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CROP BREEDING, GENETICS & CYTOLOGY

Grain Yield of Rice Cultivars and Lines Developed in the Philippines since 1966

^a Agronomy, Plant Physiology and Agroecology Division, International Rice Research Institute (IRRI), P.O. Box 3127, MCPO, 1271 Makati City, Philippines
^b Dep. of Agronomy, Univ. of Nebraska, Lincoln, NE 68583 USA
^c Plant Breeding, Genetics, and Biochemistry Division, IRRI, Lincoln, NE USA

s.peng{at}cgiar.org

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Genetic improvement in grain yield has been intensively studied in wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.]. Such information is limited in rice (Oryza sativa L.). The objective of this study was to determine the trend in the yield of rice cultivars–lines developed since 1966. Twelve cultivars–lines were grown at the International Rice Research Institute (IRRI) farm and the Philippine Rice Research Institute farm during the dry season of 1996. Seven cultivars–lines were grown at IRRI farm in the dry season of 1998. Growth analyses were performed at key growth stages, and yield and yield components were determined at physiological maturity. Regression analysis of yield versus year of release indicated an annual gain in rice yield of 75 to 81 kg ha^-1, equivalent to 1% per year. The highest yields obtained with the most recently released cultivars was 9 to 10 Mg ha^-1, which is equivalent to reported yields of IR8 and other early IRRI cultivars obtained in the late 1960s and early 1970s at these same sites. Therefore, the 1% annual increase in yield may not represent genetic gain in yield potential. The increasing trend in yield of cultivars released before 1980 was mainly due to the improvement in harvest index (HI), while an increase in total biomass was associated with yield trends for cultivars–lines developed after 1980. Results suggest that further increases in rice yield potential will likely occur through increasing biomass production rather than increasing HI.

Abbreviations: CGR, crop growth rate • HI, harvest index • IRRI, International Rice Research Institute • LAI, leaf area index • PhilRice, Philippine Rice Research Institute

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
DURING THE PAST 30 YEARS, rice production in Asia more than doubled as a result of the adoption of modern cultivars, increased investments in irrigation, greater use of fertilizer, and some expansion in cultivated area. The production environment in the future is projected to be very different. Expansion of area is limited, investments in irrigation have virtually ceased, high fertilizer use is causing concern about environmental pollution from fertilizer losses, and rice lands are being lost to non-rice uses in the major rice-growing areas of Asia. Over the next 30 yr, Asia must increase its rice production by at least 60% to meet the needs of population growth (Cassman and Pingali, 1995). To achieve this goal, our best option is to develop rice cultivars with higher yield potential through crop improvement.

IR8, the first high-yielding modern rice cultivar, was released by IRRI in 1966. This event marked the start of the "green revolution" in Asia. IR8 is a semidwarf with profuse tillering, stiff culms, erect leaves, photoperiod insensitivity, high N responsiveness, and high HI compared with traditional cultivars (Chandler, 1969). During the past 30 yr, rice breeding efforts have been directed towards incorporation of disease and insect resistance, earlier maturity, and improving grain quality (Khush, 1990). To date, 44 semidwarf indica cultivars developed by IRRI for the irrigated lowland ecosystem have been released in the Philippines. These cultivars and their derivatives have been widely grown in South and Southeast Asia and account for more than 80% of total rice production in this region. However, the yields of these rice cultivars have not been compared when grown under the same field conditions at the same time. It is important to determine the contribution of plant breeding to yield improvement because it may provide insights into the optimum strategy for attaining further gains (Wych and Stuthman, 1983).

