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Weed Seed Viability in Composted Beef Cattle Feedlot Manure
Francis J. Larney* and Robert E. Blackshaw
ABSTRACT
Schlather, 1994). At a typical local annual application
rate of 45 Mg ha
⫺
1
, approximately 340 seeds m
⫺
2
would
Manure composting has gained increased acceptance by the beef
be added to the soil seed bank.
cattle (Bos taurus) feedlot industry in southern Alberta, Canada.
Unlike fresh manure, compost is often promoted as being “weed- For these reasons, some farmers in southern Alberta
free.” Studies were conducted with five weed species in 1997 and
are reluctant to use fresh manure from nearby feedlots
thirteen in 1999 to examine the effect of feedlot manure composting
as a source of plant nutrients, especially for specialty
on weed seed viability. Weed seeds were buried in open-air compost
crops such as potato (Solanum tuberosum L.), sugar
windrows and recovered at various times during the thermophilic
beet (Beta vulgaris L.), and timothy hay (Phleum pra-
phase of composting. Windrow temperature and water contents were
tense L.). Farmers choosing to use fresh feedlot manure
also measured. Germinability was zero for all composted weed seeds
may concentrate its application on already weedy fields
at all sampling times in 1997. However, some seeds remained viable
or on fields where they can practice intensive weed
(positive tetrazolium test denoting respiration) on Day 70. In 1999,
control, if they suspect the manure contains large
only one of the thirteen species retained germinability on Day 21
amounts of weed seeds. This potentially lowers the
and only two species had respiring seeds on Day 42. Time–viability
relationships during composting were defined by exponential decay available land base for the large volumes of manure
models. Lethal temperatures to eliminate viability was species-depen-
produced in the region, leading to overapplication and
dent. In 1999, four weed species were killed in the initial 7 d of
its associated effect on soil, water, and air quality (Chang
composting at a lethal temperature of 39ⴗC while temperatures of
and Entz, 1996; Chang et al., 1998).
⬎60ⴗC were required for two species. Regression analysis on weed
Composting is a biological process, whereby regular
seed viability versus windrow temperature resulted in significant R
2
introduction of air by mechanical turning stimulates aer-
values, which showed that only 17 to 29% of the variation in viability
obic microorganisms to reduce organic materials such
was accounted for by temperature. The lack of definitive relationships
as livestock manure to a more stable material similar to
between temperature and weed seed viability demonstrated that fac-
humus (Rynk, 1992). Increasingly, composting is being
tors other than temperature may play a role in eliminating weed seeds
adopted by the beef feedlot industry in southern Alberta
during composting.
as an alternative handling method to traditional direct
haulage of fresh manure from feedlot to field (Larney
et al., 2000).
I
n southern Alberta’s beef feedlot industry, feed
Agricultural compost is often promoted as “weed-
grain is sourced locally as well as from the neigh-
free” since one of the benefits of composting is the
boring provinces of Saskatchewan and Manitoba. More
destruction of weed seed viability by the high tempera-
recently, feed corn (Zea mays L.) has been imported
tures (⬎60⬚C) achieved during the process (Eghball and
from the U.S. Midwest. Since many weed species retain
Lesoing, 2000; Tompkins et al., 1998; Wiese et al., 1998).
their viability after digestion by animals (Blackshaw and
However, Cudney et al. (1992) found that composted
Rode, 1991), new weeds may be introduced, or an in-
manure (6–8 wk old) from five California dairies con-
crease in the population of existing weeds may occur,
tained varying amounts of viable weed seeds. Churchill
when feedlot manure is land-applied. Manure from un-
et al. (1996) suggested that increased turning frequency
known feed sources may pose a greater risk of dissemi-
during composting reduced survival of all weed species,
nating weeds than manure from feed grown on one’s
probably as a result of increased temperatures. Grundy
own operation. There is speculation that introduction
et al. (1998) believed that weed seeds that survived the
of the noxious weed velvetleaf (Abutilon theophrasti
composting process did so as a result of localized “cool
Medik.) to New York farms in the 1970s was as a result
spots” caused by inefficient turning of the windrow.
of spreading manure from animals fed corn imported
Viable weed seeds may also contaminate finished com-
from the U.S. Midwest (Mt. Pleasant and Schlather,
post in open-air windrows via dissemination and deposi-
1994). Cudney et al. (1992) germinated weed seeds pres-
tion by wind.
ent in manure from five dairies in California and found
Early workers (Atkeson et al., 1934; Harmon and
viable numbers of up to 19 730 seeds Mg
⫺
1
, depending
Keim, 1934; Stoker et al., 1934) buried weed seeds in
on collection site. They recognized that this was only a
static piles of cattle, horse (Equus caballus), and chicken
fraction of the total number, due to dormancy. A survey
(Gallus gallus domesticus) manure and found that re-
of manure from 26 New York farms revealed an average
duction in viability varied by weed species. Hopkins
of 75 100 weed seeds Mg
⫺
1
manure (Mt. Pleasant and
(1936) reported that there was a critical temperature
below which moderate periods of heating had little ef-
Agriculture and Agri-Food Canada, Research Centre, 5403 1st Ave-
fect on viability and above which germination fell off
nue S., Lethbridge, Alberta, Canada T1J 4B1. Lethbridge Research
rapidly. In more recent research with composting, Wiese
Centre contribution no. 38702008. Received 12 Feb. 2002. *Corre-
sponding author (larney@agr.gc.ca).
Abbreviations: CDD, cumulative degree days.Published in J. Environ. Qual. 32:1105–1113 (2003).
