Two field experiments were conducted in a farm at the Department of Agronomy, Yezin Agricultural University, Yezin, Nay Pyi Taw, Myanmar (19˚10'N, 96˚07'E) (Figure 1). The first field experiment (dry season) was performed between January and April 2015 and the second field experiment (wet season) was performed between August and November 2015. The experimental site is located in the central dry zone of Myanmar whereas is vulnerable to drought as compared to other parts of the country. The agricultural lands receive low rainfall, intense heat and degraded soil conditions, affecting social and economic situations of the communities living in the region (Figure 2). The objective was to study the effect of combining different organic manures and inorganic fertilizers on

the growth and yield attributes of hybrid rice (Palethwe-1).

This study was performed using a split-plot experimental design and three replicates. Each experimental plot was 3 × 5 m in size. The distance between each replicate and each plot was 2 and 0.5 m, respectively. Inorganic NPK fertilizer (0%, 50%, 75%, and 100%) based on the recommended amounts of 150 kg N ha⁻¹, 70 kg P₂O₅ ha⁻¹, and 120 kg K₂O ha⁻¹ were supplied to the main plots. Organic manures [no organic manure (O₀); O_c, O_p, and O_v, 5 t・ha⁻¹, each] were su- pplied to the subplots within each main plot. Each treatment was performed in the same plot position in the two consecutive field experiments.

The land was prepared by plowing and harrowing, and then divided into three areas to form the replicates. Each of the replicate areas was further divided into four main plot areas for each inorganic fertilizer. Each main plot area was divided again into four subplots. Double bunds were constructed to prevent seepage between the adjacent plots. The full amount of each organic manure was applied when the soil was initially prepared. The inorganic fertilizers, which included urea (containing N) and muriate of potash (containing K₂O) were applied as three equal splits: one third was applied at baseline before the seedlings were transplanted, one third was applied at the active tillering stage, and the remaining third was applied at the panicle initiation stage. The full amount of triple superphosphate (containing P₂O₅) was applied at baseline.

Before performing the field experiments, initial soil samples were collected using a soil sampling tube (5 cm in diameter) from eight locations in the experimental field at depths of 0 to 15 cm. These soil samples were air dried at room temperature, crushed by hand, and passed through a 2-mm mesh sieve. The air dried soil samples were then analyzed to establish their physical and chemical properties.

The hydrated soil pH (1:5, soil:water) in mass ratio was measured with an F- 51 pH meter (Horiba Ltd., Kyoto, Japan) using the 4A1-1:5 soil water suspension method [16] . The available soil N (mg∙kg⁻¹) was extracted using the alkaline permanganate method [17] . Available P (mg∙kg⁻¹) was analyzed by 9C-Olsen’s P-malachite green method [18] using a UV-Vis spectrophotometer PD-303 (Apel Ltd., Saitama, Japan). The available soil K (mg∙kg⁻¹) was measured using the 1 N ammonium acetate extraction method [19] and analyzed using an AA-6200 atomic absorption flame emission spectrophotometer (Shimadzu Corp., Kyoto, Japan). Organic matter (%) was measured using Tyurin’s method [20] . Cation exchange capacity (CEC) was determined using the Leaching method [21] . All soil samples were analyzed at the Department of Agricultural Research (DAR) Yezin, Nay Pyi Taw.

The nutrient contents of O_c, O_p, and O_v were measured using a temperature controlled oven and analyzed gravimetrically [22] . The total N content was analyzed by Kjeldahl’s method [23] and measured using Kjeldahl’s digestion method and a Vapodest 20 s distillation unit (Gerhardt, Apparate GmbH & Co. KG, Ger- many). The total P was determined using the vanado-molybdate phosphoric acid method [24] and a 6305 UV-VIS spectrophotometer (Jenway, Stone, UK). The total K was analyzed by the wet digestion method of Stevenson and de Langen [25] using a novAA 400 atomic absorption spectrophotometer (Analytik Jena AG, Jena, Germany). Total S was determined using the turbidimetric method [26] and a 6305 UV-VIS spectrophotometer (Jenway). Organic carbon was measured by the loss on ignition method [27] using a temperature controlled oven and a muffle furnace. The chemical composition of organic manures (Table 1) was also analyzed at the DAR.

