Management of Weeds of Rainfed Lowland Rice Using Cultivar Mixture Strategies
The aim of this field study conducted in Calabar, Southeastern Nigeria was to investigate whether mixtures of rice cultivars with different characteristics and in varying proportions and deployment times could be effective in suppressing weed growth. Two lowland rice cultivars; Faro 15 (improved, semi-dwarf, profuse tillering, lodge-tolerant, early maturing),and Muduga (local, tall, lodging-susceptible ,medium maturing), were raised in nurseries and 24 day old seedlings transplanted, 2 seedlings per hill at 20x20 cm spacing. Treatments were factorial combinations of 2 planting proportions (Faro 15: Muduga at 4:1 and 3:2) and 5 times of introduction (Faro 15 introduced 2 weeks before, 1 week before, same day as, 1 week after and 2 weeks after-Muduga). The results indicate that in rainfed, low-input, lowland rice production systems, cultivar mixtures can improve the competitive ability of rice, reducing weed biomass production and diminishing rice biomass losses. Across both cultivars, the population of weeds was reduced by 39.7% when Faro 15 was introduced 2 weeks after Muduga in a 3:2 ratio, but the effect on weed biomass was not significant. The time of component cultivar introduction significantly affected the weed suppressive ability of the mixture and the best time depended on the grain preference of the farmer. On the basis of combined grain yield, introducing Muduga 1 or 2 weeks after Faro 15 gave the best results.
Received: November 24, 2011;
Accepted: December 13, 2011;
Published: December 21, 2011
Crop mixtures are a useful tool for disease management (Lannou
and de Vallavieille-Pope, 1997; Finckh et al.,
2000) and for attaining other objectives including yield stabilization and
increased yield (Bowden et al., 2001). Mixtures
may be composed of different species (inter specific) or of different genotypes
(intra specific), the later being made up of cultivars of the same species,
multilines (mixtures of genetically uniform lines of a crop species differing
in a single character) or bulk hybrids. Intra specific mixtures have been proposed
as a means of increasing crop heterogeneity thereby giving the crop a greater
capacity to adjust to the many and varied stresses that can occur, and ultimately
leading to higher yields than pure-line mono crops. They do this through mechanisms
such as complementary resource use above and below-ground (Fukai
and Trenbath, 1993), compensatory effects and facilitation which is the
positive effect of plants on the establishment and growth of other plants (Garcia-Barrios,
2003). Another benefit of intra specific mixtures is the fact that they
offer better opportunities for on-farm conservation of genetic resources because
farmers are able to cultivate varieties which, perhaps because of their low
yield potential would otherwise not be grown and may therefore become extinct.
Rice cultivar mixtures have a number of benefits from their use in low-input
systems such as practiced in Nigeria (Binang et al.,
2010a, b). While it is known that cultivar mixtures
generally stabilize crop yields and reduce lodging, their influence on weeds
have not been investigated to any significant extent. The competitive effects
of different crop cultivars against weeds vary depending on botanical characteristics,
and management practices such as time of deployment might be expected to affect
competitive ability. Estavan (2006) in a preliminary
study indicated that cultivar mixtures could improve the competitive ability
of barley, but suggested the need to devise a formula that allows us to
design correct mixtures for use in weed control. Binang
et al. (2010b) who evaluated the effect of cultivar interplanting
ratio on the productivity of rice concluded that the Muduga:Faro 15 ratios of
1:4 and 2:3 yielded highest because of the synergy of a meaningful reduction
in weed incidence and significant reduction in plant lodging. Such weed suppressive
activity could be particularly useful in subsistence farming systems where the
use of herbicides is prohibitive. Resistance to herbicides and lack of viable
control options have led to an interest in increasing the role of crop competition
as a weed control management tool. Weed-suppressive rice cultivars have been
suggested as a tool that could improve weed control and reduce the reliance
on herbicides. The use of cultivar mixtures could thus be a potent supplement
to present weed management practices and could reduce production costs and the
potential for environmental pollution, as well as alleviate some of the social
constraints associated with labour-intensive manual weeding. The aim of this
study was to evaluate the usefulness of cultivar mixtures as a strategy for
managing weeds of lowland rice in a rain-fed, low-input production system.
