**1. Introduction**

Efficient water use in rice cultivation is a prerequisite to sustain food security for the rice-consuming population of the world. In recent years, increasing water scarcity has been a major threat to rice production in Asia, where by 2025, about 15–20 million of irrigated rice is estimated to suffer [1]. If today's food production and environmental trends continue, crises in many parts of the world will arise. Action should now be taken to improve water use in agriculture to address severe water challenges for the next 50 years [2].

Rice is a key staple in the Lao People's Democratic Republic (PDR) and is an important component of food security efforts in the country. Lao PDR has one of the world's highest per capita consumption of rice, with around 179 kg per capita per year recorded in 2007 [3]. Rice production in the country is the primary source of livelihood for 724,000 producers. The rainfed lowland rice system dominates with only 13% of the total area being irrigated, considering the total paddy harvested area of 830,000 ha in 2011 [4]. Availability and access to water have been identified as the major constraints to the improvement of rice-based farming systems. Lao PDR has seen a high incidence of significant floods and droughts, which severely affected agricultural production in the country [5].

Rice is the biggest user of water in agriculture and in fact, one of the biggest users of the world's fresh water resources [1]. Most rice fields are under conventional continuous flooded conditions [6], which leads to high amounts of surface runoff, seepage, and percolation losses that account for 50%–80% of total water input [7]. With decreasing water availability for agriculture and with increasing demand for rice, water input in rice production should be reduced and water productivity must be increased. Many water-saving practices, including alternate wetting and drying (AWD), have been identified and promoted for widescale dissemination in Asia to reduce water input and increase water productivity [8]. AWD has been successfully evaluated and introduced at the farmers' demonstration fields in the drought-prone southern provinces of Lao PDR in the 2011 and 2012 dry seasons, respectively [9]. Comparison between AWD and the farmers' water management practice of continuous flooding resulted to similar yields, but a 19%–25% water input reduction with AWD was observed. These results were consistent with what had been reported in other countries that tried the technology [8]. Seedling age at transplanting is an important factor to consider in attaining the uniform crop stand [10] and for regulating growth and yield [11]. When rice seedlings are transplanted at the right age, optimum tillering and growth are achieved. However, if transplanting is delayed, fewer tillers are produced during the vegetative stage resulting to poor yield [12]. "Delayed transplanted rice" or "rice with old seedling age" is the term usually used when transplanted seedling age is more than 25 d [13]. Delayed transplanted rice is common in rainfed lowland fields or in irrigated areas in Lao PDR [5]. The annual cropping cycle in Vientiane province begins either in May or June, depending on the onset of rains, with the preparation of the nursery seedbed and the sowing of seeds for the nursery. In Lao PDR, seedlings are usually transplanted about 30 d or more after sowing. However, the untimely release of irrigation in both dry and wet seasons in irrigated areas or the late onset of rainfall in rainfed areas in the wet season for land preparation and crop establishment activities result in a delay in the transplanting of seedlings. Delayed transplanting may result in yield reduction [13,14].

In this paper, we hypothesize that increasing seedling density at transplanting and use of varieties with stronger tillering propensity ameliorates the effects of delayed transplanting on crop performance. Moreover, delayed transplanting reduces irrigation requirement and increases irrigation water productivity during the rainy season. To test this hypothesis, a field experiment was conducted to evaluate the interactive effects of seedling age, seedling density, and variety on post-transplanted rice crop development, grain yield, and water productivity.