Hybrid rice (Oryza sativa L.) variety Palethwe-1 (Myanmar hybrid rice) was cultivated for all field experiments. Seeds were obtained from the Yezin Agricultural University farm, soaked in water for 24 h and then incubated at 25˚C for 48 h. Once they had sprouted, the seedlings were sown on a prepared seed bed according to the wet bed method described by the IRRI (International Rice Research Institute). Subsequently, the water level was gradually increased to accommodate seedling height. Two 23-day-old seedlings were transplanted to each hill, with hills spaced 20 × 20 cm apart. Throughout the growing season, standard agricultural practices were used to perform irrigation, and insect, disease, and weed control. In both seasons, plants were harvested when the crops reached maturity, approximately 92 d after transplanting (DAT).

Five hills from each plot were used to determine plant height (cm), number of tillers hill⁻¹, and soil-plant analysis development (SPAD) values. These plant growth characteristics were measured at two-week intervals from 10 DAT to 50% flowering. SPAD values were measured using a SPAD-502 chlorophyll meter (Konica Minolta Inc., Osaka, Japan). The uppermost fully expanded leaf was used to measure the SPAD value before the panicle initiation stage and the flag leaf was used thereafter.

Source: Soil and Plant Analysis Laboratory, Soil Science Section, Soil Science, Water Utilization and Agricultural Engineering Division, Department of Agricultural Research (DAR).

At the active tillering stage, panicle initiation, and flowering, two hills from each plot were cut (2 - 3 cm above the ground) and used as destructive samples. These samples were oven-dried at 70˚C for 48 h and weighed immediately. The dry matter accumulation values were expressed as tons per hectare (t・ha⁻¹).

At harvest time, five hills were used to measure growth characteristics in each plot. These were harvested to determine the total dry matter (t・ha⁻¹) and yield components [(number of panicles hill⁻¹, number of spikelets panicle⁻¹, filled grain (%), thousand grain weight (g), and panicle length (cm)]. Grain yield (t・ha⁻¹) was measured using the harvest area (5 m²) of each plot. The harvest index was calculated as the ratio of economic yield (seed weight) to biological yield (total dry matter weight) [28] .

The data were summarized and subjected to an analysis of variance (ANOVA). The mean values of treatments were compared using Tukey’s honestly significant difference (HSD) test at a 5% probability level using Statistix software (ver. 8.0; Analytical Software, Tallahassee, FL, USA).

Table 2 summarizes the physicochemical properties of the soil surface (0 - 15 cm) at the experimental site.

The height of the rice plants was not significantly affected by the different inorganic fertilizer and organic manure treatments throughout either the wet or dry seasons. Rice plants grown using 100% NPK (I₁₀₀) plus organic manures attained

Source: Soil and Plant Analysis Laboratory, Soil Science Section, Soil Science, Water Utilization and Agricultural Engineering Division, Department of Agricultural Research (DAR).

the maximum plant height but did not differ from those grown using 75% NPK (I₇₅) in either season, particularly when these were supplemented with O_p (Data not shown).

The number of tillers hill⁻¹ was not significantly affected by the different inorganic fertilizer treatments in either season (Figure 3). There was no statistically significant difference in the number of tillers generated using I₁₀₀ compared with 50% NPK (I₅₀) inorganic fertilizer. However, a larger number of tillers were present in rice plants grown using I₁₀₀ compared with 0% NPK (I₀) fertilizer. The maximum tiller number was produced using I₇₅O_p in the dry season (19.22 hill⁻¹) and I₁₀₀O_p in the wet season (17.00 hill⁻¹). The O_c and O_v treatments did not produce the maximum number of tillers with any inorganic fertilizer. The O₀ plots were also unable to produce the maximum number of tillers with any inor- ganic fertilizer. The number of tillers hill⁻¹ produced using I₀O₀ was obviously low since no fertilizer was applied. The results from both seasons were similar (Figure 3(a) and Figure 3(b)).

