Reducing Weed Seed Rain with Late-Season Glyphosate Applications

1. Weed Technology 2007 21:753–758
Reducing Weed Seed Rain with Late-Season Glyphosate Applications
Chad E. Brewer and Lawrence R. Oliver*
Field trials were established in Fayetteville, AR, to determine the effects of glyphosate on biomass and seed production of
spurred anoda, entireleaf morningglory, hemp sesbania, and Florida pusley. Inconsistent control of these species by
glyphosate allows them to enrich the soil seedbank. Glyphosate rates of 0.42, 0.84, and 1.68 kg ae/ha were applied at 3, 6,
and 9 wk after emergence (WAE) and were compared with a nonsprayed control. Spurred anoda seed production was
reduced 99.9% by glyphosate at 1.68 kg/ha averaged over application timings and 99.7% by glyphosate applied at 6 WAE
averaged over rates. Entireleaf morningglory seed production was reduced 100% by glyphosate at 1.68 kg/ha at 3 WAE
and . 93% by the same rate applied at 6 or 9 WAE. Hemp sesbania seed production was reduced 100% by glyphosate at
1.68 kg/ha at 3 WAE and . 96% by glyphosate at 0.42, 0.84, and 1.68 kg/ha applied at 6 or 9 WAE. Florida pusley seed
production was reduced 99.9% by glyphosate at 1.68 kg/ha averaged over application timings and 99.4% by application at
3 or 6 WAE averaged over rates. Biomass accumulation for all species was reduced most by early applications, particularly
at 1.68 kg/ha.
Nomenclature: Glyphosate; entireleaf morningglory, Ipomoea hederacea var. integriuscula (L.) Jacq. A. Gray IPOHG;
Florida pusley, Richardia scabra L. RCHSC; hemp sesbania, Sesbania exaltata (Raf.) Cory SEBEX; spurred anoda, Anoda
cristata (L.) Schlecht ANVCR..
Key words: Seed production, seedbank, species shift.
The soil seedbank is the main source of all weeds present without regard for return of weed seed to the soil seedbank
in production agriculture (Cavers and Benoit 1989) and (Coble and Mortensen 1992). If a weed species emerges after
comprises all viable seeds present on or in the soil or the the critical weed-free period or if weed density is not sufficient
associated litter (Simpson et al. 1989). Many weed species to cause yield or quality reduction, these weeds may be left to
have emerged as problem weeds in glyphosate-based herbicide reproduce and enrich the soil seedbank. Enrichment is caused
systems. Spurred anoda control by glyphosate varies from 80 by weed seed rain, which is the reproduction and dispersal of
to 100% (Richardson et al. 2004). Entireleaf morningglory weed seed from the parent plant (Radosevich et al. 1997).
control can be as low as 64% (Starke and Oliver 1998). Weed seed rain effects were found to be a primary driving
Glyphosate rate and timing significantly influenced the force of seedbank density for wild oat (Avena spp,) and wild
control of both hemp sesbania and entireleaf morningglory radish (Raphanus raphanistrum L.) when infesting wheat
( Jordan et al. 1997; Payne and Oliver 2000). Florida pusley is (Triticum aestivum L.). Late-season herbicide applications to
another problem weed in glyphosate-based weed management prevent seed production were very effective in reducing
systems because it is not controlled consistently with seedbank densities ( Jones and Medd 2005). Weed seed rain
glyphosate ( Johnson and Mullinix 2002). is the primary input into the seedbank (Simpson et al. 1989),
Although herbicides can reduce weed interference with crop but a correlation of weed seed rain to the following season’s
production, seed production by weed escapes will replenish the seedling recruitment is species specific. Webster et al. (2003)
soil seedbank (Hartzler 1996, Schweizer and Zimdahl 1984). found that seedling recruitment for annual grass species was
Maintaining nearly zero seed production is imperative for the dependent on the seedbank density and seed rain from the
reduction of prolific seed producers in the weed seedbank previous autumn, whereas common lambsquarters (Chenopo-
(Norris 1983). Menges (1987) stated that after 6 yr of weed- dium album L.) seedling recruitment was not dependent on
free conditions, Palmer amaranth (Amaranthus palmeri S. seed rain when following no-till wheat production.
