A COMPARISON BETWEEN MCPA AND PESCO is-I5 FOR THE ...

A COMPARISON BETWEEN MCPA AND PESCO is-I5 FOR
THE CONTROL OF WEEDS IN RICE ON THE ACCRAPLAINS
By K. A. HAIZEL
Faculty of Agriculture. University of Ghana, Legon
Summary
Rice cultivation has great possibilities of
increasing production on the Accra Plains,
especially on the heavy black clays, when
water becomes available in large quantities
for irrigation from the Akosombo dam.
With expanding and intensive rice production
weed control is likely to be one of
the problems which would require attention.
. Of the various weed control measures the
use of herbicides is the most promising.
MCPA, now in common use in rice, was
compared against Pesco 18-15 which by
virtue of its composition should have a wider
spectrum of weed control.
Both herbicides proved highly toxic at
pre-emergence application, but at postemergence
MCPA showed better trend of
efficiency and is likely to hold its sway even
at much higher dosages.
Introduction
Rice production constitutes one of the
major cropping possibilities of the Accra
Plains. The Black clays of which there are
about 319,000 acres have proved pre-eminently
suitable for rice cultivation with irrigation
at the University Farm at Kpong.
When more water becomes available from the
Akosombo dam the acreage of the crop
could be increased considerably.
An important aspect of intensive rice
cultivation is the problem of keeping the
crop weed free. In Ghana the problem could
become even more pressing as rice is
seeded directly in the field and not transplanted.
The crop therefore has no initial
ad vantage over the weeds in the course of
its establishment.
AND
A. G. CARSON
Agricultural Irrigation Research Station, Kade.
Under irrigation, flooding is an adequate
measure for the control of most weeds,
particularly species that are normally adapted
to dry land conditions. Other weed species,
especially the aquatic types such as Sphenoclea
zeylanica and Cyperus difformis, can
be tough and are not so easily controlled
by normal flooding.
The use of mechanical means for the
control of weeds in rice is not easy and
often not quite practicable. Planting in rows
allows mechanical after~cultivation to a
limited extent, but close planting, as is
normally done for the crop, makes such a
venture rather hazardous to the crop.
Hand hoeing requires a lot of care because
grassy weed species are often not easily
distinguishable from the rice seedling, particularly
in the early stages of growth. When
employed on a large scale, therefore, hand
hoeing is likely to be slow, untimely, expensive
and not thorough. The future of effective
weed control in rice, therefore, seems to
lie in the use of suitable selective herbicides.
For some years MCPA and 2-4 D have
been the most popular herbicides used in rice
in most parts of the world. These two herbicides
are of the translocated type and are
effective mainly against broadleaved weeds.
Under most situations, however, it is the
grassy weeds that constitute a great problem.
Recently, Fisons Pest Control put out a
product, Pesco 18-15, which is claimed
to be selective in rice in controlling both
broadleaved weeds as well as grassy weed
species. Pesco 18-15 is, in composition, a
mixture of MCPA and TCA (Trichloro
Acetic Acid). While MCPA is essentially.
a broadleaved weed killer, TCA is mainly
a grass killer. Pesco 18-15, being a cocktail
of the two, should, therefore, have a broader
spectrum of control, destroying broadleaved
weeds as well as grassy species.
This experiment was carried out to compare
the relative effectiveness of MCPA,
already in common use in rice, and Pesco
18-15 the new product, in the control of
weeds.