1105
1106 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
the compost windrow in a factorial design. Two grass [green
et al. (1998) found that field bindweed (Convolvulus
foxtail (Setaria viridis (L.) Beauv.) and wild oat (Avena fatua
arvensis L.) was the most difficult of six weed species
L.)] and three broadleaf weed species [redroot pigweed (Ama-
to eliminate. The five other weed species were killed if
ranthus retroflexus L.), stinkweed (Thlaspi arvense L.), and
compost temperatures were maintained at 72⬚Cfor3d.
wild buckwheat (Polygonum convolvulus L.)] were selected.
Eghball and Lesoing (2000) reported that when com-
A total of 300 sample bags (6.5 ⫻ 5.5 cm), each containing
posting manure is kept moist, weed seed viability may
200 seeds, was prepared for the study. The bags were made
be destroyed even though the critical temperature is
from nylon screening with a mesh size of 500 m and an open
not reached, possibly because of compost phytotoxins.
area of 49%. This was fine enough to retain the seeds while
Ligneau and Watt (1995), Marchiol et al. (1999), and
allowing their exposure to temperature and moisture condi-
Ozores-Hampton et al. (1999) also showed that the toxic
tions within the windrow. Of the 300 bags, 225 (five weed
components of leachates from compost or soil–compost
species ⫻ five removal times ⫻ nine windrow locations) were
mixtures reduced germination of certain weeds, grasses,
buried in the compost windrow and 75 were used as control
samples (five weed species ⫻ five sampling times ⫻ three
and legumes.
replicates). The nine windrow locations were: east–top, east–
This study examined weed seed viability of common
middle, east–bottom, center–top, center–middle, center–
weed seeds found in southern Alberta during beef feed-
bottom, west–top, west–middle, and west–bottom. The east,
lot manure composting. It sought to elucidate relation-
center, and west locations corresponded to approximately 25,
ships between duration of composting and lethal tem-
50, and 75% of the length of the windrow, while the top,
peratures required to eliminate weed seed viability.
middle, and bottom locations were centered on the windrow
at approximately 0.9, 0.6, and 0.3 m from the ground.
MATERIALS AND METHODS
To facilitate weed seed recovery from the compost, the five
bags of each species were tied together with orange-colored
Compost Windrows
twine that ran to the outer surface of the windrow, where it
The study was performed at the Agriculture and Agri-Food
was labeled with the species name.
Canada Research Centre, Lethbridge, Alberta during the sum-
The control sample bags were placed in a covered plastic
mers of 1997 and 1999. Cattle manure, where barley (Hordeum
pail, which had holes to allow air entry but not moisture. The
vulgare L.) straw had been used for bedding, was removed
pail was attached to a pole at a 2-m height near the windrow.
from feedlot pens with a front-end loader. In 1997, the manure
Three control bags were removed at each sampling date for
was loaded onto a manure spreader and then deposited into
comparison with the composted samples.
windrows. The mechanical action of the manure spreader en-
At each of the five weed seed removal times (Table 1), all
sured some initial mixing on Day 0 (20 May 1997). In 1999, the
bags were recovered from the compost windrow just before
manure was loaded into a truck and deposited into compost
turning. One bag from each of the nine locations was sampled
windrows on 20 July 1999 (Day 0) with little or no mixing.
and stored at 0.5⬚C, while the remaining bags were reburied
The windrows were turned seven times over a 99-d period in
in the compost immediately after turning.
1997 and a 70-d period in 1999 (Table 1) with a tractor-pulled
EarthSaver windrow turner (Fuel Harvesters Equipment,
1999 Study
Midland, TX). This represented the active thermophilic com-
posting phase, and as such produced an almost mature com-
In 1999, the experimental design was modified to include
post. After 100 d the compost entered a mesophilic or “curing”
more weed species (13), only one replicate of the three wind-
phase (no turning) for a further 90 d until windrow tempera-
row locations (top, middle, and bottom), and only one control
ture approached ambient.
sample per weed species. The thirteen weed species included
The windrows were on an east–west orientation and varied
the five used in 1997 and eight new ones: downy brome (Bro-
in length from 13 to 15.2 m. They were about 1.6 m high and
mus tectorum L.), false cleavers (Galium aparine L.), foxtail
3.6 m wide at the base. The 1997 study was conducted outdoors
barley (Hordeum jubatum L.), green smartweed Polygonum
and hence exposed to precipitation. The 1999 study was con-
scabrum Moench), round-leaved mallow (Malva pusilla Sm.),
ducted in a roofed composting facility with no walls, so that
scentless chamomile (Matricaria perforata Merat), stork’s-bill
compost was exposed to ambient air temperatures but not pre-
[Erodium cicutarium (L.) L’Her. ex Ait.], and wild mustard
cipitation.
(Sinapsis arvensis L.).
A total of 208 nylon-mesh bags was prepared (16 bags per
Weed Seed Placement
weed species, each with 200 seeds) as described previously.
Of the 208 bags, 195 (13 weed species ⫻ 5 removal times ⫻
1997 Study
3 windrow locations) were buried in the compost (in the center
In 1997, treatments consisted of (i) weed species, (ii) time
of the windrow, 50% along its length) and 13 were used as
of removal from the compost windrow, and (iii) location within
controls. The control samples were placed as described for
1997 and removed at the last sampling date (Day 91).
Table 1. Day of turning (date in parentheses) for the 1997 and
Weed seeds were removed from compost at five times over
1999 composting trials. Weed seed sampling dates are shown
the composting period (Table 1), the same number as in 1997.
in italic type.
However, the first removal time was earlier (Day 7 vs. Day
Activity 1997 1999
14) and the last removal time later (Day 91 vs. Day 70) than
Windrow setup Day 0 (20 May) Day 0 (20 July)
in 1997.
