Yala Season Research Articles

Soil Hydraulic Characterisation under Different Cropping Patterns in Sloping Agricultural Lands in Sri Lanka

*** Abstract The highlands of Sri Lanka are generally considered as agriculturally productive, and yield potentials are high. However, in recent years, most agricultural lands in the hilly areas are being over-exploited, ignoring long-term negative impacts on the soils and the environment. Different forms of cropping can also modify soil conditions which can have positive or negative consequences on the soil quality. Thus, the objectives of this study were to evaluate the effects of different cropping patterns under different terrains in the Welimada Division of the upper watershed of Sri Lanka on soil quality using selected soil hydraulic characteristic indicators, and to identify which among them provides more positive effects on soil hydraulic quality for crop growth and production. The soil hydraulic characterisation conducted in the Yala (dry) and Maha (rainy) seasons in year 2004 involved measurements of infiltration, hydraulic conductivity, moisture retention characteristics, texture, and organic matter in selected zones of the Welimada watershed. Typical transects or hillslopes were selected based on varying cropping pattern and slope, and were divided into three or two field sites along the slope. A more integrated approach of assessing soil hydraulic quality based on soil hydraulic quality index was adopted to compare the hydraulic quality of soils under different cropping patterns. Analysis of the data gathered indicated that the effects of different cropping patterns are more pronounced in the rooted subsoil layer. In general, the potato-fallow-vegetable- fallow cropping rotation was identified as the pattern which provided more positive effects on the soil hydraulic quality in the context of enhancing crop growth and production. The more agriculturally intensive irrigated lowlands, particularly the paddy- potato-vegetable rotation, showed the potential of increasing the income of farmers without adversely affecting soil hydraulic quality.

Genotypic Variation in the Response of Rice (Oryza sativa L.) to Increased Atmospheric Carbon Dioxide and its Physiological Basis

AbstractIncreasing atmospheric carbon dioxide concentration (Ca) is one aspect of global change that will have a significant impact on the productivity of agricultural crops. Crop yields have been shown to increase with increasing Ca. The magnitude of yield response to increased Ca could vary depending on genotypic and environmental factors. The objective of this study was to determine the genotypic variation of yield response and its physiological basis in rice (Oryza sativa L.) through an initial varietal screening using 16 genotypes. The genotypes were grown under two concentrations of Ca, i.e. 370 ± 28 (ambient) and 570 ± 42 (elevated) μmol mol−1, in open top chambers under lowland field conditions at the Rice Research and Development Institute in Sri Lanka (7°50′N, 80°50′E) from May to August 2001 (yala season) and from November 2001 to March 2002 (maha). Ca within chambers was maintained around target concentrations by a computer‐based real‐time data acquisition and control system. There was significant variation between genotypes in the response of yield to elevated Ca, with absolute increases up to 530 g m−2 in yala and 347 g m−2 in maha. In relative terms, percentage yield increases from ambient to elevated Ca ranged from 4 % to 175 % in yala and from 3 % to 64 % in maha. Genotypic variation in yield showed significant positive correlations with light‐saturated net photosynthetic rate of the flag leaf during the grain‐filling stage. This indicated that increased assimilate supply and its genotypic variation contributed to the observed genotypic variation in yield response to elevated Ca. Furthermore, the capacity to develop a larger reproductive sink through increased panicle number per m2 and increased number of grains per panicle contributed to greater yield at elevated Ca. There was a significant genotype × season interaction with genotypes responding differentially to increased Ca in the two seasons. This was mainly due to inter‐seasonal variation in incident radiation during the grain‐filling stage. Our results demonstrate the significant genotypic variation that exists within the rice germplasm, in the response to increased Ca of yield and its correlated physiological parameters. A subset of genotypes from screening trials such as the present study can be used for more in‐depth analysis of the influence of elevated Ca on processes responsible for yield determination in rice and for molecular studies to elucidate the genetic basis of the response to increased Ca. This could pave the way for breeding genotypes which are more productive in a future high CO2 environment, provided that genotypes with greater flexibility in their physiology are selected to counter the genotype × environmental interaction.

