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Agronomy Journal 93:967-973 (2001)
© 2001 American Society of Agronomy

RICE

Competitiveness of Semidwarf Upland Rice Cultivars against Palisadegrass (Brachiaria brizantha) and Signalgrass (B. decumbens)

Albert J. Fischer*,a, Hector V. Ramírezb, Kevin D. Gibsonc and Beatriz Da Silveira Pinheirod

a Dep. Vegetable Crops, Univ. of California, Davis, CA 95616
b Instituto Rio Grandense do Arroz, 94930 Cachoeirinha, RS, Brazil
c Agronomy and Range Science Dep., Univ. of California, Davis, CA 95616
d EMBRAPA Arroz e Feijão, 75375-000 Santo Antonio de Goiás, GO, Brazil

* Corresponding author (ajfischer{at}vegmail.ucdavis.edu)

Received for publication January 4, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
When savannas in Latin America are brought into cultivation, rice (Oryza sativa L.) can be sown with the perennial grasses palisadegrass [Brachiaria brizantha (Hochst. ex A. Rich) Stapf] and signalgrass (B. decumbens Stapf) to harvest a grain crop while establishing a pasture to suppress weeds and provide grazing in subsequent years. However, these Brachiaria spp. can reduce upland rice yields. Rice cultivars need to be competitive with Brachiaria spp. to maintain yields but must allow Brachiaria spp. sufficient growth for pasture establishment. Field studies were conducted during 1994 and 1995 on a Typic Haplustox oxisol soil in the Eastern Plains of Colombia to evaluate the competitiveness of upland rice cultivars and to identify rice traits for competitiveness. Ten (1994) and 14 (1995) upland rice cultivars were grown with and without signalgrass in 1994 and palisadegrass in 1995. Rice cultivars differed substantially in their competitiveness. Rice yield losses ranged from 18 to 55%, and Brachiaria aboveground biomass ranged from 1.4 to 3.2 Mg ha-1 dry mass. Competition for light was critical; rice photon flux density interception, leaf area index [>=45 d after emergence (DAE)], and number of tillers (>=60 DAE) were correlated with competitiveness. No tradeoff between high yield potential and competitiveness was detected in these upland rice cultivars. Early maturity of rice is a desired characteristic for the rice–Brachiaria spp. association. The development of more-competitive cultivars appears to be a viable approach for reducing herbicide dependency and improving profitability of Latin American rice–pasture intercropping systems.

Abbreviations: CIAT, Centro Internacional de Agricultura Tropical (International Center for Tropical Agriculture) • DAE, days after emergence • LAI, leaf area index • PPFD, photosynthetic photon flux density


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
WEEDS CONSTRAIN RICE GRAIN YIELD, and weed management is a major expense in upland rice production (Moody, 1996). In Latin America and the Caribbean, 4.6 million ha of dryland rice are grown, and upland rain-fed rice accounts for 40% of the rice produced in the region (Fischer and Antigua, 1996). Upland rice in Latin America is grown under diverse management conditions, ranging from the use of itinerant slash-and-burn techniques in the forest margins to the large, mechanized farms in the savannas. Weed control in upland rice can be labor intensive (De Datta and Llagas, 1984) or involve the intensive use of herbicides (Franco, 1988). Intensive herbicide use can increase costs, pose a threat to the environment and farmers' health, and may promote the development of herbicide resistance (Valverde et al., 2000).

Upland rice varieties tolerant to acid soil can be sown in the acid, well-drained savannah soils of Latin America for the profitable production of grain while grazing land is renovated (Cadavid and Smith, 1994; Friesen, 1994; Guimarães, 1993). Signalgrass and palisadegrass are competitive perennial tropical grasses often undersown in rice in the savannas of Colombia, Venezuela, and Brazil (CIAT, 1990; Yokohama et al., 1999) to accomplish this goal. For the system to succeed, rice cultivars must be sufficiently competitive to suppress the vigorous initial growth of the pasture, and yields must be high enough to cover the cost of pasture renovation. Rice cultivars must also be competitive enough to suppress the regrowth of these well-rooted pasture species when old pastures are plowed up to sow conventional rice. Developing rice cultivars that can compete with signalgrass and palisadegrass should help upland rice farmers in Latin America and the Caribbean reduce their dependency on herbicides and decrease expenditures for weed control.

