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

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