Watts.) populations were reduced from 7.7 to 0.8 seed/100 g There have been several attempts to reduce weed seed
soil, a 98% reduction in the seedbank. However, the production with herbicides. Common lambsquarters seed
remaining 2% represented 18 million seed/ha, which indicates production was reduced by 2,4-D at 1.1 kg ai/ha applied
that the weed-free conditions must be continued to further preanthesis by killing the plant (Fawcett and Slife 1978). In
reduce the presence of Palmer amaranth seed in the seedbank. the same study, 2,4-D applied during redroot pigweed
The ability to maintain 100% weed control in commercial (Amaranthus retroflexus L.) and jimsonweed (Datura strumar-
production fields would be costly because of the associated ium L.) flowering reduced seed production by 74 and 100%,
herbicide and labor cost. Therefore, the majority of weed respectively. However, Fawcett and Slife (1978) found that
management decisions are made based on variations of an 2,4-D applied at giant foxtail (Setaria faberi L.) flowering
economic threshold that deals only with the present crop increased seed production 381%. Chlorimuron at 0.28 kg ai/
ha and imazaquin at 0.28 kg ai/ha applied to sicklepod (Senna
DOI: 10.1614/WT-06-145.1 obtusifolia L.) during the early bloom stage reduced seed
* Senior Graduate Assistant and University Professor, Department of Crop,
Soil, and Environmental Sciences, 115 Plant Science Building, University of
production nearly 100% (Issacs et al. 1989). Glyphosate
Arkansas, Fayetteville, AR 72701. Corresponding author’s E-mail: cbrewer@ application has been shown to reduce hemp sesbania and
uark.edu pitted morningglory (Ipomoea lacunosa L.) seed production 95
Brewer and Oliver: Reducing weed seed rain with glyphosate N 753

2. Table 1. Spurred anoda, entireleaf morningglory, hemp sesbania, and Florida pusley planting and emergence dates, flower initiation date, and day length at flower
initiation for 2002 and 2003 at Fayetteville, AR.
Year Species Planting date Emergence date Flower initiation Day length at flower initiation
2002 Spurred anoda June 25 June 29 August 21 13 h 19 min
Entireleaf morningglory June 25 June 29 August 21 13 h 19 min
Hemp sesbania June 25 June 29 August 22 13 h 17 min
Florida pusley July 1 July 9 August 19 13 h 24 min
2003 Spurred anoda June 16 June 20 August 11 13 h 40 min
Entireleaf morningglory June 16 June 20 August 13 13 h 36 min
Hemp sesbania June 16 June 24 August 20 13 h 22 min
Florida pusley June 16 July 18 August 11 13 h 7 min
to 98% (Norsworthy and Oliver 2002a, 2002b), common 27% sand, 61% silt, 9% clay) with a pH of 5.8 and organic
purslane (Portulaca oleracea L.) 95% (Haar and Fennimore matter of , 1%. Each year the site was fertilized according to
2003), and sicklepod 86 to 100% (Barnes and Oliver 2003). University of Arkansas recommendations for corn (Zea mays
Although most previous weed control research includes data L.) production, which included 224–112–112 kg/ha N–
on weed seed production, most discuss seed production only P2O5–K2O (Espinoza and Ross 2003). Site preparation
as a secondary aspect. included spring disking, field cultivation, and disk-hipping
When annual plants begin reproduction, the floral buds in 1-m-wide rows. Following site preparation each year,
and seeds become the greatest sinks because of their rapid cell spurred anoda, entireleaf morningglory, hemp sesbania, and
expansion and high hormonal content (Hopkins 1999). Florida pusley seeds were hand-planted on 1-m raised beds
Glyphosate accumulates in reproductive tissue because those (Table 1). Each species was thinned to a uniform density
tissues act as metabolic sinks (Goughler and Geiger 1981; across the respective trials 2 wk after emergence (WAE) to
Sandberg et al. 1980). Glyphosate has been shown to decrease ensure uniform initial intraspecific interference. The initial
free auxin in soybean [Glycine max (L.) Merr.] and pea (Pisum plant density for each species is shown as stem number per
sativum L.) (Lee 1984) and to stimulate ethylene production meter of row for the nontreated check (Tables 2–7). These
in bean (Phaseolus vulgaris L.) (Abu-Irmaileh et al. 1979). densities were chosen based on the lowest emergence density
Auxin and ethylene are vital in the formation of normal for each species.