MATERIALS AND METHODS
The experiment was conducted at the Research Farm of the University of Calabar,
Southeastern Nigeria in 2009 and 2010. The area was located in the rain forest
belt and characterized by humid tropical climate with distinct wet and dry seasons,
with a bi-modal rainfall pattern which peaks in July and September. The site
was manually cleared with machete and tilled with hoe. Stumps were uprooted
and bunds 30 cm high and 30 cm wide at the base constructed by raising the soil
around the plot. The field was then divided into 3 blocks, each with 12 experimental
plots of 5x5 m, and separated by 2.0 m wide paths. Two lowland rice cultivars,
Faro 15 (improved, semi-dwarf, mid-maturing, profuse tillering habit) and Muduga
(traditional, tall, lodging-susceptible) were used for the study. Bed nurseries
were raised and 24 day old seedlings transplanted, 2 seedlings per hill at 20x20
cm. Treatments were factorial combinations of 2 planting ratios (Faro 15:Muduga
at 4:1 and 3:2) and 5 times of introduction (Faro 15 introduced 2 weeks before,
1 week before, same day as, 1 week after, 2 weeks after-Muduga). The sole crops
were planted to permit computation of Land Equivalent Ratio (LER). All plots
received 50 kg N ha-1, 40 kg K2 O ha-1 and
40 kg P2 O5 ha-1 in the form of sulphate of
ammonia, muriate of potash and single superphosphate, respectively. Nitrogen
was split applied at transplanting and at panicle initiation stage, while P
and K were worked into the soil, one week before transplanting. Weeding was
by hand pulling and hoeing at 3 and 7 weeks after transplanting (WAT) and birds
were controlled by scarring using scare crows and bird boys.
Weed density was taken as the number of weeds from a 1 m2 quadrant
prior to each weeding operation, while weed biomass was taken as the weight
of weeds collected as described above, washed, oven-dried at 70°C for 48
h and weighed with a sensitive Mettler weighing scale. Rice straw weight
taken at maximum tillering was recorded after sun-drying to constant weight.
Productivity of the mixture was assessed by calculating the Land Equivalent
Ratios (LERs) from component cultivar yields (Mead and Willey,
1980). If LER is greater than unity, then interplanting has a yield advantage
Statistical analysis: Data collected were weed density, weed dry matter,
plant height, productive tiller number and rice straw weight and grain yield.
This data were subjected to analysis of variance (ANOVA) according to the procedure
for a factorial experiment in randomized complete block design using GENSTAT
(2003) and mean separation by Least Significant Difference (LSD) at 5% probability,
as described by Gomez and Gomez (1984).
RESULTS AND DISCUSSION
Table 1 shows the effect of cultivar ratio, time of introducing
different cultivars and the interaction effect on weed incidence. Cultivars
differed widely in the growth of weeds they permitted. At both times of weed
sampling, the effect of cultivar ratio was not significant on weed density and
weed dry matter production, although transplanting Faro 15 after Muduga tended
to have supported the production of lower weed population. At 7 weeks after
transplanting however, the most weed-suppressive combination was the introduction
of Faro 15 1 or 2 weeks after Muduga in the ratio of 3:2.
|| Weed density and dry weight as influenced by cultivar ratio
and time of cultivar introduction
Across both cultivars, per cent weed population reduction was 39.7 when Faro
15 was introduced after Muduga in a 3:2 ratio. Weed biomass on the contrary
was unaffected by the different treatments at the various times assessed, probably
because the weeds though, numerous had low weight, probably due to the effectiveness
of land preparation method adopted as well as early vigor of rice seedlings.
Significantly higher weed density was recorded with pure populations of both
cultivars than with their mixtures indicating the weed suppressive effect of
this cultivar in a mixture. Faro 15 supported the production of lower weed density
than Muduga. Weed biomass was negatively correlated with rice plant height,
tiller number, straw weight and grain yield.