Although the impact of weeds on rice production is well recognized, it has not been addressed by breeding, as have diseases and pests. The development of competitive rice cultivars would provide a safe and environmentally benign tool for integrated weed management. Contrary to complex crop management schemes requiring specialized skills and training, improved varieties have a high potential for adoption by farmers. Accordingly, weed-competitive upland rice cultivars have been developed in West Africa for areas where herbicides are too expensive or unavailable (Johnson et al., 1998). The potential for such a strategy in Latin America with irrigated and upland rice has been documented at Centro Internacional de Agricultura Tropical (International Center for Tropical Agriculture; CIAT) (Fischer et al., 1995, 1997). Other studies have also shown significant differences in competitiveness among rice cultivars. In Asia, Garrity et al. (1992) found up to a 75% difference in weed suppression among cultivars. Fischer et al. (1997) observed yield losses ranging from 27 to 60% among Latin American irrigated rice cultivars growing in competition with junglerice [Echinochloa colona (L.) Link]. Gibson et al. (2001) found that the more competitive water-seeded rice cultivar in their study required lower herbicide rates to achieve the same level of control of late watergrass [E. phyllopogon (Stapf) Koss] than the less-competitive cultivar.

The development of competitive rice cultivars requires the identification of key traits conferring competitive ability that can be used as selection criteria by breeders (Pester et al., 1999). Competitive ability in rice is often associated with traits related to light capture (Khush, 1996). Plant height can be highly correlated with competitive ability (Jennings and Aquino, 1968) but is not always important. Fischer et al. (1997) associated leaf area index (LAI) and tiller production, but not height, with the competitive ability of irrigated rice cultivars. Leaf area and leaf weight were associated with the ability of upland rice cultivars to compete with palisadegrass under greenhouse conditions (Fischer et al., 1995). Garrity et al. (1992) reported substantial differences in competitive ability among cultivars of the same height. Finally, Johnson et al. (1998) reported that their most competitive cultivar had larger leaf weight, greater specific leaf area, and earlier production of tillers than less-competitive cultivars.

The objective of this study was to expand upon earlier greenhouse research (Fischer et al., 1995) by comparing rice performance under weedy and weed-free field conditions and by identifying rice traits related to interference with the growth of Brachiaria spp.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field experiments were conducted in 1994 and 1995 at La Libertad, Meta (4°03' N, 73°29' W; 336 m above sea level), in the Eastern Plains (Llanos Orientales) of Colombia, under dryland conditions on a La Libertad clayey Typic Haplustox oxisol (4.4% organic matter, 78% Al saturation, and pH = 5). Rainfall was 1601 mm in 1994 and 1316 mm in 1995. Average daily temperatures were 25.1°C for 1994 and 25.6°C for 1995. Upland semidwarf rice cultivars differing in origin and morphological characteristics were obtained from the rice breeding program at CIAT in Colombia. All cultivars were tropical japonica types except ‘Colombia 1’, which is an indica type. Cultivar heights in monoculture ranged from 75 to 100 cm, tiller number from 220 to 390 m-2, and LAI from 2.5 to 4.0. ‘Oryzica Sabana 6’ and the improved CT lines (Table 1) are acid soil adapted cultivars developed by CIAT and the Colombian Agriculture Institute (Instituto Colombiano de Agricultura). Oryzica Sabana 6 was, at that time, widely used in monoculture or rice–pasture intercropping in the savannas of the Colombian Eastern Plains. ‘IRAT 216’ was developed by CIRAD (Centre de Coopération Internationale en Recherche Agronomique pour le Développement) in France and released as ‘JDASA6’ in the Ivory Coast and as ‘Rio Verde’ in Brazil; ‘CT 6196-33-11-1-3-M’ (Line 3) was released as ‘Oryzica Sabana 10’ in Colombia, ‘Caiapo’ is a Brazilian cultivar, ‘RHS 107-2-1-2TB-1JM’ is a drought-tolerant line developed in Mexico, and ‘CNA 7013D’ is an upland line of Latin American origin.


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  		Table 1. Rice yield, vegetative dry matter, canopy light interception, and growth of signalgrass and palisadegrass when upland rice cultivars were grown in monoculture or in competition with these species (Meta, Colombia).