Materials and Methods
The experiment was laid out on the Black
Akuse Clay soils at Kpong.
The land was ploughed, harrowed .and
levelled, but was not puddled. Treatment
plots (1/57.4 acre each) were enclosed in separate
bunds to enable independent flooding.
Paddy (variety Nickerie), treated with agrosan
at the rate of 1 oz. to 28 lb. seed, was
broadcast at the rate of 80 lb. per acre
on 20th October, 1964. The' seed was not
pre-germinated. The layout. of the experiment
was a randomised block design,
replicated three times. The treatments
consisted of two herbicides: MCP A and
Pesco 18-15, applied at two levels each: three
pts./ac. and five pts./ac, and applied at two
different times: pre-emergence and post-emergence.
The~e were compared against weeded
and unweeded rice plots.
The pre-emergence treatments were applied
two days after sowing, on the 22nd October,
1964. The post-emergence treatments were
applied on 20th November, 1964 four weeks
after sowing. The application of the herbicides
was done with a pressurised napsac
sprayer operating around a pressure of
about 60 lb./sq. inch and at a volume of
30 gallons per/acre. The plots with 'clean
weeding' treatment were kept weed free
by periodic removal of weeds by hand hoeing.
Owing to the difficulty of distinguishing
the grassy weed species from the rice crop
at the early stages, thorough weeding by
hoeing was not possible until later. In the
plots with 'no weeding' treatment weeds
were allowed to grow with the crop freely
throughout the experiment.
Observations and Discussion
Germination.-Pre-emergence application
of the herbicide delayed germination remarkably
(see Table 1 below).
TABLE 1
EFFECT OF TREATMENTS ON THE GERMINATION OF PADDY
I. MCPA at 3pts/ac. Pre-em.
II. MCPA at 5pts/ac. Pre-em.
III. Pesco 18-15 3pt/ac. Pre-em.
IV. Pesco 18-15 5pt/ac. Pre-em.
V. MCPA at 3pt./ac. Post-em.
VI. MCPA at 5pt/ac. Post-em.
VII. Pesco 18-15 3pt/ac. Post-em.
VIII. Pesco 18-15 5pt/ac. Post-em.
IX. Clean Weeding . . . .
X. No Weeding
1. Standard Error of the Mean=± 2.14
2. Shortest Significant Ranges
at 5% level
p:
(Rp)
3. Treatment Means ranked in
order
(2)
2.97
6.35
IV
23.9
Time of Germination
(50 % Thro.)
16 days
16 days
16 days
16 days
6 daysi
6 days I
6 days ~
~~:~ I
6 daysJ
(3)
3.12
6.67
I
34.1
(4)
3.21
6.87
III
36.2
31.6
10.0
·35.0
16.6
(5)
3.27
7.00
V
71.6
34.1
18.0
36.2
23.9

The germination percentages for the treatments
were transformed by the angular
transformation (LeClerg et al. 1962) and
subjected to an analysis of variance. At the
time of assessment the post-emergence
treatments had not been applied. For
purposes of this comparison, therefore, all
treatments other than pre-emergence application
should be considered as untreated.
The results of this analysis indicated
highly significant effects due to treatments.
The untreated plots gave a uniform germinanation
of about 90 per cent each, while the
pre-emergence treated plots gave well below
40 per cent germination. Within the preemergence
treatment plots, there was no
significant difference in effect due to MCPA
and Pesco 18-15 at the same dosage levels,
i.e. at 5pts/acre or 3ptsjacre. There were,
however, significant differences in effects due
to the different dosage levels of application.
At the higher dosage rate of 5ptsjacre of
either MCPA or Pesco 18-15, there was a
more significant depression in total germination.
The drastic effect of the herbicide on
germination is evidently due to the fact that
the broadcast seeds were exposed to direct
contact with the herbicide at the time of
application.
The effect of pre~emergence application is
further indicated by the total percentage
germination for treated and untreated plots
(Table 1 above). There was a considerable
drop in the plots with the pre-emergence
treated plots.
TABLE 2
Tillering.-Tillering was observed in all plots
21 days after sowing, but the degree of
tillering differed with the treated and untreated
plots. The figures in Table 2 below
sho~ the mean number of tillers per plant
per plot. These were based on counts from
10 plants taken at random in each plot.
These counts were transformed by the
square root transformation, using the
formula YV+0.5 (LeClerg et al. 1962), and
then subjected to an analysis of variance.
This showed that differences due to treatment
were just significant at the 5 per cent level.
Further examination using Duncan's (1955)
shortest significant range test revealed that the
pre-emergence treatments significantly affected
the degree of tillering in the crop. As the
post-emergence application had not been
applied at the time of assessment it was not
strange that post-emergence treatments did
not show any significant differences from the
"weeded" and "unweeded plots".
Flowering.-Flowering was delayed by preemergence
application but post·emergence
application 4 weeks after planting had no
effect on the flowering period. The delay in
flowering was about a week, being 108 days
for the pre-emergence plots as against 101days
for the post.,emergence, hand weeded and
unweeded plots. .
Crop Health.-The effect of the herbicide
application on the general growth of the
crop was assessed by guided visual scoring.
The scale used is based on the one used by
Green & Kalogeris (1963) and is given
below as Table 3.
EFFECf OF TREATMENT ON NUMBER OF TILLERS PER PLANT
(a) Standard Error of the Mean- ± 0.28
(b) Shortest SignificantRange (Rp)
p.: (2) (3) (4) (5) (6) (7) (8) (9) (10)
2.97 3.12 3.21 3.27 3.32 3.35 3.37 3.39 3.4
Rp 0.83 0.87 0.90 0.92 0.93 0.94 0.94 0.95 0.9
(c) Ranked Mean of Treatments
Post-em Post-em Post-em Clean No Post-em Pre-em Pre-em Pre-em Pre-em
Treatment
Orig.
MCPA
3ptjac
3.7
Pesco
5ptjac
3.7
pesco
3ptjac
4.7
Weeding
4.7
Weeding
5.0
MCPA Pesco Pesco
5ptjac 5ptjac 3ptjac
5.3 8.3 8.3
Pesco MCPA
3ptjac 5ptjac
8.3 11.0
Transf. 2.0 2.0 2.2 2.2 2.3 2.4 2.8 2.9 2.9 3.4