First turning Day 14 (3 June) Day 7 (27 July)
Second turning Day 21 (10 June) Day 14 (3 August)
Compost Temperature
Third turning Day 29 (18 June) Day 21 (10 August)
Fourth turning Day 50 (9 July) Day 29 (18 August)
Windrow temperatures were monitored with thermocou-
Fifth turning Day 70 (29 July) Day 42 (31 August)
Sixth turning Day 84 (12 August) Day 56 (14 September)
ples and a data logger (Sciemetric, Nepean, ON, Canada).
Seventh turning Day 99 (27 August) Day 70 (28 September)
Thermocouples were installed as soon as the windrows were
Day 91 (19 October)
formed. They were removed just before turning and reinstalled
LARNEY & BLACKSHAW: WEED SEED VIABILITY IN FEEDLOT MANURE 1107
as soon as possible after turning was completed. In 1997, 1989). Regression analysis was used to define relationships
between time and viability, and temperature and viability.temperatures were measured adjacent to weed seed bags at
six (east–top, east–middle, east–bottom, center–top, center–
middle, center–bottom) of the nine locations. In 1999, temper-
RESULTS
atures were measured at the top, middle, and bottom locations
at the east and west ends of the windrow (approximately 25
Weather Conditions
and 75% along its length). The average of these two tempera-
Total precipitation during the active composting
ture values was used to estimate the temperature that corre-
phase from 20 May–29 July 1997 was 212 mm, which
sponded to the location of the weed seed bags (50% along
may be considered as a water input to the compost
the windrow length). In both studies, temperatures were
logged every 20 min and then averaged to give daily mean
windrow. The 1999 windrow was under a roofed struc-
values.
ture, which prevented any water addition via precipita-
The degree day concept was used to integrate temperatures
tion. Mean monthly air temperatures were 11.3, 16.0,
over the composting period. Using a base temperature of 40⬚C,
and 18.2⬚C for May–July 1997 and 16.4, 18.8, 12.9, and
degree days were calculated for each location on each day as:
8.4⬚C for July–October 1999.
(mean daily temperature) ⫺ (40)
Compost Temperature
If the mean daily temperature was ⬍40⬚C, then degree days
were 0. The degree day values were then summed to give
Mean daily compost temperatures showed a rapid
cumulative degree days (CDD) for each location on each
rise in the early days of composting in 1997, with the
sampling date. Guidelines for the control of pathogens during
maximum temperature for the entire composting period
composting refer to the material maintaining a temperature
occurring on Day 2 for the middle (63.3⬚C) and bottom
of ⬎55⬚C for at least 15 d during the composting period (Cana-
(68.6⬚C) locations (Fig. 1a). The top location was much
dian Council of Ministers of the Environment, 1995). Also,
cooler than the middle and bottom locations throughout
during the high-temperature period, the windrow should be
the composting period and reached a maximum temper-
turned at least five times. We decided to use a 40⬚C base
ature of 55.5⬚C on Day 81 by which time the last weed
temperature as we believed that weed seed viability may be
seed removal date had passed (Day 70). The influence
affected at temperatures of ⬍55⬚C.
of turning on compost temperatures was denoted by the
Compost Water Content
In conjunction with turning, and hence weed seed removal
dates, compost samples (approximately 0.6 kg) were taken for
water content determination from two vertical windrow faces
exposed with a small front-end loader. In 1997, the water
content sample was a composite from five locations on the
exposed faces. In 1999, samples were taken at the top, middle,
and bottom locations of each vertical face (three locations ⫻
two replicates). Water content was expressed on a wet weight
basis after oven-drying at 60⬚C to a constant weight.
Weed Seed Viability
Weed seed viability was determined as outlined by Black-
shaw and Rode (1991). Briefly, all 200 seeds from each of
composted and control weed seed bags were placed on moist-
ened filter paper in Petri dishes in a controlled environment
chamber (temperature 20⬚C, relative humidity 40–50%) and
allowed to germinate. Seeds that did not germinate were sub-
jected to a tetrazolium test (Grabe, 1970) by placing them in
Petri dishes on filter paper moistened with a solution of 1%
tetrazolium. After 48 h at room temperature, the seeds were
examined for red staining at the growing point, an indication
of respiration and hence viability. Seeds with a positive tetra-
zolium test were summed with germinable seeds to give viable
seeds. Viable seeds were then expressed as a percent of total
seeds to arrive at a percent viability. In some cases, viability
was also expressed as a percent of the control sample
(control ⫽ 100% viability) to account for the variation in
the viability of the control samples among weed species and
between the same species in the two studies.
Statistical Analysis
The effect of vertical location on weed seed viability in 1997
Fig. 1. Effect of windrow location on temperature during composting
and on cumulative degree days in 1997 and 1999 was analyzed
in (a ) 1997 and (b) 1999. Triangle symbols on the x axes represent
turning dates.with the general linear models procedure (SAS Institute,
1108 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
sudden drop in temperature as cooler air was introduced values at the top location ranged from 12.2% of the
middle location on Day 21 to only 6.5% of the middleto the windrow, followed by a rapid rise as aerobic
microbial activity was stimulated (Fig. 1a). location on Day 91. The cooler temperatures associated
with the top location are due to the semicircular shapeIn contrast, the mean daily temperatures in the 1999
study were much cooler in the early part of the compost- of the windrow, which exposed the top location to more
ambient conditions.ing period (Fig. 1b) because manure was formed into
windrows on Day 0 and left unturned until Day 7. The In the 1997 study, it took 29 d for the bottom location
to attain 573 CDD. The cooler temperature regime ofabsence of a premixing on Day 0 precluded a rapid
early rise in temperature. Subsequently, temperatures the 1999 study is demonstrated by a similar number of
CDD (559) over a longer period (42 d).climbed steadily reaching maxima of 46.8⬚C (Day 29)
for the top location, 67.1⬚C (Day 47) for the middle
location, and 67.9⬚C (Day 49) for the bottom location. Compost Water Content
In 1997, the bottom location had significantly more
Average water content of the compost windrow was
cumulative degree days (CDD) than the middle loca-
0.71 kg kg
⫺
1
at the start of the 1997 study on Day 0
tion, which in turn had significantly more than the top
(Fig. 3a). Optimal water contents for composting range
location on Days 14, 21, 29, and 50 (Fig. 2a). By Day 70,
from 0.40 to 0.65 kg kg
⫺
1
(Rynk, 1992). Moisture content
the bottom (941 CDD) and middle (878 CDD) locations
decreased to 0.65 kg kg
⫺
1
on Day 29 and 0.51 kg kg
⫺
1
were not significantly different from each other, as the
on Day 70. A total of 212 mm of precipitation fell on
middle location showed a larger increase in temperature
the windrow during the 70-d composting period, which
following the turning event on Day 50 (Fig. 1a). How-
helped prevent excessive water loss.