Genetic improvement in grain yield has been intensively studied in wheat, barley, oat, and maize (Austin et al., 1980; Wych and Rasmusson, 1983; Wych and Stuthman, 1983; Tollenaar, 1989; Feil, 1992). Most of these studies reported a positive historical cultivar trend in grain yield. Studies of historical cultivars often show that genetic improvement in yield potential has resulted from increases in HI (Lawes, 1977; Austin et al., 1980; Riggs et al., 1981), which is associated with ideotype characters, e.g., short stature in wheat and the uniculm habit in maize and sunflower, Helianthus annuus L. (Sedgley, 1991). Others reported that the improvement in yield potential has been associated with increases in biomass yield in wheat (Waddington et al., 1986), maize (Tollenaar, 1989), oat (Payne et al., 1986), and soybean (Cregan and Yaklich, 1986). McEwan and Cross (1979), Wych and Rasmusson (1983), and Wych and Stuthman (1983) stated that the improvement in grain yield has been related to both dry matter accumulation and HI in wheat, barley, and oat. Such information is limited in rice. The objective of this study was to determine the trend in the grain yield of rice cultivars–lines developed since 1966 and associated changes in morpho-physiological traits.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Field experiments were conducted at two sites in the Philippines during the dry season of 1996: IRRI farm, Los Baños, Laguna (14°11'N, 121°15'E, 21 m above sea level) and the Philippine Rice Research Institute (PhilRice) farm, Muñoz, Nueva Ecija (15°45'N, 120°56'E, 48 m above sea level). The third field experiment was conducted at IRRI farm in the dry season of 1998. The soil at IRRI was an Andaqueptic Haplaquoll with pH 6.0, 16.2 g organic C kg^-1, 1.50 g total N kg^-1, and 32.9 cmol kg^-1 cation exchange capacity. The soil at PhilRice was a Vertic Tropaquept with pH 6.4, 11.6 g organic C kg^-1, 0.89 g total N kg^-1, and 35.5 cmol kg^-1 cation exchange capacity.

In the dry season of 1996, 10 cultivars and two lines were arranged in a randomized complete block design with four replicates. Information on these 12 entries is provided in Table 1 . The 10 cultivars were chosen because of the large area planted to them during different historical periods. The two breeding lines were included because of their superior performance in the yield trials at IRRI and in the Philippine national cooperative tests. Breeder's seeds of the 12 entries were used. The breeder's seeds were stored at 18°C and multiplied very 4 yr. Fourteen-day-old seedlings were transplanted on 13 January at IRRI and on 9 January at PhilRice. Hill spacing was 0.2 by 0.2 m with four seedlings per hill. Plot size was 5 by 6 m. Phosphorus (30 kg P ha^-1 as single superphosphate), potassium (40 and 80 kg K ha^-1 as KCl at IRRI and PhilRice, respectively), and zinc (5 kg Zn ha^-1 as zinc sulfate heptahydrate) were applied and incorporated in all plots 1 d before transplanting. Plants received a total of 200 kg N ha^-1. Fertilizer N in the form of urea was applied in four splits (60 kg as basal, 40 kg at mid-tillering, 60 kg at panicle initiation, and 40 kg at flowering) to ensure N sufficiency for all entries. The timing of N application was based on the growth stages of the individual entries. Standard cultural management practices were followed. Fields were flooded 4 d after transplanting and a floodwater depth of 5 to 10 cm was maintained until 7 d before physiological maturity of each cultivar–line when fields were drained. Pests were intensively controlled using chemicals to avoid yield loss. Whorl maggots and green and brown leaf hoppers [Nilaparvata lugens (Stal)] were controlled by applying carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranol methylcarbamate) granules at early tillering and panicle initiation stages. Chloropyrifos [O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) ester] was applied 1 wk after flowering to control rice bugs [Leptocorisa oratoria (Thunbeg)]. Benomyl was sprayed at booting and at 2 wk after booting to control sheath blight (Rhizoctonia solani Kühn).

Plants were sampled from a 0.48-m² area (12 hills) at midtillering, panicle initiation, flowering, and physiological maturity in 1996, and only at flowering and physiological maturity in 1998. Three rows were left in the border areas to avoid the border effect. All plant samples were separated into green leaf blades (leaf), culm plus sheath (stem), and panicles, when present. Leaf area was measured with a leaf area meter (LI-3000, LI-COR, Lincoln, NE) and expressed as leaf area index (LAI). Dry matter of each component was determined after oven drying at 70°C to constant weight. Crop growth rate (CGR) was calculated based on aboveground biomass accumulation over a time interval. At physiological maturity, panicles were hand threshed and the filled spikelets were separated from half filled and empty spikelets by submerging them in tap water. The filled spikelets were then oven dried at 70°C to constant weight for determining individual grain weight. Spikelets per panicle, grain-filling percentage (100 x filled spikelet number/total spikelet number), and HI were calculated. Canopy height was measured in the field at physiological maturity. Grain yield was determined from a 5-m² area in each plot and adjusted to a moisture content of 0.14 g H₂O g^-1 fresh weight. Daily weather records were obtained from the weather stations adjacent to the experimental sites.

