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X. Phytotoxicity of Herbicides and Residues
Effect of Single Herbicide and Herbicide Combinations on Upland Rice Yield versus Effect of Hand Weeded and Untreated Controls
Grain yield (tonha)
(kg a.i./ha) and its
Butachlor (2.0) fb one HW
Linuron (1.0) fb one HW
Pendimethalin (2.0) fb one HW
(A - C)
Ghosh et al. (1977)
Ahmed and Islam (1983)
Cadag and Mercado (1980)
De Datta (1972)
Munroe et al. (1981)
De Datta (1979)
Sabio and Pastores (1981)
Sankaran and De Datta (1984)
Moorthy and Dubey (1981)
Dymrone (10.0) fb butachlor
(2.0) fb bentazon (2.0)
Propanil (3.0) in 2 splits
Propanil (2.8) + 2,4-D BE (0.6)
Chattejee et al. (1971)
Bhan et al. (1972)
Okafor and De Datta (1976b)
Patro and Misra (1969)
Tosh et al. (1981)
Patro and Tosh (1974)
Dixit and Sin& (1981)
Propanil (2.0) + 2,4-D Na (1.5)
Propanil (2.0) fb one HW
Propanil (3.0) + one HW
Propanil (2.0) + thiobencarb
Propanil (3.0) bentazon (2.0)
Jones and Tucker (1978)
Propanil (8.0) 2,4,5-T (1.0)
Morales and Vargas (1976)
Filho and Carvalho (1981)
Po, followed by; HW, hand weeding.
S. SANKARAN A N D S. K. D E DATTA
rice seed germination. Surface-applied herbicide came in contact with the
germinating seed and caused toxicity, characterized by reduced plant stand,
chlorotic leaves, and stunted growth. Thiobencarb (Mamun and Shimizu,
1976) and butralin, at 2.0 kg a.i./ha, were less toxic, and diclofop-methyl was
Pande et al. (1981) summarized weed control research in rice in India and
observed that butachlor and oxadiazon caused 50% seedling mortality
without affecting the final yield. Dichiormate caused bleached, papery white
leaves. Lopez and Mercado (1978) reported that preeemergence application
of dinitramine at 1.5 kg a.i./ha gave excellent weed control and crop safety
compared to preplanting incorporation, which injured roots and reduced dry
weight by 70%.
Nako (1977) observed that shallow planting and high soil moisture after
applying thiobencarb decreased establishment and inhibited growth of
seedlings at about the one-leaf stage.
The presence of a short mesocotyl in varieties like IR40 was cited as one of
the reasons for the tolerance of certain rices to herbicides even at high doses
(Manipon et al., 1981).
Nangju (1973) and Nangju et al. (1976) tested several methods of overcoming herbicide phytotoxicity in upland rice. Seed pelleting with activated
carbon and band application of an activated carbon slurry over the rice seed
did not reduce the toxicity of chloramben and atrazine. In contrast, a 2.5-cm
layer of activated carbon and vermiculite (1:l) covering the rice seed
overcame the herbicide toxicity. Nako (1977) found that applying thiobencarb at 10 kg a.i./ha (double the normal rate) did not damage rice seedlings,
even with high soil moisture, when seeds were planted 3 cm deep.
One way to increase selectivity, lengthen the period of weed control, and
provide greater reliability under varying environmental conditions is to use
an antidote or protectant to counteract the herbicide (Blair et al., 1976).
Ruscoe and Moody (1981) reported that a 1% seed coating of the herbicide
1,8-naphthalic anhydride (NA) protected the seed from the toxic effects of
soil-incorporated butachlor and thiobencarb and increased yield. However,
NA had little effect on the reaction of rice to preemergence treatments of
Chakraborty and Majumdar (1973) reported that applying 3 kg a.i.
propanil/ha, 2,4-D, or 1 kg a.i. MCPA/ha at 21 DAS did not leave any
phytotoxic residue in upland soils and did not injure the following mustard
[Brassica nigra (L.) Koch.] crop.
Carson (1975) reported that sorghum [Sorghum bicolor (L.)Moench cv.
