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American Journal of Experimental Agriculture
5(5): 450-458, 2015, Article no.AJEA.2015.044
ISSN: 2231-0606
SCIENCEDOMAIN international
www.sciencedomain.org
Effect of Different Types of Mulching and Plant
Spacing on Weed Control, Canopy Cover and Yield
of Sweet Potato (Ipomoea batatas (L.) Lam)
S. M. Laurie
1*
, M. N. Maja
1,2
, H. M. Ngobeni
1
and C. P. Du Plooy
1
1
Agricultural Research Council, Roodeplaat Vegetable and Ornamental Plant Institute
Crop Science Division, Private Bag X293, Pretoria, 0001, South Africa.
2
National Department of Agriculture, Forestry and Fisheries, Pretoria, South Africa.
Authors’ contributions
This work was carried out in collaboration between all authors. Author MNM designed the study under
supervision of supervisors, wrote the protocol and executed the trials. Author HMN wrote the first draft
of the manuscript and arranged for statistical analysis by the Biometry division. Author SML
supervised, Author MNM during her studies, managed analyses of the results and wrote large parts of
the manuscript. Author CPDP supervised the study as program manager and made inputs at all
phases. All authors read and approved the final manuscript.
Article Information
DOI: 10.9734/AJEA/2015/12404
Editor(s):
(1)
Juan Yan, Doctorate of Horticultural Crop Biotechnology Breeding, Sichuan Agricultural University Ya’an, China.
Reviewers:
(1)
Anonymous, Institute of Agricultural Research for Development –IRAD – Cameroon.
(2)
Anonymous, Kyoto Prefectural University, Japan.
Complete Peer review History:
http://www.sciencedomain.org/review-history.php?iid=738&id=2&aid=6842
Received 30
th
June 2014
Accepted 14
th
August 2014
Published 6
th
November 2014
ABSTRACT
The aim of the study was
to determine the effects of mulching, plant spacing and other control
measures on effectiveness of weed control, canopy cover and sweet potato yield. The cultivar
Blesbok was planted in a randomized complete block design where seven treatments were
replicated four times. Field trials were established at the Agricultural Research Council -
Roodeplaat Vegetable and Ornamental Plant Institute in Pretoria, South Africa during 2006 and
2007. Seven treatments were applied: 1) HW = Hand weeding, 2) NS = Narrow plant spacing (0.5
m between the rows and 15 cm between plants), 3) CO = Organic mulch (compost), 4) PL =
Inorganic mulch (black plastic), 5) E*F = Eptam (EPTC; Thiocarbamate) followed by Fusilate
(fluazifop-p-butyl), 6) L*F = Afalon (Linuron) followed by Fusilate (fluazifop-p-butyl), and 7) CN =
Control (untreated plot). Organic mulch (compost) was omitted in the second trial as it will be
Original Research Article
Laurie et al.; AJEA, 5(5): 450-458, 2015; Article no.AJEA.2015.044
451
beneficial for both weeds and the crop making it difficult to control the weeds. Instead, grass
straws (ST) and newspaper (NP) mulches were added. Narrow row spacing, hand weeding,
plastic mulch, and newspaper mulch outperformed the other treatments and obtained more than
90% canopy cover by 5 weeks after planting. Effective reductions in weeds were detected with
plastic and newspaper mulched plots and was similar to the hand-weeded treatment, followed by
narrow spacing. Plots with newspaper mulch and narrow spacing produced marketable yields
similar to the hand-weeded treatment. Inorganic mulching and narrow plant spacing were the most
effective weed management treatments. Newspaper mulch seems to be a viable option for small
holder farmers to control weeds in sweet potato plantings, eliminating the cost of labour to conduct
hand weeding. Compost and grass mulch should not be adopted for weed control since these did
not control weeds effectively.
Keywords: Herbicides; paper mulch; plastic mulch; sweet potato; newspaper.
1. INTRODUCTION
Sweet potato (Ipomoea batatas (L.) Lam) is one
of the five most important crops in the developing
world [1]. It is believed to have originated in
Central America [2]. The crop belongs to the
Convolvulaceae or morning glory family and is a
vital staple food [3,4]. Sweet potato is used for
human consumption, livestock feed and for
industrial processing. When yellow-orange flesh
sweet potatoes are consumed vitamin A
deficiency can be combated [5]. Sweet potato
also contributes towards vitamin C and minerals
in diets [5]. Sweet potato has a large number of
existing cultivars throughout the world. The most
popular sweet potato variety in the USA is
Beauregard, because of its high yield [6,7]. The
popular cultivars grown in South Africa include
Blesbok, Bosbok, Ribbok and Beauregard [8].