ever, the top location was still significantly cooler
In the 1999 study, water content was lower at all
(494 CDD).
points in the composting process than in 1997 (Fig. 3a).
In 1999, there were no significant differences in CDD
This was due to roofed protection of the windrow from
with location on Days 7 and 14 (Fig. 2b). On Day 7,
incoming precipitation. Water content averaged 0.67 kg
there were 0 CDD at the top, 13 CDD at the middle,
kg
⫺
1
on Day 0 and decreased to 0.52 kg kg
⫺
1
on Day
and 1 CDD at the bottom location. The middle and
42 (close to the value on Day 70 for the 1997 study).
bottom locations were not significantly different from
By Day 91, compost water content had dropped to 0.29
each other on any of the five weed seed removal dates.
kg kg
⫺
1
. Larney et al. (2000) reported water mass losses
The top location was significantly cooler than the middle
of up to 75% during the thermophilic phase of summer
and bottom locations on Days 21, 42, and 91. The CDD
composting due to high evaporation rates.
Fig. 2. Effect of windrow location on cumulative degree days (CDD)
during composting in (a) 1997 and (b ) 1999. Within dates, location Fig. 3. Windrow water content (a ) during composting in 1997 and
1999 and (b ) as affected by windrow location in 1999.means with the same letter are not significantly different (P ⫽ 0.05).
LARNEY & BLACKSHAW: WEED SEED VIABILITY IN FEEDLOT MANURE 1109
In the early stages of composting in 1999, all vertical row compared with 34% for the control sample. By Day
locations behaved similarly (Fig. 3b). By Day 21, how-
70, viable seeds remained in the top (1.8%) and bottom
ever, the top location was substantially drier (0.45 kg
locations (0.2%), which may be enough to cause a weed
kg
⫺
1
) than the middle (0.59 kg kg
⫺
1
) and bottom (0.65
infestation if the compost was land-applied at this stage.
kg kg
⫺
1
). This was due to the semicircular shape of
There were 25 comparisons (five weed species ⫻ five
the windrow as the top location was more exposed to
sampling dates) of weed seed location in the compost
evaporative drying.
windrow on viability in the 1997 study. However, there
was only one comparison that showed a significant loca-
tion effect. This was wild buckwheat on Day 21, when
Time–Viability Relationships
the seeds placed near the top of the windrow showed
1997 Study
significantly lower viability (4.3%) than those placed
at the middle (13.8%) and bottom (13.3%) locations
In 1997, viability of the control samples (average of
(Table 2). A possible explanation for this might be
five sampling dates) was 32% for wild buckwheat, 86%
higher concentrations of oxygen at the top of the wind-
for green foxtail, 76% for redroot pigweed, and 75%
row than at the middle and bottom locations, which may
for wild oat. Because of a poor lot of stinkweed seed, the
have encouraged germination of wild buckwheat seeds
viability of the control samples was very low (average of
into lethal conditions, hence reducing overall viability
4.2%). Germinability was zero for all composted weed
of the recovered seeds.
seeds at all sampling times, showing that composting
had a dramatic effect on weed seed survival. However,
even though weed seeds did not germinate when sub-
1999 Study
jected to composting, some remained viable as denoted
In 1999, viability of the control samples ranged from
by a positive tetrazolium test. Since there were zero
13.5% for stinkweed to 95.5% for downy brome
germinable seeds in composted samples, all viability
(Table 3). Viability of four weed species (downy brome,
values for compost in Table 2 were entirely due to seeds
scentless chamomile, stork’s-bill, and wild mustard) had
with a positive tetrazolium test. Green foxtail, redroot
dropped to zero after just 7 d of composting. Of the
pigweed, and wild oat survival in compost was quite
remaining nine species on Day 7, germinable seed and
similar. By Day 14, the viability of these species had
respiring seed (positive tetrazolium test) contributed to
dropped to between 2 and 12% compared with 64 to
viability of five species (false cleavers, green smartweed,
89% for the control samples. By Day 29, their viability
redroot pigweed, round-leaved mallow, wild buck-
was 0.2 to 6% while at Day 70 viability was zero for all
wheat) while the viability of four species was due to
three species. Since the viability of control samples for
respiring seed only (foxtail barley, green foxtail, stink-
stinkweed was so low, comparisons with composted
weed, wild oat).
samples are not that meaningful.
By Day 14, viability of foxtail barley had dropped to
Wild buckwheat seed was more resilient to compost-
zero. Of the eight species with viable seeds on Day
ing. On Day 14, viability was similar to the control
14, round-leaved mallow was the only one to retain
samples (Table 2). Even on Day 29, viability of the
germinability. The 14% viability was comprised of
composted samples was not significantly different from
10.7% germinable seed and 3.3% respiring seed. The
the control. By Day 50, however, viability of wild buck-
wheat had dropped to ⬍2% at all locations in the wind- viability of the other seven species was entirely due to
Table 2. Effect of location and time of removal from compost windrow on weed seed viability in 1997.
