron. Differences in corn tolerance were quantified by determining the herbicide rate required to cause injury and reduce corn height by 15% (GR15). Seven hybrids were screened in the greenhouse where GR15 values indicated that the some of the corn hybrids were sensitive to foramsulfuron. All but one of these hybrids exhibited an increase in tolerance when the safener, isoxadifen-ethyl, was applied with foramsulfuron. Experiments using 14C-foramsulfuron were conducted to determine whether isoxadifen-ethyl affected foramsulfuron absorption, translocation, and metabolism in two of the corn hybrids. There was no difference in absorption between a sensitive hybrid Novartis 58D1 and a more tolerant hybrid Novartis 59Q9, but the addition of isoxadifen-ethyl increased foramsulfuron absorption in both hybrids 24 h after treatment. Less than 1 and 3% of the 14C-foramsulfuron was translocated to plant portions above and below the treated leaf, respectively. Differences in hybrid tolerance were primarily due to differential herbicide metabolism. Foramsulfuron metabolism, with and without isoxadifen-ethyl, was similar at 4 h after treatment for both the sensitive and tolerant hybrids. However, by 24 h after treatment the more tolerant hybrid metabolized foramsulfuron to more polar compounds to a greater degree than the sensitive hybrid. Nomenclature: Foramsulfuron; corn, Zea mays L.