Combined analysis of variance of two experiments at IRRI and PhilRice in 1996 was conducted according to Petersen (1994). An analysis of variance was completed first for each location. The error variance for each location was examined for variance heterogeneity by calculating the ratio of the large error variance to the small error variance (Petersen 1994). The error variances of the two locations were homogeneous in all parameters. Therefore, the pooled error of the two locations was used to test the significance of the between-locations variation, the variation among entries, and the location x entry interaction. Mean comparisons were made by the LSD (P < 0.05) on the basis of the analysis of variance (SAS Institute, 1985).

	Results

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Daily total radiation was greater at PhilRice than at IRRI (Table 3) . At IRRI, daily total radiation in 1998 was greater than in 1996. Seasonal patterns of total radiation were similar during the three field experiments. Analysis of variance revealed that the effects of location and entry were significant for most parameters measured in 1996. The interactive effects of location x entry were small but statistically significant for a few parameters. However, historical cultivar trends in these parameters were similar in the two locations. Therefore, means of the two locations in 1996 were presented, except in Fig. 1 .

View larger version (20K): [in this window] [in a new window]   Fig. 1 Relationship of grain yield (0.14 g H2O g-1 fresh weight) with harvest index (A) and total dry matter (B) for the old cultivars released before 1980 and for the new cultivars–lines developed after 1980. The twelve cultivars–lines were grown at the International Rice Research Institute (IRRI) farm and the Philippine Rice Research (PhilRice) farm in the dry season of 1996. Each data point is a mean of four replications within each location

View larger version (15K): [in this window] [in a new window]   Fig. 2 Yield (0.14 g H2O g-1 fresh weight) trend of cultivars–lines developed since 1966. (A) Twelve cultivars–lines were grown at the International Rice Research Institute (IRRI) farm and the Philippine Rice Research (PhilRice) farm in the dry season of 1996. Each data point is a mean of the two locations. The relationship between grain yield and the year of release for each location was: y = -101 + 0.056x, r2 = 0.52 (IRRI); y = -176 + 0.093x, r2 = 0.60 (PhilRice). (B) Seven cultivars–lines were grown at IRRI farm in the dry season of 1998. Vertical, capped lines represent standard error

Generally speaking, cultivars released in the first period (1966–1973) had taller stature, lower tillering capacity, higher LAI at flowering, and lower HI than did other groups (Table 4) . Cultivars released in the second period (1974–1983) had shorter stature, higher tillering capacity, lower LAI, lower total dry matter, and higher HI. Cultivars–lines developed in recent years (1985–1995) were intermediate in canopy height, tillering capacity, and LAI. Cultivars in this group were able to maintain relatively high HI although their total dry matter was the greatest among the three groups.

	Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
The annual rate of increase in grain yield based on the 12 cultivars–lines developed since 1966 was about 1%. This result is very similar to previous reports on other crop species. The estimated annual yield gains from plant breeding were 0.8% for oat cultivars released since 1923 in Minnesota, USA (Wych and Stuthman, 1983), 0.9% for malting barley cultivars since 1920 (Wych and Rasmusson, 1983), 0.5 to 0.6% for winter wheat (Johnson et al., 1968; Austin et al., 1980), and 0.5 to 0.9% for soybeans (Luedders, 1977; Boerma, 1979; Wilcox et al., 1979).

The contribution of biomass production and HI to genetic gains in grain yield potential varied among different studies. Our results indicate that the yield improvement of rice cultivars released before 1980 was associated with increases in HI, while the yield improvement after 1980 was due to increases in biomass production. The cultivars–lines developed after 1980 had relatively high HI and further improvement in HI was not achieved. Comparisons between semidwarf and traditional rice cultivars attribute improvement in yield potential to the increase in HI rather than to biomass production (Takeda et al., 1983; Evans et al., 1984). When comparisons were made among the improved semidwarf cultivars, however, high yield was achieved by increasing biomass production (Jiang et al., 1988; Akita, 1989; Amano et al., 1993). Hybrid rices have about 15% higher yield than inbreds mainly because of an increase in biomass production rather than in HI (Song et al., 1990; Yamauchi, 1994). This suggests that further improvement in rice yield potential might come from increased biomass production rather than increased HI.