flowers (Swahney and Shukla 1994). Increases in free auxin A 3 by 3 plus 1 factorial treatment structure consisting of
inhibit development of male floral anatomy in some species glyphosate at 0.42, 0.84, or 1.68 kg ae/ha applied at 3, 6, or 9
(Chailakhyan and Khrianin 1978) but promote it in others WAE and one nontreated check were arranged in a random-
(Hamdi et al. 1987). Increases in ethylene can cause male ized complete-block design with four replications. Glyphosate
sterility in Brassica species (Banga and Labana 1983). The was applied with a CO2-backpack sprayer calibrated to deliver
accumulation of glyphosate in the reproductive tissue has 94 L/ha. Each plot consisted of a single strip of plants 2 m
caused decreased pollen viability and increased distance long with 1-m alleys between plots on top of raised 1-m-wide
between the anther and stigma in cotton (Gossypium hirsutum beds. Each replication was planted on a single 37-m row, and
L.) (Pline et al. 2003). Clay and Griffin (2000) found that both adjacent rows were fallow to reduce the impact of off-
glyphosate applied at weed flowering was effective in reducing target drift. This pattern was repeated across the field for all
seed production of common cocklebur (Xanthium strumarium trials.
L.), hemp sesbania, and sicklepod. Mixtures of glyphosate and After the first killing frost, the aboveground portion of
sodium chlorate used as preharvest desiccants reduced hemp plants from the middle 1 m of each plot was hand-harvested
sesbania and pitted morningglory seed production (Bennett at ground level, placed in paper bags, and allowed to air dry at
and Shaw 2000). The objective of this study was to determine approximately 27 C. There was no noticeable seed shatter
the effect of early-, mid-, and late-season glyphosate
applications on seed rain from weeds in four distinct
Table 2. Effect of glyphosate rate on end-of-season biomass and seed production
morphological families. of spurred anoda at Fayetteville, AR, 2002 and 2003.a
Total per meter of row
Materials and Methods Glyphosate rate Biomass Stem number Seed number
Experiments were conducted in 2002 and 2003 at the kg ae/ha g
University of Arkansas Research and Extension Center in Nontreated 392 a 7a 16,218 a
Fayetteville, AR, to determine the response of four weed 0.42 78 b 5 ab 776 b
species to selected glyphosate application rates and timings, 0.84 68 b 4b 427 b
with each species evaluated in a separate experiment. 1.68 11 c 2c 10 c
Differences in weed species emergence dates prevented a
Log-transformed data were used for analysis because of unequal treatment
combining the experiments; however, the experiments were variances. Log means were separated according to Fisher’s Protected LSD, P 5
conducted concurrently. The experiments were conducted on 0.05. Detransformed means are presented with log mean separation. Means
a Taloka silt loam (fine, mixed, thermic, Mollic Albaqualfs; within a column followed by same letter are not significantly different.
754 N Weed Technology 21, July–September 2007

3. Table 3. Effect of glyphosate application timing on end-of-season biomass and Glyphosate rates at or above 0.42 kg/ha decreased means for
seed production of spurred anoda at Fayetteville, AR, 2002 and 2003.a,b biomass and seed production compared with the control
Total per meter of row (Table 2). Seed number per meter of row was reduced 99%
Glyphosate by glyphosate at 1.68 kg/ha, averaged over application timing.
timing Biomass Stem number Seed number Total biomass production, averaged across rates, was reduced
WAE g most by the two earliest glyphosate applications (Table 3).
Nontreated 392 a 7a 16,281 a
Spurred anoda total biomass and stem number were similar
3 18 b 3b 148 bc for nontreated plants and plants treated at 9 WAE. Greater
6 15 b 3b 44 c plant mortality from applications made at 3 or 6 WAE
9 158 a 5a 369 b reduced stem number compared with applications made at 9
a
Log-transformed data were used for analysis because of unequal treatment
WAE. All glyphosate application timings reduced seed
variances. Log means were separated according to Fisher’s Protected LSD, P 5 number per meter of row compared with the nontreated
0.05. Detransformed means are presented with log mean separation. Means control (Table 3). Glyphosate application at 6 WAE reduced
within a column followed by same letter are not significantly different. seed number per meter of row more than applications made at
b
Abbreviation: WAE, weeks after emergence. 9 WAE. These data indicate that applying glyphosate at 3 or 6
WAE causes biomass reduction and reduces seed production
before plant harvest for any species. Samples were processed of spurred anoda compared with the nontreated plants.