At approximately 24 DAT, before the active tillering stage, SPAD values were not significantly affected by any of the inorganic fertilizer treatments but were slightly affected at later stages of development. SPAD values produced by I₅₀, I₇₅, and I₁₀₀ treatments were similar throughout both seasons (Figure 4). The organic manure treatments produced fluctuations in the SPAD values at all growth stages. The O_p treatments performed particularly well compared with other organic manures, producing higher SPAD values at all developmental stages (Figure 4(a)). The O_c and O_v plots had lower SPAD values but these were higher

than those of the O₀ plots. A similar pattern was observed during the wet season where the O_p treatments also had a bigger effect on SPAD values than the other manures (Figure 4(b)). Without organic manure or inorganic fertilizers, optimum SPAD values could not be maintained and these were below 40 at all developmental stages throughout both seasons in the I₀O₀ treatment plots (Figure 4).

Significant differences in total dry matter accumulation (TDM) were recorded using different inorganic fertilizers throughout both seasons (Table 3). In the dry season, the I₁₀₀ treatment provided the greatest TDM at the active tillering stage (24 DAT), while the plots with no NPK fertilizer (I₀) produced less TDM. After this developmental stage, differences were observed in the TDM using I₅₀ treatments, but not between I₇₅ and I₁₀₀ treatments (Figure 5). During the wet season, significant differences in TDM produced by the different inorganic fertilizers were observed only at harvest time. However, the different organic manures significantly influenced TDM throughout both seasons. The O_p combined with inorganic fertilizer treatments produced higher TDM compared with O_v and O_c treatments until the plants flowered. At harvest time, the highest TDM (20 t・ha⁻¹) was recorded in the I₁₀₀O_p fertilizer treatments and this was similar to values recorded using I₁₀₀O_c (19 t・ha⁻¹) during the dry season (Figure 5). TDM values for the different manure treatments combined with I₇₅ did not differ significantly. Although it contained only half as much NPK, the I₅₀O_p treatment produced a similar TDM (17.51 t・ha⁻¹) to the I₁₀₀O_p treatment (19.54 t・ha⁻¹)

ns = non-significant difference.

(Figure 5). Similar results were observed during the wet season (Figure 6). Combining I₅₀ with O_p (16.18 t・ha⁻¹) and O_v (16.07 t・ha⁻¹) produced TDM similar to that produced by other combinations of organic manures with I₁₀₀ [I₁₀₀O_c (18.36 t・ha⁻¹), I₁₀₀O_p (18.03 t・ha⁻¹), and I₁₀₀O_v (18.47 t・ha⁻¹)]. The lowest TDM was recorded in I₀O₀ plots (5.3 t・ha⁻¹ and 6.1 t・ha⁻¹) in both seasons (Figure 5 and Figure 6).

The harvest index (HI) ranged from 0.42 to 0.56 for both inorganic fertilizer and organic manure treatments and there were significant differences among them. However, similar HI values were measured among I₅₀, I₇₅, and I₁₀₀ treatments throughout both seasons. For the organic manures, the O_p treatment produced the highest HI values. These were 0.54 and 0.51 in the dry and wet seasons, respectively. The lowest HI values were recorded for the I₀O₀ treatment (Table 4 and Table 5).

The number of panicles hill⁻¹ was significantly affected by different inorganic fertilizer and organic manure treatments in both seasons. The I₁₀₀ plots produced the highest mean panicle number (11.47) and this was similar to that produced

In each column, means having a common letter are not significantly different at 5% level. The numbers followed by I show the percentage of NPK applied based on 150 kg N ha⁻¹, 70 kg P₂O₅ ha⁻¹ and 120 kg K₂O ha⁻¹. I = Inorganic fertilizer; O_c = cow manure, O_p = poultry manure and O_v = vermicompost. ns = non-significant difference.

