Weeds and Weed Management
Weeds are the undesirable species of the plant world. By definition, weeds are plants that are out of place, not wanted, usually not useful, often very prolific
and persistent, competitive, and, in some cases, even poisonous. In the photograph, the white flowering plants with the rice crop are weeds.
Research at the International Rice Research Institute has shown that weed growth in unweeded plots reduced yield by as much as 34% in transplanted rice, 45% in direct-seeded rainfed lowland rice, and 67% in upland rice (IRRI 1976). As weeds interfere with agricultural operations, the potential production of rice is reduced. Weeds cause many direct damages to a crop (Moomaw et al 1966).
Weeds in cultivated fields reduce rice yield and quality by competing for nutrients, water, and light. Weeds may intensify the problem of diseases, insects, and other pests by serving as their hosts. Weeds reduce the efficiency of harvesting. Aquatic weeds interfere with flow of irrigation water. Weeds in irrigated rice account for 11–12% reduction in rice production, estimated to be about US$5.3–5.8 billion in Asia. Echinochloaspp., Cyperus difformis, C. iria, Fimbristylis spp., Monochoriavaginalis, andSphenocleaare the principal weeds in South and Southeast Asia. Weed competition during the early stages of plant growth is critical as irrigated rice needs a weed-free period during the first 30–60 days after planting. Tillage practices such as plowing followed by harrowing, coupled with flooding and/or use of herbicides, are important measures that can minimize weed growth (Krishnamurthy 1985).
Common weeds in irrigated rice
The moderate temperature and high humidity in tropical Asia encourage year-round growth of weeds in both lowland and upland rice fields. The three types of weeds most common in rice fields, as characterized by physical differences, are grasses, broadleaves, and sedges. An understanding of how these weeds grow is essential for identifying appropriate control measures (Bantilan and Harwood 1974).
Grasses
Grasses (Vergara 1979) are monocoty-ledonous plants, which have long flat leaves with parallel veins, round hallow stem, and leaves aligned up and down the stem in two rows. Young grassy weeds can be distinguished from the rice plant. Rice has both a ligule and an auricle in each leaf, whereas grasses have either a ligule but no auricle or no ligule and auricle at all. Examples are Echinochloa crusgalli(L.) Beauv, Hispidula(Retz.) Honda, and Echinochloa colona(L.).
Sedges
Sedges are similar to grasses but their leaves are aligned up and down the stem in three rows. The stems are usually solid and triangular in shape. Aside from seeds, some sedges reproduce from tubers and rhizomes (underground stem) when the top growth is killed or pulled out. The weeds are highly competitive with rice for both moisture and soil nutrients. Examples are Cyperus iria(L.), C. difformis(L.), Fimbristylis literalisGaud, and F. miliacea(L.) Vahl.
Broadleaves
Broadleaf weeds are dicotyledonous plants with various shapes of leaves and vein arrangements. The leaves are usually wider than those of grasses and sedges. The weeds are considered either perennials (requiring more than a year to complete their life cycle) or annuals (completing their life cycle in 1 year or less). Examples areMonochoria vaginalisPresl. and SphenocleazeylanicaGaertn.
Common weeds in upland rice
Grasses
Echinochloa colona(L.) Link.
Digitaria sanguinalis(L.) Scop.
Eleusine indica(L.) Gaertn.
Dactyloctenium aegyptium(L.) Beauv.
Sedges
Cyperus rotundusL.
Ipomoea triloba L.
Portulaca oleraceaL.
Commelina benghalensisL.
Weed competition
Weeds possess a remarkable ability to adapt to the environment; they have great reproductive capacity and persistence. Each plant may produce thousands of seeds, which are transported from field to field by irrigation water and farm equipment. These weed characteristics are not at all surprising as most of them are products of survival of the fittest. As such, many members of the grass family are able to withstand adverse environmental conditions that wipe out most other crops. The ubiquitous grass species in lowland rice is Echinochloa crusgalli;and in the Philippines, the most common are the broadleaf weed Monochoria vaginalisand the sedge Cyperusiria. A tremendous number of viable weed seeds are present in the soil. The presence of an equivalent of 800 million weed seeds per hectare in a lowland rice field within 15-cm depth was demonstrated in a previous study (Bantilan and Harwood 1974). C. difformiscan produce 50,000 seeds per plant and is capable of becoming the dominant weed in rice fields when the herbicides used are effective against grasses but not sedges. Other prolific weeds are C. iria,which produces up to 5,000 seeds per plant, and E. crusgalliand E. colona, which,if allowed to grow, can result in serious yield losses. Dactyloctenium aegyptium(L.) Willd. is serious in Nepal, India, Thailand, and the Philippines, producing up to 60,000 seeds per plant (Moody and De Datta 1977).