Predictability of stream flow and rainfall based on ENSO for water resources management in Sri Lanka

Summary We investigate the viability of using El Nino-Southern Oscillation (ENSO) and sea surface temperature (SST) data to predict seasonal streamflow for one of the major rivers in Sri Lanka, the Kelani, using correlation analysis, contingency tables, and principal component analysis. The agricultural seasons in Sri Lanka are Yala (April-September) and Maha (October-March). The correlation between the Kelani River streamflow during Yala and ENSO indices (r = � 0.41) is significant at 99% level. In addition, the Kelani streamflow during Yala has a correlation with the Central Indian Ocean SST (r = � 0.40) that is also sig- nificant at the 99% level. The first principal component of the Indo-Pacific Ocean SST is reminiscent of the SST associated with the ENSO mode. A prediction scheme based on this mode for the streamflow during Yala has a skill characterized by a correlation of 0.5 in a cross-validated mode. The prediction of streamflow during Maha is best carried out separately for the two halves of the season. During the El Nino phase, the rainfall during Maha is enhanced during the first half of the season (October-December) and diminished in the second half (Jan- uary-February). Rainfall rather than streamflow has a better relationship with ENSO from October to December. During the second half of the Maha season, rainfall declines with both warm and cold ENSO phases and any prediction scheme has to take into account this non-linear relationship. Overall, useful skill for seasonal streamflow predictions has been demonstrated for the Yala season and skill for seasonal rainfall predictions for the first and second half of the Maha season has been elucidated. a 2006 Elsevier B.V. All rights reserved.

Response of Growth and Yield of Rice (Oryza sativa) to Elevated Atmospheric Carbon Dioxide in the Subhumid Zone of Sri Lanka

AbstractIncreasing atmospheric CO2 is recognized as a major aspect of global climate change that would have a significant impact on the productivity of major agricultural crops. Two field experiments were done, with the objective of quantifying the response of a short‐duration rice (Oryza sativa) variety (BG‐300) to elevated atmospheric carbon dioxide, in the low elevation, subhumid zone of Sri Lanka. The experiment contained three treatments. In the elevated CO2 treatment, rice was grown at a CO2 concentration of 570 µmol/mol within open top chambers (OTC s). The ambient CO2 treatment included crops grown within OTC s, but maintained at the ambient CO2 concentration of 370 µmol/mol. The third treatment was a crop grown in the open field under ambient CO2 concentration.Grain yields of rice crops grown under elevated CO2 were 24 % and 39 % greater than the respective ambient treatments in the maha (January – March 2001) and yala (May – August, 2001) seasons. Significant increases in total biomass at harvest (23 % and 39 %, respectively, in maha and yala) were more responsible for the above yield increases than the slight increases in the harvest index (4 % and 2 %). Yields of the ambient and open field treatments did not differ significantly. Among the yield components, the number of panicles per hill was significantly higher in the elevated treatment and showed significant positive correlations with grain yield in both seasons. In addition, grain yield showed significant positive correlations with the percentage of filled grains in maha and the number of grains per panicle in yala. Significant increases in the number of tillers per hill under elevated CO2 were responsible for its greater leaf area index and the greater numbers of panicles per hill. Crops under elevated CO2 accumulated biomass faster than those grown under ambient CO2 during the vegetative and grain‐filling stages. The results of this study demonstrate that elevated CO2 causes significant yield increases in rice, even when it is grown in warm, subhumid tropical climates.

Yield performance under the rice yaya demonstration program in Sri Lanka: a case study

Launched in 1997, the rice yaya (tract) demonstration program aimed to determine how farmers’ rice yields could be further improved. The program involves extension activities to promote integrated crop management and the use of good-quality seeds and to provide credit and training facilities in selected locations across several districts in Sri Lanka. This study evaluated the impact of the rice yaya demonstration program on rice production. A field investigation was undertaken in one of the dry-zone rice-growing districts of southern Sri Lanka, Hambantota. Field work was carried out in the irrigated areas of Hambantota District. Two farmer categories in the tract program, participants and nonparticipants (40 each), were interviewed at the end of the 2001 yala (dry) season (Apr-Aug). Current and past yield data were recorded. Table 1 shows the yield performance in the past seasons of both participants and nonparticipants.