Time of removal
Weed Location Day 14 Day 21 Day 29 Day 50 Day 70
%
Green foxtail control 87.7a† 85.1a 88.7a 84.1a 82.3a
compost, top 2.7b 0.0b 0.0b 0.0b 0.0b
compost, middle 6.8b 0.0b 6.0b 0.0b 0.0b
compost, bottom 9.2b 1.2b 2.3b 0.7b 0.0b
Redroot pigweed control 72.9a 78.3a 73.8a 77.6a 77.1a
compost, top 3.5b 0.7b 0.0b 0.0b 0.0b
compost, middle 3.8b 5.7b 0.2b 0.0b 0.0b
compost, bottom 7.1b 5.3b 1.0b 0.0b 0.0b
Stinkweed control 3.4a 2.8a 2.0a 5.1a 7.7a
compost, top 0.5b 0.3a 1.2a 1.0b 4.3a
compost, middle 0.0b 0.2a 4.3a 1.2b 6.8a
compost, bottom 0.0b 0.3a 4.3a 0.0b 1.7a
Wild buckwheat control 26.5a 36.3a 33.0a 33.7a 31.9a
compost, top 32.7a 4.3c 6.4a 1.7b 1.8b
compost, middle 32.4a 13.8b 13.0a 0.7b 0.0b
compost, bottom 12.0a 13.3b 17.0a 1.0b 0.2b
Wild oat control 79.5a 64.4a 66.6a 85.4a 77.1a
compost, top 12.0b 2.7b 0.0b 0.0b 0.0b
compost, middle 5.6b 1.0b 0.0b 0.0b 0.0b
compost, bottom 2.0b 0.3b 1.3b 0.0b 0.0b
† Within columns and weed species, means followed by a different letter are significantly different (P ⬍ 0.05).
1110 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
Table 3. Effect of time of removal from compost windrow on weed seed viability in 1999.
Time of removal
Weed Control Day 7 Day 14 Day 21 Day 42 Day 91
%
Downy brome 95.5 0.0 0.0 0.0 0.0 0.0
False cleavers 54.0 2.5† 6.5‡ 0.0 0.0 0.0
Foxtail barley 79.0 0.5‡ 0.0 0.0 0.0 0.0
Green foxtail 91.0 2.8‡ 1.0‡ 1.2‡ 0.0 0.0
Green smartweed 62.5 21.0† 12.0‡ 9.0‡ 3.3‡ 0.0
Redroot pigweed 77.5 15.7† 7.2‡ 2.0‡ 0.0 0.0
Round-leaved mallow 21.0 13.5† 14.0† 6.5† 0.0 0.0
Scentless chamomile 94.5 0.0 0.0 0.0 0.0 0.0
Stinkweed 13.5 5.5‡ 8.5‡ 3.3‡ 0.0 0.0
Stork’s-bill 94.5 0.0 0.0 0.0 0.0 0.0
Wild buckwheat 52.0 14.7† 30.7‡ 15.2‡ 3.0‡ 0.0
Wild mustard 50.0 0.0 0.0 0.0 0.0 0.0
Wild oat 66.0 0.7‡ 0.8‡ 0.0 0.0 0.0
† Germinable seed and respiring seed contributing to viability.
‡ Zero germinable seed, respiring seed only contributing to viability.
respiring seeds. Viability of wild buckwheat was higher a viability of 3.5% after 14 d (Tompkins et al., 1998),
which compared favorably with our values (average of
on Day 14 (30.7%) than on Day 7 (14.7%). This increase
4.8% in 1997; 7.2% in 1999) while they reported a viabil-
in viability with time agrees with Egley (1990), who
ity of 1% for wild oat after 14 d. In comparison, after
found that sublethal temperatures promoted germina-
14 d we found an average of 6.5% wild oat viability in
tion of some weeds because they broke the dormancy
1997 and 0.8% in 1999.
of hard seeds.
Since the location effect was nonreplicated within
By Day 21, viability of wild oat was zero. Germinabil-
weed species in 1999, all weed species were averaged
ity extended to Day 21 for round-leaved mallow, al-
to examine the location effect on weed seed viability
though it was very low (0.2% germinable seed). Respir-
(Fig. 4). There were no apparent trends between wind-
ing seed (6.3%) made up the total of 6.5% viability.
row location and viability. Average viability was 4.3%
Five other species still had respiring seeds on Day 21
(⫾1.7%) at the top location, 3.3% (⫾1.6%) at the mid-
(green foxtail, green smartweed, redroot pigweed, stink-
dle location, and 10.1% (⫾4.5%) at the bottom location
weed, and wild buckwheat). Of these, wild buckwheat
on Day 7.
retained the highest viability (15.2%). By Day 42, only
two species retained viability as represented by respiring
Regression Analysis
seeds: green smartweed (3.3% viability) and wild buck-
The time–viability relationships during composting
wheat (3% viability). By Day 91, viability of all 13 weed
may be described with an exponential decay model of
species was zero.
the form:
Tompkins et al. (1998) found that weed seed viability
was zero for wild buckwheat, green foxtail, and stink-
y ⫽ a exp(⫺bx)
weed after 14 d of composting and zero for all 12 weed
where y ⫽ weed seed viability and x ⫽ days of compost-
species in their study after 28 d. In contrast, wild buck-
ing. Equations were fit to the four weed species common
wheat was one of the most resilient weeds in our studies.
to both studies (green foxtail, redroot pigweed, wild
It showed viable seed after 42 d in 1999 (Table 3) and
buckwheat, and wild oat). The control sample viability
even after 70 d in 1997 (Table 2). Redroot pigweed had
values were used for Day 0. Stinkweed was omitted
because of its low level of viability in the control sample
in 1997. All four species (Fig. 5) had highly significant
exponential decay relationships in both studies (P ⬍
0.001) except for wild buckwheat in 1999 (P ⫽ 0.08).