The photograph above shows weeds growing on a newly plowed field after one week of rain shower (author’s experience).
Weeds compete with rice for sunlight, nutrients, and water. If any of these is lacking, the other elments cannot be used effectively, even if they are present in large amounts. The competition results in poor rice growth, and thus, less grain yield (Vergara 1979). It should be noted that adequate weed control is more critical for modern varieties grown with high levels of applied fertilizer than for traditional rice grown without fertilizer. Nitrogen application favors the growth of weeds; it should not be applied if weeds are not yet controlled. There are specific recommendations with specific dates of doing weed control (i.e., number of days from seeding or transplanting).
Methods of weed control
One of the best criteria in selecting weed control methods is the cost of labor relative to that of chemicals. In most of tropical Asia, an integrated method of weed control using limited quantities of low-cost chemicals in combination with direct and indirect weed control techniques may be the most attractive alternative from the agronomic, economic, and ecological points of view (De Datta and Barker 1975).
Indirect methods
A number of practices indirectly affect the size and composition of the weed population. Examples are land preparation, puddling, water management (particularly depth of flooding), choice of variety, cropping pattern, spacing, and fertilizer application (De Datta 1973).
- Land preparation
An obvious benefit of land preparation, particularlypuddling, is minimizing weed growth. Repeated land preparation at 1-week intervals allows the remaining weeds to germinate and then be plowed under with suffi-cient time to de-compose. Farm-ers with good control of water attained higher yield with this practice most of the time. IRRI studies on the relationship bet-ween weed control and land preparation have confirmed that harrowing is critical in reducing the weed population (Barker and Abarrientos 1974, Bantilan and Harwood 1974).
2. Flooding and water depth
Submergence can control weeds, especially grasses. The emergence of weeds is closely related to the moisture content of the soil and the depth of irrigation (De Datta and Barker 1975). In an IRRI study, it was observed that the highest weed number per square meter was ob-tained with continuous soil satura-tion, whereas the lowest was seen in deep, continuously flooded plots. Under shallow continuous flooding (2.5 cm), sedges dominated over broadleaves and grasses. In 15 cm of standing water, grasses and sedges were practically suppressed but, at a depth of 7.5 cm, some broadleaved weeds and sedges were still present. Experiments repeated during the 1968 wet and dry seasons at IRRI clearly showed results similar to the above observations. Weed growth is greatly reduced in deep flooded plots (Moomaw et al 1966). Under 5–10 cm water, most grasses and sedges are prevented from growing.
3. Choice of variety
One reason for the initial success of IR8 was its high tillering capacity, which allowed it to compete effectively with weeds despite its short stature. Low-tillering varieties with the same yield potential as IR8, even under ideal conditions, did not fare well under in the field. It is generally felt that heavy tillering varieties of medium stature (120–130 cm height) such as IR34 may be better suited for both lowland and upland rice culture. The taller varieties may not respond as well to applied fertilizer in terms of yield, but they effectively shade out the weeds. This is why many farmers in tropical Asia, where land preparation and water management are inadequate, would rather use the taller 30–40-day-old seedlings than the recommended 21–25-day-old seedlings.
4. Plant spacing
In general, closer spacing minimizes weed compete-tion and will result in bet-ter yield. More weeds will grow at 25- x 25-cm spa-cing than at 15 x 15 cm because, with wider spa-cing, weeds have better chances of competing for sunlight, nutrient, and moisture. However, one must compare the added cost before deciding which weed control practice to adopt. In irrigated, direct-seeded rice, seeding rates of 80–120 kg/ha result in less weed population compared with lower rates, as practiced in the only commercial rice farm in the Solomon Islands (author’s experience).
IRRI experiments on plant spacing x variety x fertilizer rate have consistently shown that yields can be significantly improved with closer spacing. Yields of IR28 and IR30 were compared at various plant spacings (25 x 25, 20 x 20, and 15 x 15 cm). Grain yields decreased an average of 52, 30, and 18%, respectively, for the three spacings, compared with the weed-free check plots (Estorminos and Moody 1976).