Effects of Irrigation at Different Growth Stages and Source‐Sink Manipulations on Yield and Yield Components of Mung Bean, Vigna radiata (L.) Wilczek, in Dry and Intermediate Zones of Sri Lanka

Irrigation is a management option available to farmers in the subhumid zones of Sri Lanka to increase mung bean yields during the dry Yala season. The objective of this study was to quantify the yield gain in response to irrigation at different stages of the crop and thereby determine the most suitable stage/s of irrigation. Four field experiments were conducted during Yala in 1995 and 1996 at two sites, Maha‐Illuppallama (MI) and Kundasale (KS). Eight irrigation regimes consisting of all possible combinations of irrigation at three growth stages of the crop were defined. The respective growth stages were vegetative (from germination to appearance of first flower), flowering (from appearance of first flower to 75 % pod initiation) and pod‐filling (from 75 % pod initiation to maturity). The treatments which received irrigation during two or more stages had significantly higher yields (793–1396 kg ha−1) than those which received irrigation during only one stage (401–756 kg ha−1) with the lowest yield being shown by the rain‐fed treatment (227–396 kg ha−1). When at least two stages can be irrigated, irrigation during the flowering and pod‐filling stages was most effective. Irrigation during flowering produced the highest yield gain when only one stage could be irrigated. Seed yield showed a strong positive correlation with number of pods m−2. Fifity per cent de‐podding caused yield reductions at both sites, indicating sink limitation. In contrast, 50 % defoliation reduced the yields only at MI where the number of pods m−2 was greater than at KS. Hence, source limitation was present only when the number of pods was higher.

Physiology of yield determination of mung bean ( Vigna radiata (L.) Wilczek) under various irrigation regimes in the dry and intermediate zones of Sri Lanka

Drought is a major factor limiting yield improvement of mung bean ( Vigna radiata (L.) Wilczek) in the sub-humid, dry and intermediate zones of Sri Lanka. Therefore, the objective of this study was to analyze the yield response of mung bean to irrigation at various phenological stages in terms of radiation interception, radiation-use efficiency and harvest index. Four field experiments were carried out at two sites (Maha-Illuppallama and Kundasale) during the short, dry yala season over two years (1995 and 1996). The life cycle of mung bean was divided into three stages: vegetative (from germination to appearance of first flower); flowering (from appearance of first flower to 75% pod initiation); and pod-filling (from 75% pod initiation to maturity). Eight irrigation treatments were defined as all possible combinations of irrigation during the three stages. Maximum potential soil water deficits (PSWD) ranging from 127 to 376 mm developed as a result of keeping different combinations of stages unirrigated. Maximum LAI ( L m) and the fraction of incoming radiation intercepted ( F) increased significantly with the number of stages irrigated. Specifically, treatments which included irrigation during the vegetative stage achieved large L m and F. Radiation-use efficiency (RUE), maximum total biomass ( W m), harvest index (HI) and seed yield ( Y) also showed a significant positive response to the number of stages irrigated. However, all the above parameters were significantly greater in treatments which included irrigation during the pod-filling and flowering stages. The treatment which received irrigation only during the vegetative stage had significantly lower RUE, W m, HI and Y despite having higher L m and F. Therefore, irrigation is critical during pod-filling and flowering stages mainly because of the higher LAI during these periods and, consequently, the greater demand for water. Lack of irrigation during these critical stages resulted in the development of significant PSWD with adverse effects on photosynthesis and consequently decreased RUE. Moreover, water stress during flowering and pod-filling stages significantly reduced pod initiation and pod growth rates and thereby reduced HI. It is concluded that to maximize mung bean yields in the dry season of the sub-humid zones of Sri Lanka, irrigation should extend across all phenological stages, specially the pod-filling stage.

