Impact
Statement

PDF version
for printing

© 2007 Plant Management Network.
Accepted for publication 6 October 2006. Published 19 January 2007.

Herbicide Impacts on Soil Seed Bank in a Sugarbeet Rotation

Gustavo M. Sbatella, Graduate Research Assistant, Department of Plant Sciences, Stephen D. Miller, Professor, Department of Plant Sciences, and David Legg, Professor, Department of Renewable Resources, University of Wyoming, Laramie 82071

Corresponding author: Gustavo M. Sbatella. gustavo@uwyo.edu

Sbatella, G. M., Miller, S. D., and Legg, D. 2007. Herbicide impacts on soil seed bank in a sugarbeet rotation. Online. Crop Management doi:10.1094/CM-2007-0119-01-RS.

Abstract

The impact of different weed control treatments on soil seed bank dynamics was studied at Torrington, WY, for a corn/sugarbeet and barley/sugarbeet crop rotation. Common lambsquarters, hairy nightshade, and redroot pigweed accounted for 90% of the total weed seed in the soil seed bank. Samples collected after corn or barley harvest showed no significant differences in total seed numbers due to herbicide treatments; however, differences were evident in samples collected following sugarbeet harvest. Glyphosate applied postemergence at the 2 and 6 true-leaf sugarbeet stage or the conventional treatment of ethofumesate preplant plus two postemergence applications of triflusulfuron plus phenmedipham / desmedipham / ethofumesate plus clopyralid in sugarbeet reduced the total number of weed seed in soil following all corn treatments. The reduction in total seed bank numbers was due to the decrease in redroot pigweed and hairy nightshade seed density. An increase in total seed bank numbers was observed when the micro-rate treatment (three postemergence applications of triflusulfuron plus phenmedipham / desmedipham / ethofumesate plus clopyralid) in sugarbeet followed any barley treatment. No sugarbeet treatment reduced total seed numbers following untreated plots in either barley or corn.

Introduction

Crop history in agroecosystems dictates weed community composition and species densities (4). Crop rotation, herbicide treatment, and tillage are among the most frequent cultural practices used for weed control. Each practice creates a disturbance that will influence the species composition of weed communities in agricultural fields (2). Although seed density generally declines as tillage intensity increases, crop rotation is the most dominant factor influencing seed bank composition over extensive periods (1,8). The mechanism by which crop rotation reduces the size of weed seed banks results from varying patterns of resource competition, allelopathic interference, soil disturbance, and variable weed management strategies (5). Application timing of production practices is also an important issue affecting weed seed banks in agricultural fields (6). All these factors combined ultimately determine which weeds dominate in the agroecosystem (10).

Numerous species are often found in weed seed banks. The species composition and density of weed seeds is linked to the cropping history (4,18). Wilson (18) reported that few dominant species constitute 70 to 90% of the weed seed bank, while a second subset of species comprise between 10 to 20% and finally a third group formed by remnants of past crops is also observed. The most prevalent species in the soil seed bank will be the dominant weeds present in the field due to their adaptation to control measures and cropping systems (5).

Although every weed control practice affects weed populations, herbicides are perhaps the most intense selective force applied by farmers. The type and amounts of herbicides used and their application timing are dictated by the crop present in the rotation. Selection will be toward species that are less susceptible. Early reports on the impact of herbicide programs on soil seed banks are found in studies conducted by Fryer and Kirkland (11) and Roberts and Neilson (14) in the UK. Schweizer and Zimdahl (16) reported the effects of planting continuous corn and herbicide applications on weed seed banks in Colorado. Further research showed that initial weed seed density determines seed bank response to weed control practice (4). Therefore, herbicides influence both seed number and species composition of the seed bank (1).

Sugarbeet (Beta vulgaris L.) yields are greatly affected by weed competition. Dawson (9) reported sugarbeet yield loss of 49% when competing with grasses, up to94% with broadleaf weeds, and 70% for mixed populations. Sugarbeet is a slow growing crop and therefore a poor competitor with weeds. Due to crop sensitivity and limited herbicide options, alternatives for weed control are restricted. Understanding the impact of crop rotation and herbicide treatments on soil seed banks of weed species that are difficult to control in sugarbeet can lead to better management practices. The objective of this study was to evaluate the influence of weed management treatments on soil seed bank dynamics in corn (Zea mays L.)/sugarbeet and barley (Hordeum vulgare L.)/sugarbeet crop rotations.