Sweet potato crop, once established, requires
little field management apart from weed control
[9]. Manual weed control (by hand hoe) is
laborious and time-consuming. Depending on the
number of individuals in the household and the
size of the farm, weed control can take up to
several weeks, during which a large portion of
crop yield could be lost due to weed competition.
According to the findings by Mischler [10], hiring
labor to hand hoe fields was twice as expensive
when compared to other control measures.
Effective weed control is a major constraint to
small-holder farmers because of a lack of
mechanization and money to pay for chemicals
and labour.
Unfavorable conditions like rainy periods soon
after transplanting may make the weed control
measures ineffective, causing severe yield
losses due to weed competition [11].
Furthermore, weed control in sweet potato
production is difficult because of the vine-like
growth habit of sweet potato, and the availability
of only a few registered herbicides for this crop
and evidence of herbicide injury in certain
cultivars [12]. Research has shown that a weed-
free period of 2-6 weeks after planting was
required for the cultivar Beauregard. Harrison
and coworkers showed that the growth type of
the cultivar has an effect on its susceptibility to
weed interference [11]. Their results
demonstrated that a cultivar with a vigorous,
erect shoot growth habit may be less susceptible
to weed interference than cultivars with
spreading shoot growth such as Beauregard.
Weed research of sweet potato has been
insufficient over the past decade. Most weed
management strategies were developed for large
scale agriculture. As a result, these are either not
applicable or affordable to small-holder farmers.
Alternative weed control measures include
mulching, flooding, burning and cultural weed
control [13]. Mulching reduces weed growth by
preventing light (which is needed by the
germinating weed seed) from reaching the soil
surface [14]. Studies by Ossom and coworkers in
Papua New Guinea reported some benefit to
sweet potatoes dry matter yield of applying
mulches, though it was not conclusive [15]. In
addition, Aldrich reported in general for crops
that plant spacing can affect weed growth and its
ability to compete with the crop [16]. The
mentioned methods might not be suitable for
large-scale production and as effective as
chemical control, however, these can provide
small-holder farmers with useful options.
The objective of the present study was to
determine the effectiveness of different types of
mulching, narrow plant spacing and other control
measures on weed control, canopy cover and
yield of sweet potatoes.
Laurie et al.; AJEA, 5(5): 450-458, 2015; Article no.AJEA.2015.044
452
2. MATERIALS AND METHODS
Field trials were established at the Agricultural
Research Council-Roodeplaat Vegetable and
Ornamental Plant Institute (ARC-VOPI) in
Pretoria (25.56°S, 28.35°E) during 2006 (Trial 1)
and 2007 (Trial 2) planting seasons. Trial 1 was
established in January 2006. The soil type used
was a Hutton with 20-25% clay. The cultivar
Blesbok was planted in a randomized complete
block design where seven treatments were
replicated four times. The following treatments
were applied: 1) HW = Hand weeding, 2) NS =
Narrow plant spacing (0.5 m between the rows
and 15 cm between plants), 3) CO = Organic
mulch (compost), 4) PL = Inorganic mulch (black
plastic), 5) E*F = Eptam (EPTC; Thiocarbamate)
followed by Fusilate (fluazifop-p-butyl), 6) L*F =
Afalon (Linuron) followed by Fusilate (fluazifop-p-
butyl), and 7) CN = Control [untreated plot).
Afalon was applied at 2.0 L/ha and Eptam at 3.5
L/ha prior to planting to control broadleaved and
grassy weeds [17]. Fusilate, a post emergence
herbicide, was applied after emergence of grass
weeds two months after planting at the rate of 2
L/ha. Organic and inorganic mulch were placed
on the ground within 24 hours of sweet potato
planting. Plastic was placed on top of the ridges
with the edges buried under the soil to hold the
plastic in position. For all treatments sweet
potato cuttings were planted on ridges of 30 cm
high. Each plot consisted of four ridges of 3.3 m
long. Between rows spacing of 1 m and within
rows plant spacing of 0.3 m was used for all
treatments with the exception of the narrow plant
spacing treatment, where plant spacing of 15 cm
and between ridges spacing of 0.5 m was used.
Complimentary water application was applied
through overhead irrigation. Soil samples were
collected and fertilizer applied based on the
recommendations of Allemann [17]. A total of
800 kg ha
-1
of NPK fertilizer 2:3:4 (30) was
applied. Fertilizer was broadcasted and worked
into the soil before planting. Top dressing of 150
kg ha
-1
of Limestone ammonium nitrate (LAN,
28% N) was applied six weeks after planting. The
nutrient application was therefore 95 kg ha
-1
N,
80 kg ha
-1
P and 106 kg ha
-1
K.