 
Cultivars and two competition levels (systems) consisting of rice only (monoculture) and rice associated with signalgrass or palisadegrass (full-season competition) were main factor treatments within a randomized complete-block design with four replications. Plot size was 4.7 by 15 m. In mid-May, signalgrass (1994) or palisadegrass (1995) seed was broadcast at 2 kg ha-1 (20 viable seeds m-2) and lightly incorporated with a tooth harrow. Palisadegrass may be more competitive than signalgrass (Fischer, personal observation); the former was used in 1995 in an effort to increase weed pressure on the crop. Immediately after seeding signalgrass or palisadegrass, seed for each rice cultivar was drilled into rows (0.28 m apart) at 70 kg ha-1 (280 viable seed m-2). Those seeding rates had been found adequate for the establishment of a Brachiaria sp. pasture undersown with rice in the Colombian acid-soil savannas (CIAT 1990); however, rice yields were reduced significantly by grass competition (CIAT 1995). One month before seeding, 300 kg ha-1 dolomitic lime (55% CaCO3 and 33% MgCO3) was broadcast and incorporated with a disc harrow. Just before seeding signalgrass or palisadegrass, 26, 23, and 50 kg ha-1 P, N, and K, respectively, were broadcast and covered with the seeder when rice was drilled. At 40 and 60 d after emergence (DAE), 23 kg N ha-1 was topdressed. Lime and fertilizer rates used are adequate for this area (CIAT 1995).

In 1994 and 1995, two rice rows, each 2 m in length, and three adjacent 0.28- by 2-m interrow areas with signalgrass or palisadegrass were harvested at 15, 30, 45, 60, and 90 DAE. Plants were clipped at the soil surface; roots were not included in the harvest. Height, tiller number, and leaf area of rice (10 plants per sample) and signalgrass or palisadegrass were determined. Leaf area was measured with an LI-3100 meter (Li-Cor, Lincoln, NE); rice leaf area was extrapolated to the whole sampling area using the leaf area/weight ratio. Above-ground vegetative dry matter of rice and signalgrass or palisadegrass was measured for each sample. At 50% anthesis, photosynthetic photon flux density (PPFD) at ground level and incident PPFD above the crop were measured with a line quantum sensor (LI-1000, Li-Cor, Lincoln, NE) on cloudless days between 11:00 and 14:00 h. Three diagonally crossed paired readings (six per plot) were taken at ground level and averaged. In the weedy plots, signalgrass and palisadegrass were clipped at the soil surface and removed before measuring PPFD. Percent canopy PPFD interception was obtained by subtracting the ground-level reading, expressed as percent of the incident radiation above the canopy, from 100. Rice grain was hand-harvested at maturity from eight 5-m-long rows (11.2 m-2). Analysis of variance was conducted for grain yield and growth parameters. Correlation analysis for cultivar means (Wortmann, 1993) was performed to relate plant traits to competitiveness and yield.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Competition Effects
Competition with Brachiaria spp. reduced rice yields and biomass in both years (Table 1). Signalgrass and palisadegrass had similar effects on rice, and there was no difference in mean biomass between species (Table 1). Rice biomass was significantly lower by 60 DAE in the weedy system than in the weed-free system in both years (Table 2). Biomass of signalgrass and palisadegrass was positively correlated with reductions in rice biomass (Table 3). There was substantial variability among rice cultivars in their ability to compete with signalgrass and palisadegrass (Table 1). The cultivars most tolerant of the weed were also the most suppressive. Oryzica Sabana 6 and Line 3 consistently maintained yields under competition, with average yield reductions across years of 20% for Line 3 and 25% for Oryzica Sabana 6. Conversely, Colombia 1 and the RHS line were always among the worst competitors; their average yields over 2 yr were reduced by 52 and 43%. Biomass of signalgrass and palisadegrass under these lines was about 72% greater than under Line 3 and Oryzica Sabana 6 (Table 1). Maximum rice tolerance to signalgrass and palisadegrass was observed in ‘CT 11891-2-2-7-M’, a newly developed line included in 1995.