ADAPTATION OF EXTRA-SHORT-DURATION PIGEONPEA IN THE SHORT RAINY SEASON OF A TROPICAL BIMODAL RAINFALL ENVIRONMENT

The adaptation of extra-short-duration (ESD) and short-duration (SD) pigeonpea (Cajanus cajan) genotypes to the short rainy season (late March to late May; yala) was studied in Sri Lanka in 1995 and 1996. Eight ESD and two SD genotypes were sown on two dates in April and May, with and without supplementary irrigation. In addition, pigeonpea ESD genotype ICPL 88039, and SD genotype ICPL 86012, sesame (Sesamum indicum cv. M.I.3), blackgram (Vigna mungo cv. M.I.1) were compared under rainfed zero-tillage conditions after a rice crop at two locations in 1996. Grain yields of the pigeonpea genotypes under rainfed conditions ranged from 0.07 to 0.47 t ha−1 in the early sowings and 0.09 to 0.42 t ha−1 in the late sowings. The irrigated yields reached 1.7 t ha−1 in early sowing and 1.3 t ha−1 in the late sowings. Irrigation differentially influenced the performance of pigeonpea genotypes in both the sowings in 1996. Analysis of pooled data showed highly significant genotype × environment (year–sowing–irrigation combination) interactions. The interaction pattern among genotypes was associated with differences in the length of the reproductive period and the partitioning coefficient of genotypes. ICPL 88039, with the shortest reproductive period and one of the best in partitioning efficiency, recorded maximum mean yield. As a rainfed post-rice crop in farmers’ fields, ICPL 88039 yielded 0.43–1.4 t ha−1 which exceeded yields of SD pigeonpea (0.02–0.88 t ha−1), blackgram (0.03–1.00 t ha−1) and sesame (0–0.09 t ha−1). The results suggest scope for introducing ESD genotypes as a sole crop in the yala season in Sri Lanka since genotypes with short reproductive periods and high partitioning efficiencies are likely to have better adaptation to this season.

An Economic Assessment of On-Site Effects of Soil Erosion in Potato Lands in Nuwara Eliya District of Sri Lanka

Soil erosion is believed to be a serious problem in many parts of Sri Lanka. It reduces soil organic matter, fine clays, water retention capacity of the soils and plant nutrients of the soils. Potato lands in Nuwara Eliya district are highly susceptible to soil erosion due to several factors such as slopy nature, high rainfall and improper land use. This study looks into the economic aspects of on-site soil erosion in potato lands in Nuwara Eliya District. The Universal Soil Loss Equation (USLE) was used to assess the rate of soil loss and the replacement cost approach was used to assess on-site cost due to soil erosion. This approach is based on the costs of repairing damage to productive assets caused by improper on-site land/soil management practices. Estimated Soil loss was 15 ton/ha in Maha season and 9 ton/ha in Yala season. Under the replacement cost approach two different alternatives were considered to assess on-site damages due to potato cultivation. According to the first alternative, on-site d...

Population sampling of planthoppers, leafhoppers and insect predators on broadcast rice, treated with carbofuran, in Eastern Sri Lanka

A survey of planthopper and leafhopper assemblages and insect predators was conducted in the rice fields of Kaluvanchikudi in Batticaloa District and Kohabana and Central Camp in Amparai District during the dry (yala) season in 1989. Nilaparvata lugens (Stal) was the predominant planthopper species in these rice fields; Sogatella furcifera (Horvath) comprised less than 10% of the sample in any field. The leafhopper species collected included Nephotettix nigropictus (Stal), Nephotettix virescens (Distant), Recilia dorsalis (Motschulsky), and Cofana (=Tettigella) spectra (Distant). Nephotettix species were predominant in Kaluvanchikudi and Kohabana, while R. dorsalis was dominant in Central Camp. Insect predators included the mirid, Cyrtorhinus lividipennis Reuter, an anthocorid, Orius tantillus (Motschulsky) and two coccinellids, Harmonia octomaculata (Fabricius) and Micrapsis discolor (Fabricius). Application of furadan granules reduced the planthopper and leafhopper populations drastically, but did not appreciably reduce the insect predator population.