Weed control effectiveness was determined
based on visual estimation of the percentage
area free of weeds. The percentage canopy
closure per plot were determined by visual
estimation. Both parameters were recorded at
weekly intervals. Unfortunately early frost killed
of the plants in Trial 1 before the storage roots
reached maturity.
Trial 2 was planted to determine the effect of
weed control treatments on root yield and was
established in December 2006 and harvested
April 2007. The experimental design, planting
method, watering, fertilization and data collection
was the same as the first trial. The treatments
were altered as follows. Organic mulch
(compost) was omitted since it favours the
growth of weeds, making it difficult to control the
weeds. Instead, grass straws (ST) and
newspaper (NP) mulches were added.
Herbicides were also excluded. The treatments
applied were therefore as follows: 1) Hand
weeding (HW); 2) Narrow plant spacing (0.5 m
between the rows and 15 cm between plants)
(NS); 3) Organic mulch (grass straws) (ST); 4)
Inorganic mulch (black plastic) (PL); 5)
Newspaper mulch (NP); and 6) Control
(untreated plot) (CN). Five months after planting,
sweet potatoes were harvested manually. The
storage roots were sorted as follows: Good
quality roots were grouped as extra-large (800-
1200g), large (500-800g), medium (250-500g)
and small (100-250g). The unmarketable classes
consisted of unmarketable large (>1200g),
unmarketable small (< 100g), rat damaged,
insect damaged, rotten, long curved and
sprouted tubers.
An ANOVA (analysis of variance) was conducted
with the statistical program Genstat 2003 to test
for differences among treatments for each data
set. Treatment means were separated using
Fisher’s t-test least significant differences (LSD)
at the 5% level of significance [18].
3. RESULTS
3.1 Canopy Closure and Weed Control
Effectiveness
The results of Trial 1 showed that narrow row
spacing (NS), plastic mulch (PL), and hand
weeding (HW) had similar results with more than
90% canopy cover by 5 weeks after planting,
while the control treatment had less canopy
cover by that time (Fig. 1). The canopy cover for
the narrow plant spacing (NS) plots was faster
than that of other treatments. In Trial 1, at week 2
and 3, narrow spacing exceeded (P=.05) the
canopy cover of most other treatments, and in
Trial 2 there was a tendency of higher cover at
week 2 and 3 (Figs. 1 and 2). In Trial 2, by week
Laurie et al.; AJEA, 5(5): 450-458, 2015; Article no.AJEA.2015.044
453
4 the narrow row spacing (NS), plastic mulch
(PL) and newspaper much (NP) performed
significantly better than grass straws (GS) and
the untreated control (CN) (Fig. 2). Together with
the hand weeding (HW), these treatments
obtained more than 90% cover by week 5
(Fig. 2).
Plastic (PL) were similar to the hand-weeded
(HW) treatment in effectiveness of weed control
as seen from the results of Trial 1 (Fig. 3), while
the narrow plant spacing (NS) and herbicide
treatments successfully controlled most of the
weed population. In Trial 2, newspaper mulch
(NP), plastic mulch (PS) and narrow spacing
(NS) controlled the weeds as effectively as hand
weeding (HW) from week 3 onwards (Fig. 4).
Weed control efficiency of compost (CO) and
grass straw (ST) mulch plots deteriorated weekly
as from week 3 (Fig. 4), which showed that these
treatments were ineffective in suppressing the
weeds.
NS=Narrow row spacing CO=Organic mulch HW=Hand weeding CN=Untreated control
PL=Plastic E*F=Eptam L*F=Afalon WK=Week
Fig. 1. Weekly canopy closure (%) of sweet potato at different weed control treatments
for Trial 1
Bars marked by different letters differ significantly at the P=.05 significance level
Fig. 2. Weekly canopy closure (%) of sweet potato at different weed control treatments
for Trial 2
Bars marked by different letters differ significantly at the P=.05 significance level
HW=Hand weeding PL=Plastic ST=Grass straws WK=Week
NS=Narrow plant spacing NP=Newspaper CN=Untreated control *NS=Non significant
Laurie et al.; AJEA, 5(5): 450-458, 2015; Article no.AJEA.2015.044
454
3.2 Yield
In Trial 1 there were no significant differences in
crop yield with various treatments from the
control treatment because plants were killed by
frost before the storage roots reached maturity.