5. Application of nitrogen fertilizer
Even where HYVs are grown, grain yields are lower because farmers lack an appreciation of the interdependence of factors in production. For example, heavy fertilization without weed control may be more damaging than poor fertilization with good weed control. To increase fertilizer efficiency at low rates, it is imperative that weeds be adequately controlled (De Datta 1976).
A study conducted by IRRI economists in the Philippines and Thailand revealed close complimentarity between fertilizer input and weed control. The benefits from added fertilizer increased markedly with higher levels of weed control (IRRI 1973). But, even at a low rate of 30 kg N/ha, with proper weed control, IR26 yield reached 5 t/ha in the farmer’s field; it was 4 t/ha for the same HYV without nitrogen application. These results clearly demonstrate the need to improve cultural practices such as weed control and water management, if fertilizers were to be effectively used (De Datta 1975).
6. Cropping system
Various crop husbandry practices applied over time in an intensive cropping system can eliminate troublesome grasses and sedges with minimum cash inputs. A crop’s competitive ability, as when corn was intercropped with mungbean, was enhanced as the level of weed control was reduced (Bantilan and Harwood 1974).
The effects of three cropping systems and three weeding methods on the population density of Scirpus maritimus, a difficult weed in lowland rice, have caused the shift from weed species that are difficult to control to those that are easier to control. Further, the results demonstrate the advantage of growing crops in a planned sequence or rotation and of reducing to a minimum the yield losses caused by weed competition. In integrated weed management systems, chemical weed control is often necessary, even though the rate of herbicide is decreased (Lacsina and De Datta 1975).
Direct methods of weed control
Direct methods of weed control include hand weeding, mechanical weeding (by rotary weeder in lowland paddies or by animal-drawn harrow under upland conditions), and chemical weed control (either preemergence or postemergence). In Asia, hand weeding is the dominant method of weed control. However, low-cost chemicals such as 2,4-D are being effectively used, often in combination with limited hand weeding. This proved economical in many situations. Further, hand weeding may not be adequate for some forms of rice culture, such as in direct-seeded rice, especially when the crop is fertilized (Moomaw et al 1976).
Critical to a successful weed control is timeliness of weeding. Because weeds provide the greatest competition and cause the most damage at the early crop growth stage, early control is important. Early-maturing varieties must be weeded earlier than late-maturing ones. Barnyard-type grass weeds grow fast and compete aggressively with rice for space and soil nutrients. If preemergence herbicide is used, however, growth duration may not be a critical factor in irrigated rice but may still pose problems in rainfed rice (De Datta 1981).
1. Hand or mechanical weeding
Straight-row planting may cost a little more than random planting, but it is worth the additional cost as it makes weeding much easier and more efficient. The critical period for weed control of HYVs with short to medium maturity is between 15 and 30 days after transplanting. Hand weeding of plants in straight rows takes 15–17 man-days per hectare or 120–130 hours per hectare.
The use of a rotary weeder is more efficient than hand weeding, but it requires straight-row planting to accommodate the equipment. Standing water should be drained from the field before using the rotary weeder in order to incorporate the weeds into the mud. It takes about 70 hours per hectare to use the rotary weeder.
Hand weeding is the most common method of weed control in all tropical rice-growing areas, especially for transplanted rice. One or two hand weedings should be sufficient to control weeds adequately. Increase in labor cost is noted if hand weed-ing is done beyond 40 days after transplanting for short-maturing 120-day varieties and yield is significantly reduced. One properly timed hand weeding should be ade-quate to reduce weed populations enough to obtain high yields with HYVs; doing this between 15 and 30 days after transplanting will require less labor because weed population is still low at this stage.
2.Chemical weed control
A herbicide is a chemical used to kill or prevent the growth of weeds. Herbicides are most effective against broadleaved weeds, sedges, and grasses. However, weeds which are not controlled chemically must be eliminated by mechanical means. Weeds that survive the herbicide treatment may grow more vigorously due to less competition from other weeds (Moody 1981).
Other features of herbicides
Based on selectivity. Selective herbicides will kill certain plants only, like 2,4-D, whereas nonselective herbicides will kill all plants, like PCP.
Based on time of application. Herbicides can be applied before the weed seedling come out (preemergence) or after the weed seedlings are out (postemergence).
