*3.5. Water Input and Water Productivity*

Rice plants that were planted earlier received more rainfall than those planted later because more rainfall events occurred from July to early September (Table 6 and Figure 1). In 2014, total rainfall received by SA15 was 65 mm, 123 mm, and 363 mm more than SA25, SA35, and SA45, respectively. Seedling age and variety significantly affected seasonal irrigation, and total water (irrigation plus rainfall) inputs in both years, while seedling density and interactions among the treatments were not significant. In 2015, the differences in total rainfall between SA15 and the other seedling age treatments were 96 mm (SA25), 192 mm (SA35), and 279 mm (SA45). In general, more rainfall was received by plants in 2014 than in 2015 in all SA treatments, ranging from 4% to 20%, with the highest percent difference observed in SA35 and the lowest in SA45.

**Table 6.** Irrigation and rainfall water input during 2014 and 2015 under different seedling age by a variety treatments, ARC, Vientiane, Lao PDR 1.


<sup>1</sup> In a column and within the same treatment, means with the same letter (lower case) are not significantly different at *p* < 0.05. In the interaction significance, ns means that the F-value is not significant.

With the abundance of rain in the wet season, total irrigation inputs received by the plants were 30%–45% of the total water input in 2014 and 26%−48% in 2015, with delayed transplanted rice receiving a higher fraction of the water input from irrigation than early transplanted seedlings. The lowest amount of irrigation input was received by SA15, whereas older seedlings (SA35 and SA45) received more irrigation input in both years (Table 6). The total irrigation input was higher by 36% in SA45, 15% in SA35, and 7% in SA25 in 2014 in comparison with SA15. Higher percent differences between irrigation inputs were observed in 2015, wherein against SA15, irrigation input was higher by 82%, 65%, and 44% in SA45, SA35, and SA25, respectively. On a per season basis, the total water input was 1244–1449 mm in 2014 (while it was 1201–1333 mm in 2015). Among SA treatments, SA45 received

the lowest and SA15 received the highest total water input in 2014; in 2015, total water input was similar among SA treatments. V3 received the highest water input among the varieties tested and V1 received the lowest, particularly from irrigation, in both years. However, we did not find any significant difference in total rainfall and total water input between varieties during the field experiment.

Irrigation water productivity (WPI) and total water productivity (WPI+R) were only significantly influenced by V and by SA × V interactions in both years. In 2014, WPI and WPI<sup>+</sup><sup>R</sup> values ranged from 0.78 to 1.01 kg m−<sup>3</sup> and from 0.23 to 0.40 kg m<sup>−</sup>3, respectively; in 2015, the corresponding range were 0.80–1.01 kg m−<sup>3</sup> and 0.32–0.40 kg m−3. Comparing SA at each level V in 2014, the WPI of SA45 was significantly higher than those of SA35 and SA15 in V1 and significantly higher than that of SA15 in V2. However, SA45's WPI under V3 became significantly lower than those of SA35 and SA15 (Table 7). Comparing V at each level of SA, V1 had higher WPI than V2 at all levels of SA; it was also higher than V3 at SA15 and SA35 levels, respectively, in 2014. In 2015, there were no significant differences in WPI among SA treatments in each variety level. However, comparing WPI of the different varieties V at each level of SA, V1 had the lowest and V3 (although not significantly different from V2) had the highest WPI values. In terms of WPI+R, the significantly highest WPI<sup>+</sup><sup>R</sup> value was found in SA45 in all levels of varieties in 2014 (Table 7). The lowest values were observed in SA15 and SA35 under the V1 level and in SA15 and SA25 under the V3 levels; SA15, SA25, and SA35 were similar under the V2 level. Comparing WPI<sup>+</sup><sup>R</sup> of the different varieties V at each level of SA in 2014, V1 had the lowest values, particularly under SA15 and SA35 levels, respectively, while, V2 and V3 were similar (except in SA45). In 2015, there were no significant differences in WPI<sup>+</sup><sup>R</sup> among SA treatments under V1 and V3 levels. Significant differences were only observed under the V2 level, wherein the most delayed transplanted seedlings (SA45) produced higher WPI<sup>+</sup><sup>R</sup> than did the earliest transplanted seedlings (SA15) in 2015 (Table 7). Comparing varieties at each level of SA, lower values of WPI<sup>+</sup><sup>R</sup> were observed with V1 under the SA25 and SA35 levels, whereas no significant differences in WPI<sup>+</sup><sup>R</sup> were found among varieties under SA15 and SA45.


**Table 7.** Seedling age × variety (SA × V) interaction on irrigation water productivity (WPI) and total water productivity (WPI+R) during 2014 and 2015 wet seasons, respectively 1.

<sup>1</sup> In a column and within the same year, means with the same lowercase letter are not significantly different (comparison of SA at each level of V); in a row and within the same year, means with the same uppercase letter are not significantly different (comparison of V at each level of SA).