TABLE 3
CROP HEALTH SCALE
Crop stand as healthy as that of weeded plot
Slight stunting or delay in development
Slight necrosis of leaves, chlorosis or reduced vigour of
crop
Considerable leaf scorch, chlorosis or considerable reduction
in vigour of crop.
Crop severely damaged, stunted or deformed
Crop killed.
The results of the scoring taken at 3 different times in the life of the crop are given in Table
4 below:
Pre-em. MCPA 3ptsjacre
Pre-em. MCPA 5ptsjacre
Pre-em. Pesco 18-15 3ptsjacre
Pre-em. Pesco 18-15 5ptsjacre
Post-em. MCPA 3ptsjacre ..
Post-em. MCPA 5ptsjacre ..
Post-em. Pesco 18-15 3ptsjacre
Post-em. Pesco 18-15 5pts/acre
Hand Weeding
No Weeding ..
TABLE 4
SCORE FOR CROP HEALTH
The immediate effect of the herbicide on the
development of the crop was evident soon
after application, during the germination of
the seed. The seedlings that emerged from
the seed in the pre-emergence treatment plots
were all unusually yellow and were considerably
deformed. The extreme effect of the
herbicides on the seedlings at this stage is
evidently due to the fact that the spray fell
more or less directly on the seed. Perhaps
when the seed is dibbled or drilled at depth
the adverse effect will not be so drastic.
By the 4th of November, 12 days after pre-
"mergence application, when the first scoring
was done, the figures (table 4) indicate that
the crop still showed a severe check in growth
from the pre-emergence application. Recovery
to normal growth was rather slow with
all pre-emergence treatments, except with
the lower level of MCPA at 3pts/acre. In this
case recovery was fairly quick and by the 18th
of November, 26 days after application, the
crop had almost completely recovered.
Growth was normal under all other treat .•
ments except in the 'hand weeded' plots. In
these plots growth and establishment
deteriorated a bit after the first scoring. This

was apparently due to the periodic disturbances
in the plot at times of weeding, resulting
from trampling and also possible mechanical
damage to the crop during hoeing.
Weed Control.- The relative effectiveness
of different treatments on the control of weeds
was also assessed by scoring at the same time
as scoring was done for crop health. The
following scale (Table 5), also a modification
of one used by Green and Kalogeris (1963) in
Arusha, was used.
PeE MCPA 3 pts/ac 1
PeE MCPA 5 pts/ac 2
PeB Pesco 3 pts/ac 3
PeE Pesco 5 pts/ac 4
PoB MCPA 3 pts/ac 5
PoE MCPA 5 pts/ac 6
PoE Pesco 3 pts/ac 7
PoE Pesco 5 pts/ac 8
Hand Weeding 9
No Weeding '10
TABLE 6
SCORES OF WEED DENSITY
Weeding Control.-On the 4th of November,
12 days after pre-emergence application
weed growth in the treated plots were much
less than in the untreated plots with the
exception, of course, of the 'hand weeded'
plots. On the 18th of November, 26 days
after pre-emergence application the effect
of the herbicide seemed to have waned
.off a bit with the lower levels of pre-emergence
application of either MCPA or
Pesco 18-15, but control in the pre-emergence
plots was still very much better than in
the untreated plots, except the 'hand weeded'
':p1ots'. At this time the post-emergence
application had not been given should be
, oonsidered untreated.
TABLE 5
WEED DENSITY SCALE
Score Reference
5 No weeds
4 Few weeds
3 Moderate stand of weeds
showing symptoms of suppression
by herbicides
2 Dense stand of healthy weeds
1 Very dense stand of healthy
weeds
o Complete cover by weeds.
Table 6 below gives the scores recorded on the
three different dates for the various treatments.
By the 11th of February, i.e. 92 days
after pre-emgergence application, the effect
of the herbicide applied pre-emergence
seemed to have faded away completely.
This was reflected in the predominance of
weeds in these plots. The post-emergence
application, which had been applied 41
days before, showed a remarkably better
control, compared with the pre~emergence
application plots. .
The life of a herbicide after its application
in the field determines the duration of its
effectiveness. Leaching and biological
activities are the two main factors leading