Based on formulation or form. Herbicides can be granular, liquid, and wettable powder.
Based on mode of action. As contact herbicide, it will kill only the plant parts that were sprayed. As systemic, the herbicide can move inside the plant and can therefore poison and kill the whole plant.
Herbicide computation
Herbicide recommendation rates are often expressed in terms of kilograms of active ingredients per hectare (kg ai/ha). These rates have been carefully studied and evaluated; herbicides must be applied at the correct dosage at the right time, following manufacturer’s instructions carefully. Overdose of the recommended rate might damage the rice crop while under dose will not provide a proper weed control, and may even encourage weeds to grow.
The ai of a herbicide is the principal chemical compound that acts on the weed. For herbicides that are salts of organic acids, their acid equivalents (ae) are considered. If we have sodium MCPA formulation containing 22.2% of sodium MCPA, this formulation would contain 22.2 percent weight/volume of ae. In solid formulations (wettable powder [WP], water-soluble powder [WSP], granules), the concentration is expressed as a percentage of the weight of ai relative to the weight of the commercial solid herbicide (weight/weight).
Examples:
Hedonal 3.2% granules contain 3.2 g of isopropyl/ester of 2, 4-dichlorophenoxyacetic acid in every 100 g of Hedonal. In commercial liquid formulation (emulsifiable concentrate [EC]), the concentration is expressed as weight of ai by volume of the commercial EC herbicide (weight/volume).
MCPA sodium, EC 2 lb ai/gallon contains 2 pounds of 2-methyl, 4-chlorophenoxy acetic acid in every gallon of the MCPA liquid herbicide (or 240 g/liter).
Herbicides in the form of EC, WP, or WSP must be diluted with water or other carriers such as oil. If proper dilution is not made, the effect of herbicide may either be too weak or too strong. It is possible that the herbicide may not control the weed effectively or it may even damage the crop. The amount of water for dilution, however, depends on the type of herbicide and calibration of the sprayer, but the amount of herbicide is fixed, given a certain rate and the size of area to be treated.
Computations
The basic data needed to calculate the amount of formulated product, either in powder, liquid, or granular form, are recommended rate of application (RR, kg ai/ha); area to be treated in (A, ha); and concentration of ai in the herbicide formulation (C, %) (Calderon 1970).
To compute for the amount of herbicide for a given A, with the chemicals’ ai and concentration (%) known and in either solid (kg) or liquid (liter) preparations, the following formulas are used:
Solid (WP, WSP, granules
Amount (kg) = [RR (kg ai/ha) x A (ha) x 100] ÷ C
Liquid (EC)
Amount (li) = [RR (kg ai/ha) x A (ha) x 100] ÷ C
1. WP: To apply 2 kg ai/ha of PCP WP 85% to a 2,500-m2field, how many
kilograms of herbicide are needed?
RR = 2 kg ai/ha
A = 2,500 m2or 0.25 ha
= 85%
Required weight of PCP WP 85% = (RR x A x 100) ÷ C
= (2 x 0.25 x 100) ÷ 85
= 0.588 kg
2. EC: How many liters of MCPA EC 4 lb/Imp. gal. are required to spray a 500-m2plot at the rate of 0.8 kg ai/ha?
RR = 0.8 kg ai/ha
A = 500 m2or 0.05 ha
C = 4 lb/Imp. gal. or 400 g/li = 40%
Required volume of MCPA (li) = (RR x A x 100) ÷ C
= (0.8 x 0.05 x 100) ÷ 40
= 0.10 li
3. Granular: How much EptamM (EPTC/MCPA) granules are needed for a field of 3.5 ha? Rate of application is 1.75 kg ai/ha for Eptam, and EptamM contains 4.38% (EPTC) 5% and 10% granules and EC 6 lb/gal.
RR = 1.75 kg ai/ha
A = 3.5 ha
C = 4.38 %
Weight of EptamM required = (RR x A x 100) ÷ C
= (1.75 x 3.5 x 100) ÷ 4.38
= 139.84 kg
Some granular formulations often contain more than one ai, as in the previous example. These are called combination herbicides, used to eliminate all types of weeds. The details of these calculations are found in the Training manual for rice production (Xuan, B. and Ross V. 1972). Always refer to the manufacturer’s instructions or consult an extension worker to know the recommended herbicides for transplanted rice and their effectiveness against different weed species.