In each column, means having a common letter are not significantly different at 5% level. The numbers followed by I show the percentage of NPK applied based on 150 kg N ha⁻¹, 70 kg P₂O₅ ha⁻¹ and 120 kg K₂O ha⁻¹. I = Inorganic fertilizer; O_c = cow manure, O_p = poultry manure and O_v = vermicompost.

by the I₇₅ plots (11.32). Fewer panicles were produced by I₅₀ treatments compared with I₇₅ and I₁₀₀ treatments but fewer still were produced by I₀ treatments. However, different inorganic fertilizer levels had no significant effect on the length of panicle, except I₀ (control) (Table 4). Similar results were recorded for inorganic fertilizer treatments during the wet season (Table 5). Among the organic manures, the O_p treatment produced the highest mean panicle numbers (10.58 and 8.2) and the highest panicle length (24.46 cm and 23.29 cm) in the dry and wet seasons, respectively, followed by the O_c and O_v treatments.

The use of I₁₀₀ fertilizer significantly increased the number of spikelets per panicle, producing the highest values in both the wet (126.14) and dry (154.39) seasons. Similar numbers of spikelets were produced by I₅₀ and I₇₅ treatments during the wet season (Table 5). The fewest spikelets were produced by I₀ treatments in both the dry (112.63) and wet (95.95) seasons. For the organic manures, the O_p treatment produced the highest number of spikelets in both the dry (147.58) and wet (125.99) seasons, followed by the O_c and O_v treatments. Lower spikelet numbers were produced by O₀ treatments in the dry (126.87) and wet (110.75) seasons due to severe deficiencies in N at the vegetative stage (Table 4 and Table 5).

The different inorganic fertilizer treatments produced significant differences in the percentage of filled grains. The highest filled grain percentage was produced by I₁₀₀ treatments during the dry (91.62%) and wet (81.04%) seasons. Similar values were produced by I₅₀ and I₇₅ treatments, with the I₀ treatment producing the lowest filled grain percentages in both seasons (Table 4 and Table 5). The O_p treatment produced the highest filled grain percentage in both the dry (88.18%) and wet (80.67%) seasons but these values did not differ significantly from those recorded for the O_c and O_v treatments. The O₀ treatment produced the lowest percentages in both season (Table 4 and Table 5).

Thousand grain weight was unaffected by the different inorganic fertilizer treatments in either season. However, the thousand grain weight produced by the I₀ treatment was significantly lower than that produced by I₅₀, I₇₅, and I₁₀₀ treatments. Although the different organic manure treatments did not have a significant effect on thousand grain weight, the O_p treatment produced the highest values in both the dry and wet seasons (Table 4 and Table 5). Conversely, panicle length was not significantly affected by the different inorganic fertilizer treatments. However, the I₀ treatment produced the shortest panicles in both seasons. Although panicles of similar length were produced by different organic manure treatments, the O_p treatment produced slightly longer panicles. Similar trends were observed during the wet season (Table 5).

Significant differences in yield were produced by the different inorganic fertilizers and organic manures in both seasons. The highest yields were produced by the I₁₀₀ treatments (9.56 t・ha⁻¹ in the dry and 8.46 t・ha⁻¹ in the wet season) and these were similar to those produced by the I₇₅ treatments (9.55 t・ha⁻¹ in dry and 8.36 t・ha⁻¹ in the wet season). The I₅₀ treatment plots produced lower yields but these were still higher than in the I₀ plots. For the organic manures, the O_p treatments produced the maximum yields in both the dry (8.77 t・ha⁻¹) and wet (7.78 t・ha⁻¹) seasons, followed by the O_c and O_v treatments. Combining inorganic fertilizers and organic manures demonstrated that the O_p plus I₅₀, I₇₅, and I₁₀₀ treatments produced the highest yields (9.78, 10.17, and 10.27 t・ha⁻¹, respectively). However, there were no significant differences between these values in the dry season (Figure 7(a)). Similar observations were made for the wet season (Figure 7(b)). Organic manures alone (O₀) did not produce high yields in either season.