Plant heights at 10 WAT when Faro 15 headed, were similar in both cultivars. However, given that Muduga stayed longer in the field, its height at maturity was much taller than that of its companion cultivar. The earlier the introduction of a cultivar, the taller it tended to be perhaps because of the transplanting shock which the introduced component would have to overcome. However, whereas cultivar proportion did not influence the height of Faro 15, the 3:2 Faro 15 to Muduga ratio resulted in significantly taller Muduga plants (Table 2). Significant differences between cultivars were observed in tillering ability, as the improved, semi-dwarf Faro 15 expectedly bore many more tillers than the local Muduga cultivar. When grown in mixture, either cultivar tillered most when introduced before the other, suggesting an intra specific competition for resources between them.
|| Rice yield and some other parameters as influenced by competition
Although, there is no universal agreement about how tillering affects the plants
competitive ability (Fischer et al., 1995; Dingkhun
et al., 1999), both the plant height and tiller number are a good
measure of plant vigor and it is thought that rice cultivars that compete well
against weeds are tall and rapid in early growth and have high specific leaf
area. The tall erect Muduga probably complemented the semi-dwarf Faro 15 in
forming a more effective canopy than the respective mono crops, which prevented
sunlight from reaching the underlying weeds and thereby smothering them. Therefore,
weed growth suppression could be attributed to resource competition, although,
this analysis did not take into account allelopathic differences between the
two rice cultivars. This probably explains the superior weed-suppressive ability
of these cultivars in mixtures than as pure populations.
Rice straw yield of Faro 15 was highest when the variety was sown in a 3:2
ratio, 1 or 2 weeks before Muduga, while the treatment combination that gave
the highest Muduga straw weight was also the 3:2 Faro 15 to Muduga proportion
but with Faro 15 being introduced 1 or 2 weeks after Muduga. Given that plant
biomass at tillering is the best predictor of modern cultivar competitiveness
against weeds (Fischer et al., 1995), the best
weed-suppressing mixture would be the 3:2 Faro 15 to Muduga mixture with the
later component introduced after the former. The high straw weight of Muduga
relative its grain yield was due to the fact that this data was taken at maximum
tillering which was much earlier than the heading to grain-filling stage at
which the cultivar is most susceptible to lodging. It however, demonstrates
the yield potential of the cultivar if effective lodging-reducing measures are
adopted in its cultivation.
Cultivar ratio as well as its interaction with cultivar time of deployment
did not affect grain yield significantly (p = 0.05) but the time of introduction
did influence the yield of Faro 15 (Table 2). The grain yield
of Faro 15 ranged from 2.37 to 5.16 t ha-1 while that of Muduga was
much lower, and ranged from 1.35 to 2.32 t ha-1 when interplanted
with Faro 15. Sole Muduga only gave an average yield of 0.69 t ha-1.
The actual yield of sole Faro 15 exceeded that of the mixture, but the yield
of Muduga was increased by between 48.9 and 71.8% relative to the pure population,
when interplanted, because of a reduction in lodging brought about by the physical
support provided by Faro 15 (Binang et al., 2010a).
Cultivar weed-competitiveness is a function of weed tolerance, or the ability
to maintain high yields despite weed competition, and weed-suppressive ability,
or the ability to reduce weed growth through competition. Differences in cultivar
weed competitiveness have been demonstrated in barley (Christensen,
1995) and rice (Fischer et al., 2001; Haefele
et al., 2004), amongst other crops. Although these individual cultivars
possessed weed-suppressive traits such as early vigorous growth, tall plant
stature and high plant biomass, growing them in mixture was more effective in
suppressing weeds because of complementary resource use which ensured vigorous
early growth due to intra specific competition that led to the development of
a better canopy cover. Planting geometry did not seem to affect mixture weed-competitiveness
because the cultivars were similar in growth habit at least, up to maturity
of the early-maturing Faro 15. The time of cultivar introduction was however,
more influential on mixture weed-suppressiveness and this differed between mixtures.
In terms of effect on rice grain yield, the optimum competitive mixture was
the introduction of Muduga was 1 or 2 weeks after Faro 15, but if the absolute
yield of Muduga were to be considered important, the preferred time would be
its introduction before Faro 15.
It is concluded that cultivar mixtures could be an effective weed management
strategy in rainfed, low-input lowland rice systems, but for successful adoption
of this weed control method, the time of cultivar deployment is of critical
importance. In addition, cultivars for inclusion should be carefully selected
to reduce intra specific competition by ensuring that they have different plant
architectures and maturity periods.
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