Cultivars released during the first period (1966–1973) had poor grain filling and/or low grain weight which reduced HI and grain yield. Improvements in grain filling and HI were achieved in the cultivars released during the second period (1974–1983). These improvements were probably associated with the reduction in canopy height, growth duration, and spikelets per panicle compared with the cultivars released in the first period. There was a negative relationship between HI and canopy height for the 12 entries (r² = 0.66). Tsunoda (1962) pointed out that short stature increases HI in rice. Vergara et al. (1966) found a negative relationship between HI and growth duration. Akita (1989) reported that HI decreased from 55 to 35% as growth duration increased from 95 to 135 d. The grain yield of cultivars released in the second period was lower than recently developed material due to lower biomass production. Reduced total growth duration and plant height were probably the causes for the lower biomass production. Yield per day was not significantly different between cultivars–lines developed in the second and third periods. Akita (1989) found a linear increase in total biomass of recent IRRI cultivars as total growth duration increased from 95 to 135 d. Recent studies indicated that plant height of semidwarf rice and wheat may limit canopy photosynthesis and biomass production (Kuroda et al., 1989; Gent, 1995). A taller canopy has better ventilation and therefore higher CO₂ concentration inside the canopy (Kuroda et al., 1989). However, we cannot select for tall plants to increase biomass production if lodging resistance and HI cannot be maintained.

Cultivars–lines developed during the most recent period (1985–1995) were intermediate in canopy height, total growth duration, panicle size, and tillering capacity. High grain yield of these new materials was due to their ability to maintain a moderately high HI even though they had high biomass production. This ability was associated with high CGR during the ripening phase.

To date, 44 semidwarf indica cultivars developed by IRRI for the irrigated lowland ecosystem have been released in the Philippines. The test cultivars for this study were chosen because of the large area planted to them during different historical periods. The differences in morphological traits among the test cultivars–lines developed during the three periods do not necessarily apply to all other cultivars which were not tested in this study. For example, IR42, IR44, and IR48 developed by IRRI during 1974–1983 had medium-tall stature and medium growth duration. However, they were not as successful as IR30, IR36, IR50, and IR60 in terms of areas planted to them.

In this study, the difference in grain filling duration among the cultivars–lines was small despite large difference in total growth duration among them. Therefore, the yield trend of the 12 entries was not associated with changes in grain filling duration. Recently developed materials had high CGR during grain filling period. The 12 entries had different growth durations and their flowering dates differed by as much as 28 d in 1996. They might have experienced different climatic environments during the grain filling period. The average daily total radiation and daily mean temperature during the grain filling period in 1996 were similar for the cultivars–lines developed during the periods of 1966–1973, 1974–1983, and 1985–1995 (23.3, 22.9, and 23.1 MJ m^-2; 27.7, 27.2, and 27.3°C for the cultivars developed in the first, second, and third period, respectively). Therefore, differences in CGR during grain filling period were not associated with the climatic conditions.