by manually removing seeds after recording stem number and However, if a plant escapes early- season control, the best
total sample dry weight. To quantify seed production, seeds application timing to reduce seed production is at 6 WAE,
from each sample were weighed and total seed number per which coincided with the flower initiation in this experiment.
meter of row was calculated based on the average weight of These results concur with Puricelli et al. (2004), who reported
100 seeds for each sample. that glyphosate at 0.54 kg/ha achieved excellent control of
Because of unequal treatment variances, data analysis spurred anoda and greatly reduced seed production. However,
included logarithmic (log) transformation of individual data there was still a linear relationship between spurred anoda
points. Studentized residuals were plotted for each treatment biomass and seed production.
to determine whether the transformed data satisfied the Entireleaf Morningglory. The rate by timing interaction was
assumption of homogeneity. Log transformed data were then significant for all parameters for entireleaf morningglory.
analyzed using ANOVA by the PROC GLM function in Glyphosate applied at 1.68 kg/ha at 3 WAE reduced entireleaf
SAS.1 Year was analyzed as a random variable to make broader morningglory biomass production and stem number per
inferences about treatment effects (Carmer et al. 1989). meter of row compared with the nontreated control and lower
Treatment log mean separation was performed by Fisher’s rates (Table 4). Seed production per meter of row of entireleaf
Protected LSD at P 5 0.05. Data reported are detransformed morningglory was reduced $ 88% by glyphosate at 0.84 or
treatment means that are separated according to log mean 1.68 kg/ha at 9 WAE compared with the nontreated control.
separation. Glyphosate applied at 3 WAE did not reduce seed number
unless glyphosate rate was 0.84 or 1.68 kg/ha, which caused
plant death or severe stunting that persisted throughout the
Results and Discussion
season. Applications made at 6 or 9 WAE did not reduce stem
Spurred Anoda. For spurred anoda, the rate by timing number per meter of row or biomass per meter of row.
interaction was not significant for any response variable. Glyphosate applied at 6 WAE released the lateral buds from
Table 4. Effect of rate and timing of glyphosate application on end-of-season biomass and seed production of entireleaf morningglory at Fayetteville, AR, in 2002
and 2003.a,b
Total per meter of row
Glyphosate rate Timing Biomass Stem number Seed number
kg ae/ha WAE g
Nontreated 741 a 10 a 3,890 a
0.42 3 224 ab 10 a 1,413 ab
6 447 ab 10 a 2,455 a
9 741 a 12 a 1,413 a
0.84 3 62 b 5b 371 cd
6 288 ab 12 a 617 bc
9 562 a 12 a 457 c
1.68 3 2c 1c 0e
6 186 ab 10 a 251 d
9 537 ab 12 a 263 d
a
Log-transformed data were used for analysis because of unequal treatment variances. Log means were separated according to Fisher’s Protected LSD, P 5 0.05.
Detransformed means are presented with log mean separation. Means within a column followed by same letter are not significantly different.
b
Abbreviation: WAE, weeks after emergence.
Brewer and Oliver: Reducing weed seed rain with glyphosate N 755

4. Table 5. Effect of rate and timing of glyphosate application on end-of-season biomass, plant density and seed production of hemp sesbania at Fayetteville, AR, 2002
and 2003.a,b
Total per meter of row
Glyphosate rate Timing Biomass Stem number Seed number
kg ae/ha WAE g
Nontreated 891 a 10 a 7,943 a
0.42 3 427 b 7c 1,148 ab
6 269 c 9 ab 155 bcd
9 525 ab 10 a 126 bcd
0.84 3 174 c 6 bc 537 bc
6 214 cd 10 ab 229 bcd
9 617 a 9 ab 302 bcd
1.68 3 3e 2d 0e
6 95 d 9 ab 49 cd
9 417 b 8 ab 28 d
a
Log-transformed data were used for analysis because of unequal treatment variances. Log means were separated according to Fisher’s Protected LSD, P 5 0.05.