These relationships were also significant for the re-
maining species in the 1999 study: false cleavers (P ⫽
0.04), foxtail barley (P ⬍ 0.001), green smartweed (P ⫽
0.001), round-leaved mallow (P ⫽ 0.01), and stinkweed
(P ⫽ 0.04) (data not shown). Obviously, relationships
could not be fitted for the four weed species in 1999,
where viability had dropped to zero by Day 7 (downy
brome, scentless chamomile, stork’s-bill, and wild
mustard).
Temperature–Viability Relationships
Lethal Temperatures
Using daily mean temperature values and viability
Fig. 4. Effect of windrow location on weed seed viability (average of
13 weed species) in 1999. data, lethal temperatures were estimated as the mini-
LARNEY & BLACKSHAW: WEED SEED VIABILITY IN FEEDLOT MANURE 1111
Fig. 6. Relationship between cumulative degree days (CDD) and via-
bility of wild buckwheat seed in 1997 and 1999. Viability expressed
as percent of control samples.
The lethal temperature for wild oat was 53.8⬚C. This
compares with the 48⬚C reported by Thompson et al.
(1997) to prevent over 90% germination of wild oat in
a heated soil. Thompson et al. (1997) believed that the
maximum temperature required to prevent germination
was of greater importance than the duration of heating.
If high windrow temperatures were associated with
the elimination of weed seed viability, then we would
expect to observe location effects on viability since loca-
tion had a highly significant effect on windrow tempera-
ture, especially in 1997. However, this was not the case.
In fact, the only significant effect of location on viability
in 1997 (wild buckwheat on Day 21, Table 2) could
not be explained by temperature differences. The top
location, which had significantly lower viability (4.3%)
than the middle and bottom locations (13.3–13.8%) was
significantly cooler (161 CDD) than the middle (347
Fig. 5. Relationship between days of composting and weed seed via-
CDD) and bottom locations (466 CDD).
bility for (a) green foxtail, (b) redroot pigweed, (c ) wild buck-
wheat, and (d ) wild oat in 1997 and 1999.
Regression Analysis
Regression analysis on weed seed viability versusmum temperature required to ensure complete loss of
viability. Since the 1997 compost attained its tempera- windrow temperature (expressed as CDD) was con-
ducted on all five weed species in 1997 and on nine ofture maxima in the first week of composting, lethal
temperatures were more apparent from the 1999 study. the thirteen species in 1999. The four weeds (downy
brome, scentless chamomile, stork’s-bill, and wild mus-Lethal temperatures varied widely among weed species.
The lethal temperature for four weeds (downy brome, tard) where viability had reached zero after just 7 d of
composting in 1999 were excluded. The only significantscentless chamomile, stork’s-bill, and wild mustard) that
lost viability in the initial 7 d in 1999 was only 38.9⬚C linear regressions were for wild buckwheat in 1997 and
1999 (Fig. 6) and wild oat in 1997 (data not shown).(top location of the windrow). Grundy et al. (1998)
reported low viability and germination of white clover The relationships showed that weed seed viability de-
creased as CDD increased. However, there was a large(Trifolium repens L.) seed after only3dat35⬚C.
At the other end of the scale, the lethal temperature amount of scatter in the data and R
2
values showed that
only 17% (wild oat, 1997), 20% (wild buckwheat, 1997),for green smartweed was 66.3⬚C, followed by 62⬚C for
wild buckwheat. Green foxtail, redroot pigweed, round- and 29% (wild buckwheat, 1999) of the variation in
viability was accounted for by CDD. This demonstratesleaved mallow, and stinkweed all had lethal tempera-
tures of 56⬚C. Grundy et al. (1998) found that none of that factors other than temperature (e.g., germination
into lethal conditions or pathogen infestation) may af-eight weed species recovered from compost maintained
at 55⬚C for 3 d (a much shorter duration than in our fect weed seed viability during composting. Ammonia
toxicity to viable weed seeds is another possible mecha-study) remained viable.
1112 J. ENVIRON. QUAL., VOL. 32, MAY–JUNE 2003
Table 5. Weed seed viability of the five species common to bothTable 4. Temperature and water content conditions at top, mid-
dle, and bottom locations in the initial 14 d of composting in studies under three different temperature–water content re-
gimes in 1997 and 1999.1997 (Regime 1), initial 14 d of composting in 1999 (Regime
2), and initial 7 d of composting in 1999 (Regime 3). Standard
Weed species Regime 1† Regime 2 Regime 3
errors of the temperature means are in parentheses.
% of control
Average Maximum Average
Green foxtail 7.1 1.1 3.1
Location temperature temperature water content
Redroot pigweed 6.6 9.2 20.2
ⴗCkgkg
⫺
1
Stinkweed 8.3 63.0 40.7
Wild buckwheat 95.8 59.0 28.2
1997, 0–14 d (Regime 1)
Wild oat 8.3 1.3 1.0
Top 47.0 (⫾2.1) 55.1 –
Middle 55.8 (⫾2.7) 63.3 –
† Regimes 1, 2, and 3 are defined in Table 4.
Bottom 61.1 (⫾3.1) 68.6 –
Average 54.6 62.3 0.705†
after 14 d, while in 1999, overall viability was lower
1999, 0–14 d (Regime 2)
(6.2%), despite the 1999 study having lower tempera-
Top 38.0 (⫾1.4) 46.4 0.634
Middle 43.8 (⫾2.6) 53.8 0.645
tures and slightly drier conditions.