## **4. Discussion**

In our experiment, seedling age significantly affected the tillering dynamics of the rice plants. Younger seedlings (SA15) had the highest tiller count, while older seedlings (SA45) had the least in both years, regardless of planting density and variety used. This indicates that rice seedling age is vital in determining tiller occurrence and confirmed the findings of [18] that, with increasing seedling age, tillering was depressed, thereby resulting in reduced tiller number. Increasing seedling

density ameliorated the effects of delayed transplanting on tillering performance in our experiment. Transplanting with three (SD3) to five (SD5) seedlings per hill produced more tillers per hill than with one seedling per hill (SD1), and therefore the tiller number of older seedlings could be induced by increasing seedling density at transplanting. The result is in line with the findings of [13,19], but the effect may vary with seasons [20].

Seedling age also affected crop growth duration. In the present study, the phenological development of the rice plants was delayed with the transplanting of older seedlings, resulting in longer crop duration. The panicle initiation and physiological maturity were delayed by 5–11 d and 5–7 d, respectively, for every 10-d delay of transplanting in both years. Similar findings were reported by [14]: An increase of 6–9 d of vegetative phase (sowing to panicle initiation) with 10-d-old seedlings vs. 30-d-old seedlings in Central Luzon Philippines during two dry seasons, and by 8 d during one wet season of field experimentation. With increased total crop duration (sowing to physiological maturity) using older seedlings, the expected shortening of the stay of the crop in the main field did not correspond to the number of days of delay in transplanting. In our study, crop duration in the main field was only shortened by 3–5 d for every 10-d delay in transplanting. Seedling density did not show any effect on the total rice growth duration, regardless of the variety used. Seedling age could be an important factor in determining phenological stages and crop duration. Indisputably, varietal characteristics dictated the tiller number, plant height, and growth duration.

Many research studies have shown that the use of younger seedlings (not older than 25 d old) produced positive impacts on grain yield [21–23], although many authors have also contradicted this [24–26]. In our study, a significant SA × V interaction on grain yield was found in both years. This means that the influence of seedling age on grain yield depends on the variety used. The use of varieties with a stronger tillering propensity ameliorated the effects of delayed transplanting on crop yield.

Availability of water is crucial in deciding whether to transplant rice seedlings at an early or later stage of the season. When the water supply is uncertain, delay in transplanting becomes inevitable. Rainfall is abundant during the wet season in Lao PDR. However, much of these rains occur from May to September, when crops are in their vegetative growth stage. In our study site, the rains were not evenly distributed during the rainy months and the number of dry spell periods interrupted the regular monsoon rainfalls, which occurred particularly during critical crop growth stages. Therefore, supplemental irrigation is vital to avoid water stress during periods with no rains. In our study, we hypothesized that the shortening of stay in the main field had implications on total water input of the rice crop. However, our results indicated that the shortened stay of older seedlings in the main field did not translate in irrigation reduction. Higher rainfall occurred from July to August in 2014, and from July to September in 2015, which favored early transplanting. Although with relatively shorter stay in the main field, older seedlings were harvested later than younger seedlings; an additional one or two supplemental irrigations were applied (data not shown) before terminal irrigation. Compared with older seedlings (SA45), younger seedling age treatment (SA15) received about 56% more rainfall and about 26% less irrigation in 2014 and about 45% more rainfall and 45% less irrigation in 2015. At the end of the season, total water input (rainfall plus irrigation) was similar among the SA treatments, considering that older seedlings received less rainfall but more irrigation, while younger seedlings received more rainfall but less irrigation. A similar study conducted in the Philippines during the dry season (with comparable seasonal rainfall in Lao PDR) has indicated that the total irrigation input decreased with older seedlings but not in the wet season when rain could not be easily controlled in the field [14]. A significant difference in irrigation input was also found among the varieties used in the experiment. As expected, irrigation received by the shorter duration variety was lower than that of the longer duration variety (i.e., V1 > V2 > V3) in both years because of one additional irrigation needed for V3 before terminal drainage was implemented (data not shown). In terms of water productivity, a significant SA × V interaction was found in both years.

There is a common understanding among farmers that planting old seedlings (especially with TDK 8) could result in yield losses. The experiment results further confirmed their suspicion. The inference from the results can be given only to the selected varieties used in the experiment. However, as the seeding age at transplantation is more important for better crop productivity, breeders should take note on testing this effect more systematically before releasing the varieties for rainfed lowland rice cultivation.

The overall water management practice in the experimental field was safe AWD and although rainfall was abundant, the field experiments attained an average of five to six wetting and drying cycles in the main field. Safe AWD was developed to reduce irrigation water input without sacrificing rice grain yields particularly in water-short areas and seasons. In our experiment, we have demonstrated that safe AWD is possible during the wet season, although the wetting and drying cycles were fewer compared to what we expected during the dry season with better control of irrigation.