Field Experiment Design and Implementation

The study was conducted in 2002 and 2003 at Torrington, WY (elevation 1246 m, 42°05’N, 104°13’W) on a Valentine sandy loam (Ustic, Torripsamment, mixed mesic, 78% sand, 13% silt, 7% clay, 1.1% organic matter, and a pH of 7.3).

The crop rotations in the study consisted of corn/sugarbeet and barley/sugarbeet, both common among farmers in southeastern Wyoming. The two cropping sequences selected were located in adjacent experimental fields. In 2002 a total of sixteen plots (9 by 12 m) each were planted with either corn (Dekalb 4628 RR) or barley (Steptoe variety). Cultural practices used by local farmers were employed in each rotation. These included plow, disk, and roller harrow for seed bed preparation and furrow irrigation. All crops received spring fertilization with nitrogen and phosphorous based on soil analysis. Herbicide treatments were applied broadcast in all crops with a knapsack sprayer calibrated to deliver 187 liter/ha at 276 kPa pressure and a speed of 5 km/h. Detailed description of corn and barley treatments, herbicide rates, and crop stage at time of application is presented in Table 1.

Table 1: Herbicide treatments used in corn and barley in 2002.

 ^x Abbreviations: Dif = Diflufenzopyr; Dic = Dicamba; Rim = Rimsulfuron; Thifen = Thifensulfuron; NIS = Non Ionic Surfactant; Triben = Tribenuron.

 ^y Abbreviations: v/v = volume/volume.

 ^z See references 13 and 19 for growth stage information.

In 2003, all 9- by 12-m plots were split into four 3- by 9-m plots and planted to sugarbeet (Hilleshog HM 1605 RR). Sugarbeet herbicide treatments, rates, and application timing are described in Table 2. Weed seed bank samples were collected at three different periods in each rotation: first, after planting corn or barley in the spring of 2002, but before herbicide treatments; second, following harvest of corn and barley in the fall of 2002; and finally, after sugarbeet harvest in the fall of 2003. Five soil samples were taken with a 10-cm diameter tulip bulb planter to a depth of 15 cm following an X-shape pattern in each plot. Sub- samples were combined and bagged; each soil sample weighed approximately 1 kg. Excess air was removed from the bagged sample to reduce the risk of seed germination during storage. Samples were stored in a refrigerator at approximately 5°C (±2), and processed 15 to 20 days after collection. Seeds were separated from soil using a semi-automatic N.C. Elutriator (NC-E1). Seeds were collected on 300 µ mesh screens during the elutriation process. Material retained on the screens was washed onto filter paper placed on a Buchner suction funnel and excess water removed. Weed seeds were identified and counted by species using a dissecting scope with 10× magnification. Seed viability was determined by applying gentle pressure to each seed with tweezers. Seed resisting pressure were recorded as potentially viable (2,6,15). Seed numbers were expressed as seed/m² of soil.

Table 2. Herbicide treatments used in sugarbeet in 2003.

 ^x Abbreviations: Phen/desm/etho = Phenmedipham/desmedipham/ethofumesate.

 ^y Abbreviations: v/v = volume/volume.

 ^z See reference 12 for growth stage information.

In 2002, corn and barley treatments were arranged in a randomized complete block design with four replications. In 2003, each plot was split into 4 sub-plots where sugarbeet treatments were randomly assigned. Thus, the experimental design was a split-plot RCBD with 4 replications, with whole plots consisting of corn or barley treatments, and split-plots consisting of sugarbeet treatments for a total of 16 possible treatment combinations for each crop rotation. Weighted least squares analysis was utilized in the analysis to stabilize variance. Comparisons among sampling periods were conducted using linear contrasts at 0.05 probability level. All statistical procedures were performed using SAS version 8e (SAS Institute Inc., Cary, NC).

Effects of Weed Control Programs Applied in Corn/Sugarbeet and Barley/Sugarbeet Rotations on Soil Seed Banks

Hairy nightshade (Solanum physalifolium Rusby), common lambsquarters (Chenopodium album L.), and redroot pigweed (Amaranthus retroflexus L.) accounted for 90% of the total weed seed in the soil seed bank during the course of the study. Common purslane (Portulaca oleracea L.), green foxtail [Setaria viridis (L.) Beauv.], kochia [Bassia scoparia (L.) A.J. Scott] and stinkgrass [Eragrostis cilianensis (All.) Vign. ex Janchen] were the species that composed the remaining 10%, with none individually accounting for more than 3% of the total weed seed bank.