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  		Table 2. Analysis of variance for grain yield and four growth parameters of rice growing in monoculture and in competition with signalgrass and palisadegrass.{dagger}§

 

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  		Table 3. Correlation coefficients (r) between rice parameters and signalgrass or palisadegrass, recorded 90 d after emergence (DAE), when rice was grown in competition with these species.

 
Rice Morphology and Competitiveness
Rice competitiveness was associated with its ability to intercept light. Thus, rice LAI, number of tillers, and PPFD interception were negatively correlated with biomass of signalgrass and palisadegrass when rice was grown with weeds (Table 3). Rice LAI and tiller number were positively correlated with PPFD interception in weedy plots in both years (Table 3). Rice LAI measured as early as 45 DAE in both years was negatively correlated with biomass of signalgrass and palisadegrass at 90 DAE (Table 4). This is illustrated in Fig. 1 for one of the most (Line 3) and least competitive (Colombia 1) cultivars. In contrast to Colombia 1, Line 3 had substantially greater LAI than signalgrass or palisadegrass for most of the growing season (Fig. 1). Competition with signalgrass and palisadegrass modified tillering late in the season (Table 2), and rice tiller number only became negatively correlated with growth of signalgrass at 90 DAE in 1994 (Table 3) and palisadegrass at 60 DAE in 1995 (Table 4).


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  		Table 4. Correlation coefficients (r) between rice growth parameters recorded in competition at four dates and signalgrass or palisadegrass matter harvested at 90 d after emergence (DAE).{dagger}

 


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  		Fig. 1. Leaf area index (LAI), tiller number, and aboveground biomass of signalgrass and palisadegrass (open symbols) and rice (closed symbols) when the grasses were sown with a weakly (Colombia 1) and strongly (Line 3) competing rice line. Error bars are standard error of the mean.

 
Rice cultivars differed in height (data not shown), but height was only associated with rice competitiveness in 1995 until 30 DAE (Table 4). Rice height at 90 DAE was not significantly correlated with weed growth or reductions in rice yield in either year (Table 3). By 45 DAE in both years, signalgrass and palisadegrass were taller than rice (data not shown), and rice height did not substantially increase in response to weed competition (Table 2). These results agree with Fischer et al. (1995), who found that the height of upland semidwarf rice cultivars was not related to interference with growth of palisadegrass under greenhouse conditions. Modern semidwarf rice plant types have erect leaves that allow substantial light penetration deep into the canopy (Peng et al., 1994). Therefore, with such plant types, variations in height should not affect light penetration as much as they would in leafier canopies with droopy leaves, as in tall traditional cultivars. A relationship between rice height and competitive ability was found when tall traditional cultivars were compared with short lines (Jennings and Aquino, 1968; Jennings and de Jesus, 1968; and Jennings and Herrera, 1968). A study by Fischer et al. (1997) with irrigated rice cultivars of heights ranging from 80 to 128 cm also showed no relationship between rice height and competitiveness against junglerice.

Significant system x cultivar interactions occurred in both years for rice growth parameters and PPFD interception (Table 2). Rice trait expression when competing with signalgrass or palisadegrass was different than in monoculture; trait expression in monoculture was not related to competitiveness with the weed (Table 5). This lack of correspondence between traits measured in monoculture and in competition was also noted in greenhouse experiments (Fischer et al., 1995) and field experiments with junglerice as the target weed (Fischer et al., 1997). Wall (1983) suggested that trait evaluation under competition might result in more progress in selecting competitive genotypes than evaluation in monoculture. Our results with upland rice germplasm adapted to the acid-soil savannas of Latin America support this approach.


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  		Table 5. Correlation coefficients (r) between rice parameters (in monoculture) and signalgrass or palisadegrass dry matter at 90 d after emergence (DAE), and rice yield reduction from signalgrass or palisadegrass competition.{dagger}

 
In addition to selecting for traits related to competitive ability, a breeding program should also focus on cultivars with consistent performance. Cousens and Mokhtari (1998) reported substantial seasonal and site variability among wheat (Triticum aestivum L.) cultivars in their ability to tolerate weed competition. The authors argued that inconsistent performance of cultivars might make it difficult to provide information to farmers. In our study, Oryzica Sabana 6 and Line 3 were among the most competitive cultivars in both years of the study while Colombia 1 was the least competitive. However, there was variability between years in competitive ability of other cultivars although the same suite of traits was important in both years. Additional research may be necessary to better understand variability in cultivar performance across seasons and in different environments.