However in Trial 2, almost all treatments
exceeded the unweeded control in marketable
root yield as well as total root yields
(Figs. 5 and 6). Hand weeding (HW) and narrow
spacing (NS) produced the highest total yields of
above 70 t/ha (Fig. 5). Plastic (PL) and
newspaper (NP) mulched plots produced total
yields of 60 t/ha, which were not significantly
lower than hand weeding (HW). The control
treatment produced total yields of less than 40
t/ha (Fig. 5).
Hand weeding, narrow spacing (NS) and
newspaper (NP) plots yielded the highest
marketable root yield, exceeding 50 t/ha as
compared to 24.5 t/ha for the unweeded control
(CN) (Fig. 6). Plots mulched with plastic (PL) and
grass straws (ST) produced average marketable
root yields, around 40 t/ha (Fig. 6). This may be
due to weed competition at early stages of
growth in the case of grass straws. The lower
marketable yield detected with plastic mulch was
due to the number of sprouted storage roots
(data not shown).
HW=Hand Weeding
NS=Narrow Row Spacing
E*F=Eptam
CN=Untreated Control
PL=Plastic L*F=Afalon CO=Organic Mulch WK=Week
Fig. 3. Weekly weed control effectiveness (WCE) at different weed control treatments of sweet
potato for Trial 1
Bars marked by different letters differ significantly at the P=.05 significance level
Fig. 4. Weekly weed control effectiveness (WCE) of sweet potato for Trial 2
Bars marked by different letters differ significantly at the P=.05 significance level
HW=Hand weeding
PL=Plastic
ST=Grass straws
WK=Week
NP=Newspaper NS=Narrow plant spacing CN=Untreated control
Laurie et al.; AJEA, 5(5): 450-458, 2015; Article no.AJEA.2015.044
455
HW=Hand weeding NP=Newspapers ST=Grass straws
NS=Narrow plant spacing PL=Plastic CN=Untreated control
Fig. 5. Total root yield of sweet potato at different weed control treatments for Trial 2
Bars
marked by different letters differ significantly at the P=.05 significance level
HW=Hand weeding NP=Newspapers ST=Grass straws
NS=Narrow plant spacing
PL=Plastic
CN=Untreated control
Fig. 6. Marketable root yield of sweet potato at different weed control treatments for Trial 2
Bars marked by different letters dffer significantly at the P=.05 significance level
3.3 Size Classes of Roots
There were no significant difference detected in
Trial 2 for percentage roots per marketable size
class among treatments, except for extra-large %
for which hand weeding (HW) and newspaper
(NP) plots had the highest percentages
(Table 1). Despite the non-significant differences
in different treatments on the other three size
classes, the untreated control showed a
tendency of higher percentage for small-sized
roots and smaller percentage large-sized roots
(Table 1).
4. DISCUSSION
There is a paucity of research of weed control of
sweet potato, compared to other major staple
crop, and more so in terms of methods of control
related to small-holder farming. The present
study established that narrow row spacing (NS),
newspaper mulch (NP) and plastic mulch (PL)
are good options for weed control in sweet
potato. These treatments reduce the cost of
labour to execute hand weeding and should be
useful for small-scale production.
Laurie et al.; AJEA, 5(5): 450-458, 2015; Article no.AJEA.2015.044
456
Table 1. Mean percentage roots per marketable size classes achieved by sweet
potato at different weed control treatments during year 2007
Treatments
Means per size class
Small %
Medium %
Large %
Extra-large %
Hand weeding (HW) 38.1 27.4 21.0 13.60 a
Newspapers (NP) 42.1 37.2 14.5 6.25 b
Narrow plant spacing (NS) 41.2 37.4 18.6 2.90 b
Grass straws (ST) 41.4 37.7 17.5 3.40 b
Plastic (PL) 41.0 37.1 19.7 2.23 b
Control (CN) 48.3 38.9 11.1 1.73 b
P=.05 0.714 0.332 0.147 0.020*
CV % 21.2 21.1 30.8 2.44
MSE 79.2 57.2 27.6 21.3
Means followed by the same letter do not differ significantly at the P=.05 significance level
* F-probability significant at P=.05, CV% = Coefficient of variance in percentage, MSE = Mean Square Error
Most reported weed management research in
sweet potato were based on large scale
agriculture. Chemical weed control is generally
considered to be effective in crop production, but
has constraints such as the cost of chemicals
and the application thereof, phytotoxic effects on
the crop and environmentally sustainability. For
example, in a study by Steven and coworkers
[19] at nine days after transplanting sweet potato,
20% injury was observed of the application of
flumioxazin. Lewthwaite and coworkers [20]
tested application of a number of herbicides (i.e.
acetochlor, dimethenamid and alachlor) for weed
control in sweet potato due to occurrence of
paraquat-resistant black nightshade. In that
experiment none of the chemicals tested could
be justified by improved economic returns and, in
addition, herbicide phytotoxicity was detected. In
the present study, herbicide application
combinations did not provide advantages in
terms of canopy cover and weed control
efficiency above narrow row spacing (NS) and
mulching with newspaper (NP) or plastic (PL).