The analysis of samples collected following corn or barley harvest compared to the initial samples pulled after planting showed that herbicide treatments had no significant impact on total seed numbers (Table 3). Initial seed counts indicated that weed seed densities in the seed bank were quite low and dominated by species that usually exhibit high levels of seed dormancy. Buhler (4) reported that these conditions can result in a more stable seed bank. Even though herbicides reduced seed production, stable seed banks dominated by dormant seeds can be maintained with low annual seed input (17).

Table 3. Differences in total weed numbers between sampling times.

 ^x Tendency denotes whether a significant increase or decrease in the number of seeds was observed between the sampling times designated in each comparison. N/S = not significant; * = significant at 0.05.

Numerous differences were observed when the samples collected following sugarbeet harvest were compared to seed bank values obtained in corn or barley (Table 3). Results differed with rotation. In the corn/sugarbeet rotation, total seed were reduced after glyphosate or conventional treatments were applied in sugarbeet, regardless of the herbicide treatment used in corn; however, seed numbers were not significantly reduced with the micro-rate treatment in sugarbeet (Fig. 1). Since three species composed 90% of the seed bank in this study any changes in the total seed bank number can be explained by their impact on these species. Both the glyphosate and conventional treatments applied in sugarbeet had similar effects. These treatments reduced hairy nightshade and redroot pigweed seeds, while having no impact on common lambsquarters seed (Figs. 2 and 3). Different results were observed in the barley/sugarbeet rotation. No sugarbeet treatment reduced total seed numbers after barley treatments. Instead, the micro-rate treatment in sugarbeet resulted in a significant increase in seed bank numbers following all barley treatments (Fig. 4). The effect on dominant species from micro-rate applications differed by species. The micro-rate treatment reduced hairy nightshade seeds numbers; however redroot pigweed and common lambsquarters seeds increased significantly (Fig. 5).

Fig. 1. Impact of sugarbeet treatments on total seed numbers after corn treatments. Abbreviations: Conv. = conventional sugarbeet treatment; Glyph = glyphosate. * = significant difference compared to seeds in corn at α = 0.05.

Fig. 2. Impact of conventional sugarbeet treatments on main species. Abbreviations: SOLSA = hairy nightshade; AMARE = redroot pigweed; CHEAL = common lambsquarters; * = significant difference compared to seeds in corn at α = 0.05.

Fig. 3. Impact of glyphosate in sugarbeet on main species. Abbreviations: SOLSA = hairy nightshade; AMARE = redroot pigweed; CHEAL = common lambsquarters; * = significance at P = 0.05.

Fig. 4. Impact of sugarbeet treatments on total seed numbers after barley treatments. Abbreviations: Conv. = conventional sugarbeet treatment; Glyph = glyphosate. * = significant difference compared to seeds in barley at α = 0.05.

Fig. 5. Impact of micro-rate treatment in sugarbeet on main species. Abbreviations: SOLSA = hairy nightshade; AMARE = redroot pigweed; CHEAL = common lambsquarters; * = significant difference compared to seeds in barley at α = 0.05.

Sugarbeet treatments had no significant impact on total seed numbers when applied after weedy check treatments either in corn or barley (Table 3). This suggests that in highly infested fields planted to sugarbeet, no herbicide treatment will effectively reduce weed seed bank numbers.

A rapid increase in weed seed density of the dominant species can be expected when weeds are allowed to produce seed or left unchecked (5,7). In our study we observed this pattern only for common lambsquarters (Fig. 6 and 7) in both rotations.

Fig. 6. Species changes in weedy checks for corn / sugarbeet. Abbreviations: SOLSA = hairy nightshade; AMARE = redroot pigweed; CHEAL = common lambsquarters; * = significant difference compared to seeds in corn at α = 0.05.

Fig. 7. Species changes in weedy checks for barley / sugarbeet. Abbreviations: SOLSA = hairy nightshade; AMARE = redroot pigweed; CHEAL = common lambsquarters; * = significant difference compared to seeds in barley at α = 0.05.

Conclusions

No significant impact in total seed numbers of soil seed banks were observed after corn or barley regardless of weed control treatments (Table 4). However, differences were evident after sugarbeet harvest. Weed surface counts in sugarbeet untreated checks indicate higher weed emergence for plots under corn/sugarbeet rotation (Table 4). No herbicides were applied in these plots; therefore, differences in emergence between rotations can be attributed to different levels of seed dormancy. Temperature, photoperiod, and light wave length during seed formation can affect seed dormancy (3). Results suggest that seed formation conditions under each cropping system were different affecting seed dormancy.