Evaluation of Sesbania speciosa as a green manure for lowland rice in the dry zone, Sri Lanka

Sri Lankan rice farmers rarely practise green manuring and they depend mostly on chemical fertilizers for the nutrient requirement of their crops. With the removal of government subsidy on fertilizers since 1990, they are now faced with the dilemma of meeting the cost of production to sustain previous yield targets. Therefore the present study devotes to evaluate alternative cheaper nutrient sources like green manures for rice. Results of recent work in Sri Lanka have shown that Sesbania speciosa Taub. ex Engl. is a promising green manure for lowland rice in the dry zone. It was evaluated to adopt a suitable green manuring system for rice. Its year-round biomass production, nutrient accumulation during the vegetative growth and nitrogen fixation were monitored. Field trials were conducted to investigate its ability to suppress weed growth when grown during fallow periods and its contribution to soil fertility and effect on rice yield. Highest biomass of around 100 g dry matter per plant, was achieved when S. speciosa was seeded in August and harvested at flowering in November. When this was planted during a fallow period at a spacing of 0.25×0.25 m (16 plants m-2), percentage weed reduction was as much as 40%. Its N2 fixation activity was similar to that of S. sesban (L.) Merr., Nitrogen concentration in S. speciosa increased until 70 days after establishment and then decreased towards flowering. Phosphorus and K accumulation was also effective during this period. Therefore suitable time for soil-incorporation of S. speciosa is around 70 days after establishment. S. speciosa-manured and chemically fertilized rice crops were comparable in terms of grain yield, indicating the effective nutrient supplying potential of this green manure. Straw biomass was significantly high under S. speciosa, because of the high N-allocation to vegetative growth of rice under this. This evaluation shows that S. speciosa not only supplies N, but is also an effective and complete supplier of other nutrients for rice. A scheme to introduce this into rice production system is proposed, where S. speciosa is seeded/broadcast two weeks before crop harvest (around January) in Maha (major) season as well as Yala (minor) season (around June) so that its biomass can be incorporated into soil during land preparation of subsequent seasons.

Four types of relationships between rainfall and paddy production in Sri Lanka

Due to the absence of marked thermal differences in Sri Lanka, the paddy-climate relationship has been confined to rainfall alone in this study. Increase in total paddy production is highly attributed by the increase in paddy lands rather than yields, particularly in the drier parts, called Dry Zone. The effective rainfall period for the two principal cropping seasons in Sri Lanka, Maha and Yala, were determined according to the crop calendar of the Districts. The deviation values from the trend lines of 20 years (1961–1980) observations in sown, harvest and yield were well correlated with the deviations from the mean rainfall. The effects of rainfall on paddy production are well pronounced in the Dry Zone during the Maha season and in the Wet Zone during the Yala season. From the statictical analyses, four types of relationship, were observed: (1) Type I is the positive relationship, (2) Type II is the negative relationship, (3) Type III is the no realtionship and (4) Type IV a complex one which has a positive relationship below certain critical rainfall value and no or negative relationship above the critical rainfall value. The combination of Types I and IV shows the areas which need irrigation facilities with proper water management while the combination of the Types II and III indicates the areas which are prone to frequent floods and waterloggings.

Rainfall and Paddy Production in Sri Lanka

Sri Lanka experienced severe droughts and floods during the recent years which resulted much crop losses in the paddy production. Effective rainfall periods for the major paddy producing seasons, Maha and Yala seasons are determined according to the crop calendar of the Districts. Here, the impact of rainfall on the paddy production is analysed for each Districts of Sri Lanka during the period from 1961 to 1980. Significant correlation coefficients reveal the clear effect of rainfall on the paddy production, especially in harvest acreage in the Wet and Dry zones. The regions in the Wet Zone have significant positive relationship in the Southwest monsoon season and the Dry Zone in the second intermonsoon and Northeast monsoon seasons as their principal cropping seasons. The Maha rain effects on the sown extent and harvest acreage not only in the same Maha season, but also in the subsequent Yala season sometimes. The excess rainfall amount does not increase the paddy acreage and causes floods even in the drier parts of the country, indicating a negative relationship between rainfall and sown or harvest acreage. Therefore, we can recognize four types of relationships between rainfall and paddy production: (1) significant positive relationship, (2) significant negative relationship, (3) no significant relationship and (4) others, for instance, there is a significant positive relationship below certain critical values and no relationship above the values.