Detransformed means are presented with log mean separation. Means within a column followed by same letter are not significantly different.
b
Abbreviation: WAE, weeks after emergence.
apical dominance, whereas entireleaf morningglory had Florida pusley were reduced equally by application of
already attained maximum biomass by 9 WAE. Sublethal glyphosate at 0.42 and 0.84 kg/ha (Table 6).
glyphosate applications have been observed to remove lateral Glyphosate application timing was crucial for maximum
bud dormancy in soybean and pea seedlings (Lee 1984). effect on total biomass, stem number, and seed number of
However, Lee (1984) found that hand removal of the apical Florida pusley (Table 7). Total biomass per meter of row was
meristem did not increase indole-3-acetic acid metabolism, reduced by glyphosate applied at 3 and 6 WAE. Similarity
but sublethal glyphosate application did, which reduced levels between the nontreated control and the 9 WAE applications
of free indole-3-acetic acid. Reducing the amount of free indicates that the Florida pusley had reached its full size by 9
indole-3-acetic acid may alter the delicate balance of plant WAE, and any application made at this time had little effect
hormones and cause changes in flower morphology and loss of on biomass accumulation. Seed number per meter of row was
apical dominance. reduced by all application timings. Glyphosate applied at 3
For entireleaf morningglory, plants not controlled early in WAE caused plant death or severe stunting, whereas
the season, glyphosate applications of 1.68 kg/ha at 6 or 9 applications made at 6 WAE coincided with Florida pusley
WAE were necessary to reduce seed production approximately flower initiation and reduced the reproductive capability of
93%. These data indicate that a single glyphosate application the plant. The best glyphosate application timing to reduce
at 1.68 kg/ha during entireleaf morningglory flowering is Florida pusley seed production . 99% was 3 or 6 WAE.
sufficient to dramatically reduce seed production. Entireleaf Glyphosate was effective at reducing seed production of
morningglory biomass was reduced 92% and 99.7% by spurred anoda, entireleaf morningglory, hemp sesbania, and
glyphosate at 0.84 and 1.68 kg/ha, respectively, when applied Florida pusley. Although the application timings were species
at 3 WAE. specific, applications made near weed flowering reduced weed
Hemp Sesbania. Compared with nontreated plants, biomass seed production of each of these species with the greatest
was reduced by glyphosate, except at 0.42 and 0.84 kg/ha reduction occurring at the high rate. Adequate control of these
applied at 9 WAE (Table 5). Glyphosate at 1.68 kg/ha
applied at 3 WAE caused the greatest reduction in seed per
meter of row and biomass accumulation because of plant Table 6. Effect of glyphosate rate on end-of-season biomass, plant density, and
seed production of Florida pusley at Fayetteville, AR, 2002 and 2003.a
death. However, applications that did not cause plant death
reduced seed production $ 93%, with the exception of Total per meter of row
0.42 kg/ha applied at 3 WAE. It was observed that Glyphosate rate Biomass Stem number Seed number
applications at 6 WAE slowed the growth and development
of hemp sesbania immediately before flowering, whereas kg ae/ha g
applications made at 9 WAE were after flower initiation. Nontreated 389 a 7a 16,080 a
Bennett and Shaw (2000) reported that glyphosate at 1.65 kg/ 0.42 76 b 5b 715 b
0.84 59 b 3 bc 315 b
ha used as a preharvest desiccant reduced hemp sesbania seed 1.68 11 c 2c 10 c
production by 57%. This is in contrast to the level of seed
reduction seen in our study. a
Log-transformed data were used for analysis because of unequal treatment
variances. Log means were separated according to Fisher’s Protected LSD, P 5
Florida Pusley. The rate and timing main effects were 0.05. Detransformed means are presented with log mean separation. Means
significant for all parameters. All measured parameters for within a column followed by same letter are not significantly different.
756 N Weed Technology 21, July–September 2007

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and 2003.a,b treatment designs. Agron. J. 81:665–672.
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The authors would like to express appreciation to the Payne, S. A. and L. R. Oliver. 2000. Weed control programs in drilled
Arkansas Soybean Promotion Board for funding this research. glyphosate-resistant soybean. Weed Technol. 14:413–422.
Pline, W. A., K. L. Edmisten, J. W. Wilcut, R. Wells, and J. Thomas. 2003.
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