The present study reports novel results of the
effectiveness of the use of newspaper mulch for
control of weeds in sweet potato. Some research
on the use of mulch for control of weeds in sweet
potato has been conducted by Ossom and
coworkers, who found grass mulch to lower the
number of weed species and reduce weed
weight [15]. However, they also emphasized the
danger of grass establishment in fields and
therefore, did not recommend its use. In their
experiment, the use of sawdust was not effective
as compared to grass and coffee pulp mulch.
These authors furthermore reported a decrease
in soil temperature due to mulching but found
non-significant effect on dry matter yield. In
another study by Gawronski [21] newspaper
mulch was tested on silver beet, and the authors
found the mulch to be potentially useful.
However, there were confounding effects in the
experiment causing the results to be
inconclusive. Sangakkara and coworkers [22]
found that rice straw and grass mulch increased
root yield, leaf area and crop growth rates of
sweet potato and reduced the time for storage
root initiation significantly. Management aspects
of mulching should also be taken into account.
Organic mulch, e.g. grass mulch, allows some
flexibility in fertilization and irrigation, since the
water can infiltrate and the mulch can be raked
back from the plants, and organic mulch
decomposes naturally. Inorganic mulches, e.g.
plastic, do not decompose and the plastic need
to be removed by hand after the growing season,
and as seen in the present study, may need
management of soil moisture to avoid sprouting.
Newspaper mulch (Fig. 7) provides an option
similar to plastic mulch but has the advantage of
decomposing naturally.
The use of narrow row spacing (NS) is also an
attractive option for weed control in sweet potato.
Most studies have shown the benefits of
reducing row spacing on early canopy closure
that increases the capability of crops to compete
with weeds for sunlight, nutrients and water [16].
In this regard it will be important to consider the
effect of plant population in relation to the
cultivar. Blesbok, the cultivar used in the present
study, did not show significant yield reduction
with narrow spacing (NS). The case may be
different for other cultivars, e.g. those which are
slow maturing or more reactive to plant
population. Du Plooy and coworkers found that
closer spacing between plants influenced size
class achieved in certain cultivars while not
influencing others [23].
Fig. 7.
Newspaper mulch plot at four weeks after planting showing the effective
5. CONCLUSION
This study showed that mulching (plastic and
newspapers) and narrow plant spacing could be
used to improve weed management in sweet
potato, since these provided effective weed
control and earlier canopy closure. Newspaper
mulch and narrow spacing particula
be a viable option for small holder farmers to
control weeds in sweet potato plantings and
resulted in high marketable root yield, similar to
hand weeding.
Future research may further
investigate the influence of mulching and narrow
spacing o
n root size and should
specifically per cultivar before making production
recommendations.
ACKNOWLEDGEMENTS
The authors are grateful for funding for the study
provided by the Department of Science and
Technology and the Agricultural Research
Co
uncil; valuable advice received from Prof
Charley Reinhardt (at that time based at the
University of Pretoria), Dr Konanani Lipadzi (at
that time based at the Agricultural Research
Council), and for Mr Andre van den Berg for
technical advice.
COMPETING INTERESTS
Authors have declared that no competing
interests exist.
Laurie et al.; AJEA, 5(5): 450-458, 2015
; Article no.AJEA.20
457
Newspaper mulch plot at four weeks after planting showing the effective
weed control
This study showed that mulching (plastic and
newspapers) and narrow plant spacing could be
used to improve weed management in sweet
potato, since these provided effective weed
control and earlier canopy closure. Newspaper
mulch and narrow spacing particula
rly, seems to
be a viable option for small holder farmers to
control weeds in sweet potato plantings and
resulted in high marketable root yield, similar to
Future research may further
investigate the influence of mulching and narrow
n root size and should
be tested
specifically per cultivar before making production
The authors are grateful for funding for the study
provided by the Department of Science and
Technology and the Agricultural Research
uncil; valuable advice received from Prof
Charley Reinhardt (at that time based at the
University of Pretoria), Dr Konanani Lipadzi (at
that time based at the Agricultural Research
Council), and for Mr Andre van den Berg for
Authors have declared that no competing
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© 2015 Laurie et al.; This is an Open Access article distributed under the terms of the Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Peer-review history:
The peer review history for this paper can be accessed here:
http://www.sciencedomain.org/review-history.php?iid=738&id=2&aid=6842