Naga White] responded to 1 ppm of thiobencarb, butachlor, and propanil
with a good linear relationship between the growth response and herbicide
concentration. There were no detectable residues in the bioassay test 9
WEEDS AND WEED MANAGEMENT
months after applying the herbicides. Deleterious effects of butachlor, propanil, and thiobencarbon soil pH and organic matter at 6.0,4.0, and 3.0 kg
a.i./ha, respectively, were not observed.
XI. ECONOMICS OF WEED CONTROL IN UPLAND RICE
In the Philippines, Singh et al. (1981) reported that the rolling weeder,
high-wheel cultivator, and hand hoe were time-saving compared with hand
weeding. Hoe weeding produced the highest net income and hand weeding
the lowest net return. In India, the sweep hoe gave excellent weed control and
a high net return compared with conventional hand weeding (Singh et al.,
1976). Although repeated weedings gave effective control and increased grain
yield, there was a net loss in income compared with hoeing plus hand weeding
at 3 and 6 weeks (Upadhyay and Choudhary, 1979). Sabio et al. (1980) found
the farmers' practice of two cultivations followed by one hand weeding was
more remunerative than two timely hand weedings.
A preemergence spray of 2.0 kg a.i. butachlor/ha (Tasic et al., 1980; Sabio
and Pastores, 1981) followed by one hand weeding (Singh and Chauhan,
1978; Fisher et al., 1980; Ahmed and Islam, 1983) yielded a higher income
than conventional hand weeding. Ghosh et al. (1977) found that although
chemical weed control (preemergence butachlor at 2.5 kg a.i./ha) was more
expensive than mechanical weeding, the net income from butachlor-treated
plots was higher than for other weed control treatments (Table XVII). In Fiji,
Mandal (1977) reported that even butachlor at 3.6 kg a.i./ha was more
efficient and economical than traditional mechanical weeding.
Grain Yield with Different Methods of Weed Control in Upland Rice and Their Relative
Mechanical weeding (four times)
Postemergence propanil (2.5 kgfia)
Preemergence butachlor (2.5 kg/ha)
Adapted from Ghosh et al. (1977).
* In a column, means followed by the same letter are not significantly different at the
5 % level by DMRT.
S . SANKARAN AND S. K. DE DATTA
A postemergence application of propanil at 2.0 or 4.0 kg a.i./ha (Doming0
and Palis, 1966; Raghavulu and Sree Ramamurthy, 1973) followed by either
2,4-D (Akhanda, 1966; Chakraborty and Majumdar, 1973) or one hoeing
and weeding (Kolhe and Mittra, 1981) gave higher net returns than conventional hand weeding.
A combination of 1.0 kg a.i. 2,4-D sodium salt/ha and MCPA followed by
one hoeing and weeding (IRRI, 1967; Misra and Roy, 1970; Singh and Khan,
1972) gave a higher net return than the weed-free check in upland rice.
The choice of alternative weed control methods can be based on the
relative cost of labor versus chemicals. The daily wage rate in a given area and
the approximate time required for one hand weeding determine the cost per
hectare of hand weeding. This cost can be compared to the cost of herbicides
to determine the economical choice.
Hand weeding is still the major form of weed control in upland rice in
Indonesia, Bangladesh, and India, where wage rates are low (De Datta and
Barker, 1977). Considering the labor-hours saved and the returns from
herbicide use, herbicides may soon be adopted in many areas.
XII. CRITICAL RESEARCH NEEDS
Many research papers are available on upland rice weeds and their
management in different countries and under different ecological conditions.
However, there are still questions the answers to which will provide greater
understanding of the ecophysiology of upland weeds and permit formulation
of appropriate control measures. Certain areas should be studied.
1. Weed competition will be greatly minimized by methods to improve
crop establishment of upland rice by refining seeding methods and identifying
other limiting 'factors. Preplant weed control should be developed to aid
stand establishment and moisture utilization.
2. Studies should evaluate the biology, ecology, and control of weeds in
specific upland rice regions. Survival mechanisms of weeds, including adaptation to soil disturbance, and crop competition must be examined to develop
cultural practices to minimize weed competition. Vegetation analyses are
necessary to understand why changes in weed population occur with time
and different management practices.
3. Understanding the effect on the plant environment of continuous use of
a single weed control method and the formulation of effective controls for
resistant weed ecotypes that will develop because of such continuous