Bottom 38.1 (⫾2.3) 46.9 0.635
Temperature and water content conditions for the
Average 40.0 49.0 0.638
0- to 7-d period in 1999 offer further insight into interac-
1999, 0–7 d (Regime 3)
tions between temperature, water content, and weed
Top 34.5 (⫾1.6) 39.1 0.664
seed viability (Table 4). This period represented an even
Middle 36.3 (⫾2.7) 43.5 0.644
Bottom 31.0 (⫾1.9) 38.9 0.651
cooler temperature regime (average temperature,
Average 33.9 40.5 0.653
33.9⬚C; maximum, 40.5⬚C) over a shorter duration (7
† Water content not sampled by location.
vs. 14 d), but seeds were maintained under quite wet
conditions (0.65 kg kg
⫺
1
). However, these conditions
led to an average overall weed seed viability of 5.9%,
nism as high concentrations of ammonia are present in
which was slightly lower than that for the 0- to 14-d
the early stages of manure composting (Rynk, 1992).
periods in 1997 and 1999 (6.2–8.8%).
The weed seed viability of the five weed species com-
Initial 14 Days of Composting
mon to both studies were examined for the three com-
Since most of the effects on viability occurred in the
posting regimes outlined in Table 4. Viability was ex-
first 14 d of the composting process, the temperature
pressed as percent of control sample viability to
and water content conditions during that period were
normalize differences between weed species and within
examined in more detail.
weed species for 1997 and 1999. If viability is strictly
Weed seed viability may be affected more by a combi-
related to temperature, then redroot pigweed was the
nation of high temperatures and wet conditions rather
only weed to behave in the expected fashion, that is,
than high temperatures alone. Bloemhard et al. (1992)
viability was lowest (6.6%) in the hottest composting
suggested that dry seeds are more resistant to heat while
regime (Regime 1) and increased as the regimes cooled
Thompson et al. (1997) indicated that heat treatment
(Table 5). In contrast, wild buckwheat showed highest
of weed seeds may have more severe effects on fully
viability values in the hottest regime (95.8%) and lowest
imbibed seed. Egley (1990) found that species tolerance
values in the coolest and shortest regime (Regime 3,
to heat and moisture varied. Some weed seeds, heated
28.3%).
to 70⬚C in dry soil (0.02 kg kg
⫺
1
), were killed after 7 d.
Some heated in moist soil (0.19 kg kg
⫺
1
) survived for
DISCUSSION
upto3dat70⬚C,orupto7dat60⬚C, while others
were killed after3dat50⬚C. Some weed species have Compost temperatures as low as 39⬚C achieved over
a 7-d period without turning proved lethal for some“hard seed,” which prevents germination and protects
against decay. The combination of moisture and heat weed species in our study. Further experiments concen-
trating on this lower temperature range may be requiredmay render hard seed permeable to water. Horowitz
and Taylorson (1984) found that soaking velvetleaf seed under both moist and dry compost conditions, with and
without turning. Our compost water contents did notfor 1 h at 70⬚C reduced the number of hard seeds from
99 to 15%. vary enough in the early stages of composting to fully
test the effect of water content on viability. All waterIn 1997, average temperatures and water contents in
the 0- to 14-d period of composting were higher than contents were in the wet range (0.60–0.70 kg kg
⫺
1
). It
may be worthwhile to examine the behavior of weedin 1999 (Table 4). Overall, average temperature was
14.6⬚C higher in 1997 (54.6 vs. 40.0⬚C) and the maximum seeds in drier conditions since manure is often dry
(⬍0.40 kg kg
⫺
1
) at pen cleaning in southern Alberta.temperature attained was 13.3⬚C warmer in 1997 (62.3
vs. 49.0⬚C) and average water content was slightly wetter However, microbial activity and hence temperature is
suppressed if the substrate is too dry (Rynk, 1992) and(0.71 versus 0.65 kg kg
⫺
1
). To compare overall viability
between the two studies, the total viable seeds were water is generally added to ensure optimum composting.
We did not examine the effect of compost leachatessummed for all five species in 1997 and all 13 species
in 1999. This value was then expressed as a percentage on weed seed germination. Production of water-soluble
organic phytotoxins such as short-chain fatty acids (e.g.,of the total number of seeds buried in the compost. In
1997, 8.8% of all composted weed seeds remained viable acetic acid), during composting has been reported (Kirch-
LARNEY & BLACKSHAW: WEED SEED VIABILITY IN FEEDLOT MANURE 1113
Blackshaw, R.E., and L.M. Rode. 1991. Effect of ensiling and rumen
mann and Widen, 1994). Shiralipour et al. (1997) found
digestion by cattle on weed seed viability. Weed Sci. 39:104–108.
an inhibitory effect of acetic acid on the germination of
Bloemhard, C.M.J., M.W.M.F. Arts, P.C. Scheepens, and A.G. Elema.
cucumber (Cucumis sativus L.). Ozores-Hampton et al.
1992. Thermal inactivation of weed seeds and tubers during drying
(1999) found that germination of ivyleaf morning glory
of pig manure. Neth. J. Agric. Sci. 40:11–19.
Canadian Council of Ministers of the Environment. 1995. Guidelines
(Ipomoea hederacea L.), barnyard grass (Echinochloa
for compost quality. Part 1. Environment Canada, Ottawa, ON.
crus-galli L.), and common purslane (Portulaca oleracea
Chang, C., C.M. Cho, and H.H. Janzen. 1998. Nitrous oxide emission
L.) was delayed and decreased by extracts from 3-d-,
from long-term manured soils. J. Environ. Qual. 27:677–682.
4-wk-, and 8-wk-old composts as compared with a ma-
Chang, C., and T. Entz. 1996. Nitrate leaching losses under repeated
cattle feedlot manure application in southern Alberta. J. Environ.ture compost extract (1 yr old). This was attributed to
Qual. 25:145–153.
higher amounts of acetic acid in the immature composts.