Competitiveness and Yield Potential
Rice competitiveness was not exclusive of high yield potential in our study. No significant negative correlation was found between rice grain yields in monoculture and the ability to tolerate competition; likewise, rice yields in monoculture were not negatively associated with the suppression of signalgrass or palisadegrass in our study (data not shown). For example, monoculture yields for the highly competitive varieties Oryzica Sabana 6 and Line 3 were greater within each year than those of the least competitive cultivar, Colombia 1 (Table 1). Short-stature and erect-leaf canopies are sought for high productivity in modern rice plant types (Peng et al., 1994). In earlier work in Asia, researchers associated low productivity in rice with the tall and leafy canopies of traditional land races (Jennings and Aquino, 1968; Jennings and de Jesus, 1968; and Jennings and Herrera, 1968) as opposed to the high yield potential of short but less competitive modern plant types. By comparing traditional vs. modern cultivars, researchers concluded that rice competitiveness conferred a yield penalty and that there would be little reason to develop competitive cultivars as long as alternative weed control methods were available (Jennings and Aquino, 1968). However, more-recent studies comparing competitiveness among modern rice cultivars found that rice could be both high yielding and competitive with weeds (Garrity et al., 1992; Fischer et al., 1997). Dingkuhn et al. (1999) suggest that some tradeoffs between competitiveness and yield potential may exist but could be reduced by specifically expressing traits for competitiveness at an early developmental stage.

Growth of signalgrass and palisadegrass substantially increased toward the end of the season, and rice faced an ever-increasing competitive pressure at the time of grain filling (Fig. 1). The most competitive cultivar in the 1995 study, CT 11891-2-2-7-M, is a short-season cultivar (50% flowering by approximately 56 DAE vs. an average of 87 DAE for the other cultivars). This cultivar may have minimized the effect of signalgrass or palisadegrass interference by completing grain filling while the weeds were still relatively small. In addition to maintaining yields, CT 11891-2-2-7-M also reduced biomass of signalgrass and palisadegrass as much as the competitive full-season cultivars Oryzica Sabana 6 and Line 3 (Table 1). Thus, partial avoidance of competition, in addition to high light interception and yield potential, may have resulted in CT 11891-2-2-7-M being the most competitive rice cultivar in 1995.

Competitive ability can be conceptualized as the ability of a cultivar to suppress weed growth or to maintain crop yields by tolerating weed competition (Jordan, 1993). Researchers have argued in favor of breeding programs that select for suppression or tolerance (Jordan, 1993; Callaway and Forcella, 1992). In our study, the most suppressive cultivars were also the most tolerant, suggesting that in a rice–pasture cropping system, genotypes could be selected for both tolerance and weed suppression. However, it should be noted that complete suppression of Brachiaria spp. by rice would prevent the establishment of a pasture. Traits related to tolerance of Brachiaria spp. are probably more useful in this system than traits related to weed suppression.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Our results suggest that tall and lodge-prone cultivars are not needed to achieve significant levels of competitiveness in rice and that combining high yield potential and competitiveness into rice cultivars is a viable goal. Competition for light was a critical component of rice–Brachiaria spp. interference in this and in an earlier greenhouse study (Fischer et al., 1995). Similar results were found in a field study of irrigated rice competition with junglerice (Fischer et al., 1997). In the field studies, rice LAI and tiller density were positively correlated with PPFD interception, indicating that traits may need to be combined to enhance light capture and competitiveness in rice. Our work supports the idea that enhanced rice competitiveness can provide a safe and low-cost tool for integrated weed management that results in reduced need for herbicide use. Competitive rice cultivars may be particularly useful in rice–pasture intercropping systems where rice grain yield provides additional revenue to support the cost of implanting a pasture for grazing.


   ACKNOWLEDGMENTS
 
This study was supported by the Holdback Project R5969 (H) of the Overseas Development Administration (ODA) of the United Kingdom. The authors acknowledge the useful suggestions to this research provided by Dr. Elcio P. Guimarães and Mr. Marc Châtel.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Contribution of CIAT (ciat{at}cgiar.org).


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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