For the cultivars released in the first period, low CGR during grain filling was associated with poor grain filling percentage although we cannot determine which one was the cause. In this study, IR8 produced 8.4 Mg ha^-1 at IRRI and 6.1 Mg ha^-1 at PhilRice in 1996, and 7.2 Mg ha^-1 at IRRI in 1998. The low yield of IR8 was due to poor grain filling and low HI. Grain filling was 63% at IRRI and 43% at PhilRice with HI of 39.1% at IRRI and 24.3% at PhilRice in 1996. In 1998, IR8 had the grain filling of 68% and HI of 35.8% at IRRI. Yield of 9 to 10 Mg ha^-1 was often reported for IR8 in the late 1960s and early 1970s at the same sites as this study (Chandler, 1969; De Datta et al., 1968; Yoshida and Parao, 1972). Such yield was achieved under total N input of 120 kg ha^-1 and with the grain filling of 85% to 90% and HI of about 50%. Crop management, such as transplanting spacing and plant density, was similar to that of today. The highest yield obtained with the most recently released cultivars–lines was also 9 to 10 Mg ha^-1. Therefore, the 1% annual increase in yield may not represent genetic gain in yield potential. We cannot exclude the possibility that there have been new biotic and/or abiotic constraints in the intensive irrigated lowland rice ecosystems, and the more recent cultivars are better adapted to these changes. Identification of abiotic and/or biotic factors that caused the reduction in yield potential of the old cultivars is beyond the scope of this study. Future studies should be conducted to determine the causes of poor grain filling and low CGR during the grain filling period of IR8 and other older cultivars in the present environment.

	ACKNOWLEDGMENTS

 
Part of this research was conducted as a collaborative research project between the International Rice Research Institute (IRRI) and the Philippine Rice Research Institute (PhilRice). We are grateful to Dr. Santiago Obien, Director General of PhilRice, for providing excellent facilities for conducting the field experiment.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Contribution of IRRI.

Received for publication March 4, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 