to the disappearance of herbicides in the
field. Both MCPA and Pesco 18-15 are
highly soluble in water and are likely to
leach out of the top soil very quickly. Unde.r
the flooded conditions of irrigated rice
fields, leaching is likely to be even more
intensive and the residual effect of these
herbicides will not be. long. The residual
effect of such herbicides in the soil will
generally be related to their quantity of
active ingredient per unit land surface.
The evidence from this study confirms
this. At. the higher dosage rate of 5ptsjacre
the length of the weed control period was
longer with the pre-emergence application
of both MCPA and Pesco 18...:15than at
the lower dosage rate of 3ptsjacre. For
longer residual effects, perhaps larger dosage
rates may be used if the crop tolerance is
high. A delayed application of the herbicide
relative to the life of the crop also affords
an opportunity of extending weed control
into the latter part of the crops life. This
TABLE 7
was attempted with the post-emergence
application but the stage of application in
relation to the weeds was perhaps so late
that effective kit! was slow to achieve.
The effect of the herbicide on the composition
of the weed flora was assessed rather
empirically and note was only made of the
most striking species in the plots. The
results of this assessment cannot therefore,
be very conclusive. lschaemum rugosum,
a mono cot, was significant in all plots;
Sphenoclea zeylanica was significant in all
pre-emergence plots and also· in the unweeded
plots; Cyperus difformis was particularly
significant in the 'unweeded' plots.
Yields.-The yields obtained for the
different treatments were subjected to an
analysis of variance. This showed significant
differences due to treatments. Table 7
below shows the mean yield of treatments
arranged in ranked order for comparison,
using Duncan's multiple range test.
EFFECTS OF TREATMENT ON THE YIELD OF MAIZE
(a) Standard Error of the mean - ± 4.01
(b) Shortest significantRange (Rp)
p: (2) (3) (4) (5) (6) (7) (8) (9) (10)
Rp 2.97 3.12 3.21 3.27 3.32 3.35 3.37 3.39 4.41
(c) Ranked Means of Treatment Yields
Pre-em Pre-em Pre-em Pre-em No Post-em Post-em Clean Post-em Post-em
Treatment: MCPA Pesco Pesco MCPA Weed- Pesco MCPA Weed- Pesco MCPA
18-15 18-15 ing ing
5pt/ac 5pt/ac 3pt/ac 3pt/ac 5pt/ac 3pt/ac 3pt/ac 5pt/ac
Yield/plot 1.8 6.8 7.5 8.7 11.7 21.3 23.2 25.7 27.2 28.2
lb./acre 101.5 387.5 430.5 498.0 669.9 1224.3 1330.0 1473.5 1559.6 1617.0
This comparison showed clearly significant
differences in yields between the pre-emergence
application on the one side and the
post-emergence application on the other
side. Yield from the unweeded treatment
was, however, not significantly different
from those with post-emergence application
of Pesco 18-15 at 5ptsjacre and those with
post-emergence application of MCPA at
3ptsjacre.
Two principal factors accounted for yield
differences of the different treatments. These
are (a) the relative crop tolerance to the
herbicide and (b) the degree of effective weed
control. The effect of weed control without
any interference from herbicide can be
obtained by direct comparison between the
unweeded and the clean weeded treatments.
These showed a remarkably significant
difference due to competition from weeds.
The highest mean yields were obtained
from the post-emergence application of
Pesco 18-15, at 3ptsjacre, and MCPA, at
5ptsjacre, although these were not significantly
better than the mean yield of the
clean weeded treatment.
It was quite evident that yields were
significantly affected by crop tolerance. In
this respect time of application was a more
significant factor. The most significant

evidence for this came from MCPA. While
MCPA at 5ptsjacre produced the highest
yield among all treatments when applied
post-emergence, the same dosage produced
the least at pre-emergence. Within the preemergence
application there was also an
indication that the crop's tolerance of both
herbicides was lower at the higher dosage
rate of 5ptsjacre than at 3pts/acre. With
post-emergence application there was
evidence of increasing toxicity with Pesco
18-15 at higher dosage (5ptsjacre) than at the
lower dosage rate (3ptsjacre) in terms of
yields. MCPA, however, showed greater
efficiency with increasing dosage, and may be
tried at higher rates for more effective weed
control.
References
LE CLERG,B.L., LEONARD,W.H. & CLARK,
A. G. (1962) Field Plot Technique, Burgess
Publishing Company, Minnesota.
DUNCAN, D.B. (1955) Multiple F-tests.
Biometrics, 11, 1-42.
GREEN, D.H. & KALoGERIS,B (1963) A
preliminary Herbicide screening trial on
Ground. Miscellaneous Report No. 418.
Tropical Pesticides Research Institute,
Arusha.