Churchill, D.B., S.C. Alderman, G.W. Mueller-Warrant, L.F. Elliott,
The lack of viable weed seeds makes compost an
and D.M. Bilsland. 1996. Survival of weed seeds and seed pathogen
attractive soil amendment, especially in organic farming
propagates in composted grass seed straw. Appl. Eng. Agric. 12:
where weed control with herbicides is not an option.
57–63.
Cudney, D.W., S.D. Wright, T.A. Shultz, and J.S. Reints. 1992. Weed
However, our study demonstrated that care should be
seed in dairy manure depends on collection site. Calif. Agric. 46(3):
taken to ensure that the composting process is complete,
31–32.
with adequate turning, as some weed seeds remained
Eghball, B., and G.W. Lesoing. 2000. Viability of weed seeds following
viable even after 70 d of composting (wild buckwheat,
manure windrow composting. Compost Sci. Util. 8:46–53.
Egley, G.H. 1990. High-temperature effects on germination and sur-
1997).
vival of weed seeds in soil. Weed Sci. 38:429–435.
The question is often posed as to whether application
Grabe, D.F. 1970. Tetrazolium testing handbook for agricultural seeds.
of compost rather than fresh manure will lead to lower
Contribution no. 29. Handbook on seed testing. Assoc. of Official
herbicide inputs to cropping systems since introduction
Seed Analysts, Las Cruces, NM.
of viable weed seeds is potentially lower. The size of
Grundy, A.C., J.M. Green, and M. Lennartsson. 1998. The effect of
temperature on the viability of weed seeds. Compost Sci. Util.
the soil seedbank and the annual input from weeds
6(3):26–33.
already present in the field will determine the relative
Harmon, G.W., and F.D. Keim. 1934. The percentage and viability
advantage of compost. If soil seedbank numbers are low
of weed seeds recovered in the feces of farm animals and their
and manure seed counts are high, then composting may
longevity when buried in manure. J. Am. Soc. Agron. 26:762–767.
Hopkins, C.Y. 1936. Thermal death point of certain weed seeds. Can.alleviate a potential weed problem. However, if soil
J. Res. Sect. C 14:178–183.
seedbank numbers are high and manure weed seed
Horowitz, M., and R.B. Taylorson. 1984. Hardseededness and germi-
counts are low, then composting may not affect weed
nability of velvetleaf (Abutilon theophrasti) as affected by tempera-
populations. Mt. Pleasant and Schlather (1994) believed
ture and moisture. Weed Sci. 32:111–115.
than when exotic weeds are present in manure, which
Kirchmann, H., and P. Widen. 1994. Fatty acid formation during
composting of separately collected organic household waste. Com-
may occur if feed is sourced from a different ecozone,
post Sci. Util. 2(1):17–19.
manure application may introduce these exotics to the
Larney, F.J., A.F. Olson, A.A. Carcamo, and C. Chang. 2000. Physical
soil seed bank. Few seeds are required to develop into
changes during active and passive composting of beef feedlot ma-
a major infestation, perhaps necessitating the use of a
nure in winter and summer. Bioresour. Technol. 75:139–148.
Ligneau, L.A.M., and T.A. Watt. 1995. The effects of domestic com-new herbicide and increasing herbicide inputs. In this
post upon the germination and emergence of barley and six arable
scenario composting may be beneficial.
weeds. Ann. Appl. Biol. 126:153–162.
Marchiol, L., C. Mondini, L. Leita, and G. Zerbi. 1999. Effects of
municipal waste leachate on seed germination in soil–compost mix-
CONCLUSIONS
tures. Restor. Ecol. 7:155–161.
Mt. Pleasant, J., and K.S. Schlather. 1994. Incidence of weed seed in
Our study showed that while composting dramatically
cow (Bos sp.) manure and its importance as a weed source for
reduced weed seed viability, the exact mechanism was
cropland. Weed Technol. 8:304–310.
unclear. Temperature required to achieve complete
Ozores-Hampton, M., P.J. Stoffella, T.A. Bewick, D.J. Cantliffe, and
elimination of viability was species-dependent, as was
T.A. Obreza. 1999. Effect of age of cocomposted MSW and biosol-
ids on weed seed germination. Compost Sci. Util. 7(1):51–57.
the duration of exposure to those temperatures. The
Rynk, R. 1992. On-farm composting handbook. Publ. NRAES-54.
lack of definitive relationships between compost tem-
Northeast Reg. Agric. Eng. Serv., Ithaca, NY.
perature and weed seed viability suggests that other
SAS Institute. 1989. SAS/STAT user’s guide. Version 6. Vol. 2. 4th
mechanisms, such as phytotoxic leachates, may play an
ed. SAS Inst., Cary, NC.
Shiralipour, A., D.B. McConnell, and W.H. Smith. 1997. Phytotoxic
important role in the destruction of viability.
effects of a short-chain fatty acid on seed germination and root
length of Cucumis sativus cv. ‘Poinset’. Compost Sci. Util. 5(2):
ACKNOWLEDGMENTS
47–52.
Stoker, G.L., D.C. Tingey, and R.J. Evans. 1934. The effect of different
The authors thank the Alberta Agricultural Research Insti-
methods of storing chicken manure on the viability of certain weed
tute (Farming For the Future Matching Grants Program, Proj-
seeds. J. Am. Soc. Agron. 26:600–609.
ect no. 97M-179) for partial financial support of this study.
Thompson, A.J., N.E. Jones, and A.M. Blair. 1997. The effect of
The technical help of Greg Semach, Andrew Olson, and Paul
temperature on viability of imbibed weed seeds. Ann. Appl. Biol.
DeMaere is gratefully appreciated.
130:123–134.
Tompkins, D.K., D. Chaw, and A.T. Abiola. 1998. Effect of windrow
composting on weed seed germination and viability. Compost Sci.
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