• Akita, S. 1989. Improving yield potential in tropical rice. p. 41–73. In Progress in irrigated rice research. International Rice Research Institute, Los Baños, Philippines.
• Amano T., Zhu Q., Wang Y., Inoue N., Tanaka H. Case studies on high yields of paddy rice in Jiangsu Province, China. I. Characteristics of grain production. Jpn. J. Crop Sci. 1993;62(2):267-274.
• Austin R.B., Bingham J., Blackwell R.D., Evans L.T., Ford M.A., Morgan C.L., Taylor M. Genetic improvements in winter wheat yields since 1900 and associated physiological changes. J. Agric. Sci. (Cambridge) 1980;94:675-689.
• Boerma H.R. Comparison of past and recently developed soybean cultivars in maturity groups VI, VII, and VIII. Crop Sci. 1979;19:611-613.[Abstract/Free Full Text]
• Cassman K.G., Pingali P.L. Intensification of irrigated rice system: learning from the past to meet future challenges. GeoJournal 1995;35:299-305.
• Chandler R.F., Jr. Plant morphology and stand geometry in relation to nitrogen. In: Eastin J.D., et al. , ed. Physiological aspects of crop yield. Madison, WI: ASA, 1969:265-285.
• Cregan P.B., Yaklich R.W. Dry matter and nitrogen accumulation and partitioning in selected soybean genotypes of different derivation. Theor. Appl. Genet. 1986;72:782-786.
• De Datta S.K., Tauro A.C., Balaoing S.N. Effect of plant type and nitrogen level on growth characteristics and grain yield of indica rice in the tropics. Agron. J. 1968;60:643-647.[Abstract/Free Full Text]
• Evans L.T., Visperas R.M., Vergara B.S. Morphological and physiological changes among rice varieties used in the Philippines over the last seventy years. Field Crops Res. 1984;8:105-125.
• Feil B. Breeding progress in small grain cereals - A comparison of old and modern cultivars. Plant Breed. 1992;108:1-11.
• Gent M.P.N. Canopy light interception, gas exchange, and biomass in reduced height isolines of winter wheat. Crop Sci. 1995;35:1636-1642.[Abstract/Free Full Text]
• Jiang C.Z., Hirasawa T., Ishihara K. Physiological and ecological characteristics of high yielding varieties in rice plants. I. Yield and dry matter production. Jpn. J. Crop Sci. 1988;57(1):132-138 (in Japanese, with English abstract).
• Johnson V.A., Shafer S.L., Schmidt J.W. Regression analysis of general adaptation in hard red winter wheat (Triticum aestivum L.). Crop Sci. 1968;8:187-191.
• Khush G.S. Varietal needs for different environments and breeding strategies. In: Muralidharan K., Siddiq E.A., eds. New Frontiers in rice research. Hyderabad, India: Directorate of Rice Research, 1990:68-75.
• Kuroda E., Ookawa T., Ishihara K. Analysis on difference of dry matter production between rice cultivars with different plant height in relation to gas diffusion inside stands. Jpn. J. Crop Sci. 1989;58(3):374-382.
• Lawes D.A. Yield improvement in spring oats. J. Agric. Sci. (Cambridge) 1977;89:751-757.
• Luedders V.D. Genetic improvement in yield of soybeans. Crop Sci. 1977;17:971-972.[Abstract/Free Full Text]
• McEwan, J.M., and R.J. Cross. 1979. Evolutionary changes in New Zealand wheat cultivars. p. 198–203. In S. Ramanujam (ed.) Proc. International Wheat Genetics Symposium, 5th. 23–28 Feb. 1978. Indian Society of Genetics and Plant Breeding, New Delhi, India.
• Payne T.S., Stuthman D.D., McGraw R.L., Bregitzer P.P. Physiological changes associated with three cycles of recurrent selection for grain yield improvement in oats. Crop Sci. 1986;26:734-736.[Abstract/Free Full Text]
• Petersen R.G. Agricultural field experiments: Design and analysis. New York: Marcel Dekker, 1994.
• Riggs T.J., Hanson P.R., Start N.D., Miles D.M., Morgan C.L., Ford M.A. Comparison of spring barley varieties grown in England and Wales between 1880 and 1980. J. Agric. Sci. (Cambridge) 1981;97:599-610.
• SAS Institute. SAS User's guide: Statistics, 5th ed Cary, NC: SAS Inst, 1985.
• Sedgley R.H. An appraisal of the Donald ideotype after 21 years. Field Crops Res. 1991;26:93-112.
• Song X.F., Agata W., Kawamitsu Y. Studies on dry matter and grain production of F₁ hybrid rice in China. I. Characteristic of dry matter production. Jpn. J. Crop Sci. 1990;59:19-28 (in Japanese, with English abstract).
• Takeda T., Oka M., Agata W. Studies on the dry matter and grain production of rice cultivars in the warm area of Japan. I. Comparison of the dry matter production between old and new types of rice cultivars. Jpn. J. Crop Sci. 1983;52:299-306 (in Japanese, with English abstract).
• Tollenaar M. Genetic improvement in grain yield of commercial maize hybrids grown in Ontario from 1959 to 1988. Crop Sci. 1989;29:1365-1371.[Abstract/Free Full Text]
• Tsunoda S. A developmental analysis of yielding ability in varieties of field crops. IV. Quantitative and spatial development of the stem-system. Jap. J. Breed. 1962;12:49-55.
• Vergara B.S., Tanaka A., Lilis R., Puranabhavung S. Relationship between growth duration and grain yield of rice plants. Soil Sci. Plant Nutr. 1966;12:31-39.
• Waddington S.R., Ransom J.K., Osmanzai M., Saunders D.A. Improvement in the yield potential of bread wheat adapted to northwest Mexico. Crop Sci. 1986;26:698-703.[Abstract/Free Full Text]
• Wilcox J.R., Schapaugh W.T., Jr., Bernard R.L., Cooper R.L., Fehr W.R., Niehaus M.H. Genetic improvement of soybeans in the Midwest. Crop Sci. 1979;19:803-805.[Abstract/Free Full Text]
• Wych R.D., Rasmusson D.C. Genetic improvement in malting barley cultivars since 1920. Crop Sci. 1983;23:1037-1040.[Abstract/Free Full Text]
• Wych R.D., Stuthman D.D. Genetic improvement in Minnesota-adapted oat cultivars since 1923. Crop Sci. 1983;23:879-881.[Abstract/Free Full Text]
• Yamauchi M. Physiological bases of higher yield potential in F₁ hybrids. In: Virmani S.S., ed. Hybrid rice technology: New developments and future prospects. Los Baños, Philippines: International Rice Research Institute, 1994:71-80.
• Yoshida S., Parao F.T. Performance of improved rice varieties in the tropics with special reference to tillering capacity. Exp. Agric. 1972;8:203-212.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (30) Citing Articles via Google Scholar Google Scholar Articles by Peng, S. Articles by Khush, G.S. Search for Related Content PubMed Articles by Peng, S. Articles by Khush, G.S. Agricola Articles by Peng, S. Articles by Khush, G.S.