Table 4. Total seed numbers in soil after corn and barley
harvest; surface weed counts in sugarbeet untreated checks.

Reduction in soil seed banks observed only in the corn/sugarbeet rotation were probably the combined effects of high weed emergence and subsequent effective weed control with the conventional treatment and glyphosate applications in sugarbeet.

The species prevailing in the seed bank indicate that cultural practices and the sugarbeet crop structure create an environment favorable for summer annual weed species. Under local conditions, common lambsquarters showed a capacity for high seed production after several herbicide management programs.

Literature Cited

1. Ball, D. A. 1992. Weed seed bank response totillage, herbicides, and crop rotation sequence. Weed Sci. 40:654-659.

2. Ball, D. A., and Miller, S. D. 1993. Cropping history, tillage, and herbicide effects on weed flora composition in irrigated corn. Agron. J. 85:817-821.

3. Bewley, J. D., and Black, M. 1985. Seeds: Physiology of Development and Germination. Plenum Press, New York, NY.

4. Buhler, D. D. 1999. Weed population responses toweed control practices. I. Seed bank, weed populations, and crop yields. Weed Sci. 47:416-422.

5. Buhler, D. D., Hartzler, R. G., and Forcella, F. 1997. Implications of weed seedbank dynamics toweed management. Weed Sci. 45:329-336.

6. Buhler, D. D., Kohler, K. A., and Thompson, R. L. 2001. Weed seed bank dynamics during a five-year crop rotation. Weed Technol. 15:170-176

7. Burnside, O. C., Moonmaw, R. S., Roeth, F. W., Wicks, G. A. and Wilson, R. G. 1986. Weed seed demise in soil in weed-free corn (Zea mays) production across Nebraska. Weed Sci. 34:248-251.

8. Cardina, J., Herms, C. P., and Doohan, D. J. 2002. Crop rotation and tillage system effects on weed seedbanks. Weed Sci. 50:448-460.

9. Dawson, J. H. 1965. Competition between irrigated sugar beets and annual weeds. Weeds 13:245-249.

10. Froud-Williams, R. J. 1988. Changes in weed flora with different tillage and agronomic management practices. Pages 213-227 in: Weed Management in Agroecosystems: Ecological Approaches. M. Altieri and M. Liebman, eds. CRC Press, Boca Raton, FL.

11. Fryer, J. D., and Kirkland, K. 1970. Field experiments toinvestigate long-term effects of repeated applications of MCPA, tri-allate, simazine and linuron: Report after six years. Weed Res. 10:133-158.

12. Holen, C. D., and Dexter, A. G. 1997. A growing degree day equation for early sugarbeet leaf stages. Online. Sugarbeet Res. and Ext. Rep. 27:152-157. Sugarbeet Research and Extension Board of Minnesota and North Dakota, Fargo, ND.

13. Ritchie, S. W., Hanway, J. J., and Benson, G. O. 1997. How a corn plant develops. Spec. Rep. No. 48. Coop. Ext. Serv., Iowa State Univ. of Sci. and Tech., Ames, IA.

14. Roberts, H. A., and Neilson, J. E. 1981. Changes in the soil seed bank of four long-term crop/herbicide experiments. J. Appl. Ecol. 18:661-668.

15. Roberts, H. A., and Rickettes, M. E. 1979. Quantitative relationships between the weed flora after cultivation and the seed population in the soil. Weed Res. 19:269-275.

16. Schweizer, E. E., and Zimdhal, R. L. 1984. Weed seed decline in irrigated soil after six years of continuous corn (Zea mays) and herbicides. Weed Sci. 32:76-83.

17. Thompson, K. 1992. The functional ecology of seed banks. Pages 231-258 in: The Ecology of Regeneration in Plant Communities. Commonw. Agric. Bureaux Int., Wallingford OX, UK

18. Wilson, R. G. 1988. Biology of weed seed in the soil. Pages 25-39 in: Weed Management in Agroecosystems: Ecological Approaches. M. Altieri and M. Liebman, eds. CRC Press, Boca Raton, FL.

19. Zadoks, J. C., Chang, T. T., and Konzak, C. F. 1974. A decimal code for the growth stages of cereals. Weed Res. 14:415-421.