ReviewWeed management challenges in rice (Oryza sativa L.) for food security in Bhutan: A review
Introduction
Agriculture is the most important sector for the economy of Bhutan, a small country in the Himalaya of South Asia, providing livelihood to more than 60% of the population (Ministry of Agriculture and Forests, 2014). Rice is the most important cereal crop in Bhutan, grown over an area of 19,549 hectares and producing 77,038 tonnes (Department of Agriculture, 2014). It is preferred for human consumption and plays a significant role in Bhutanese culture, traditions, religion and as the key staple, is linked to food security (Ghimiray and Katwal, 2013). The majority of rice is irrigated with transplanting occurring in bunded terraces due to the steep terrain. The rainfed lowland rice crop, which occupies about 30% of the total rice area, is also transplanted. The upland or dryland rice culture is insignificant as farmers traditionally cultivate maize in their drylands.
The area under cultivation has been almost static for the last decade or so due to limited scope of further expansion. At the same time, there is a continuous threat of wetland conversion to other commercial purposes due to urbanization and a growing population. The national average productivity of 3.9 t ha−1 (Department of Agriculture, 2014) is less than the global average production of 5.4 t ha−1 under irrigated conditions (Global Rice Science Partnership, 2013). Thus, there is a great challenge to improve the productivity of rice from the existing area of production to ensure food security for the Bhutanese people in the face of the growing human population and increasing demand for cereal staples, such as rice.
Rice production in Bhutan is constrained by a number of abiotic and biotic factors, depending on the rice growing ecosystems. One of the main biotic factors that impedes rice production is weed competition. In both irrigated and dryland cultures, weeds compete for resources such as nutrients, water, and sunlight (Ampong-Nyarko and De Datta, 1991) which would otherwise be available for the crop. In addition, weeds harbour pests and diseases. Across Asia, the annual loss of rice yield due to weeds is estimated to be 37% in India (DWSR, 2011 cited in Rao et al., 2015), 17–47% in Nepal (Ranjit, 2007 cited in Bhurer et al., 2013), 10–20% in China (Zhang, 2003), 8.4–43% in Vietnam (Khanh et al., 2006), and 10–35% in Malaysia (Karim et al., 2004). On a global scale, 37% of the rice yield is considered to be lost to weeds (Oerke, 2006).
Weeds have been documented as a constraint to production in potato, a major crop in Bhutan (Roder et al., 2009), as well as in minor cereals (Dukpa, 2006). Although there have been no national systematic studies or publications on rice yield losses due to weeds in Bhutan, the problem of weeds impeding production has been acknowledged. Karma and Ghimiray (2006), during a survey to document the constraints of rainfed rice production at lower elevations, reported that 90% of rice farmers experienced weeds as a problem, causing yield losses of up to 50% in extreme cases. Similarly, Nidup and Wangdi (2000) reported that the perennial broadleaf semi-aquatic Potamogeton distinctus A. Benn., which is widely prevalent and invading mid to high altitude rice growing areas in Bhutan, has the potential to reduce rice yields by up to 37%. In a recent survey by Dorji et al. (2013) in three western rice growing districts, up to 19 species of weeds were identified as affecting the transplanted rice crop. Considering these constant threats, the Ministry of Agriculture and Forests in the 11th Five Year Plan (2013–2018) Program document have highlighted weed management as one of the key strategies to increase rice production (Ministry of Agriculture and Forests, 2014). These data indicate that weeds pose a considerable challenge to Bhutanese rice farming and needed productivity increases, and yet there has been virtually no published research conducted on weed ecology and management in rice in Bhutan to date. Furthermore, expertise in weed science in Bhutan is limited and both the general public and government tend to allocate weeds a low priority (Weiss et al., 2004).
The aim of this review is to describe the current weed management practices in rice in Bhutan, and from a targeted review of relevant research and experience from neighbouring Asian countries, identify challenges and prospects to guide future weed control research in rice in Bhutan.
Section snippets
Important rice weeds
Naturalized weed species account for only a relatively small proportion (less than 5%) of the flora of Bhutan, compared with many other countries. In part, this is due to the fact that Bhutan has never been colonized or had acclimatization societies planting exotic species, and has only been opened to the outside world since the mid 1960s (Weiss et al., 2004). Nevertheless, similar to other rice growing countries, important annual and perennial weeds abound in the rice ecosystem in Bhutan (
Physical methods
Physical methods have been the main form of weed management for Bhutanese rice farmers in the past and continue to play a vital role currently. As in other developing Asian nations, manual weeding through hand pulling is the dominant form of weed control. Farmers who are unable to afford and have limited access to herbicides have effectively managed weeds with this labour-intensive method. However, the trend is gradually switching to chemical methods as farm labour shortages become intense and
Potamogeton distinctus management
Among the weed species in Bhutan, the perennial broadleaf P. distinctus, known for its fast growing and deep distribution of rhizomes in the soil, is becoming a major problem of transplanted rice at altitudes of 1200–2500 m in some of the most productive irrigated rice growing areas of the country. In those areas average rice yields are generally higher than other areas due to the availability of suitably improved rice varieties (Table 2). Other Asian countries such as China (Su and Ahrens, 1997
Future research
Despite advances in science and technology, weeds will continue to threaten rice production, and a developing country such as Bhutan is particularly vulnerable. Having reviewed the available literature on rice weeds both in Bhutan and abroad, the following aspects of weed management research need to be considered in the national interest.
Acknowledgments
This paper was first drafted when the first author was on an Endeavour Postgraduate Award Scholarship provided by the Australian Government at the University of New England, Armidale, Australia. We thank the Australian Government for providing financial support, and the Royal Government of Bhutan for administrative support.
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