Water that sustains life is no more abundant and the changing climate is making it all the more difficult to find water for cultivation of crops. This requires a paradigm shift in the manner we cultivate major crops, especially that of rice. In future, we need to economize the available water and reap better harvests. Water use efficient cultivars of rice are the need of the hour. These varieties have to be cultivated like an irrigated crop such as wheat and maize under aerobic conditions where oxygen is available to the root system. An integrated concept of zero tillage, direct seeding, herbicide tolerance and aerobic cultivation will revolutionize rice production and productivity in addition to protecting valuable resources, environment and biodiversity. All these interlinked components are briefly described in the following account.
For several millennia after human beings domesticated crops, land is usually prepared for planting the seed by turning over the soil by plough or by tractors. This act is called tillage. Tillage is primarily a form of weed control. It disrupts the life cycle of any pre-existing weeds and pests. Tillage is labour intensive. We now realize that the tillage practices injure both agricultural land and the environment. When the land is ploughed as much as a foot deep of soil is overturned, leading to a loss of crop residue from the top soil. This practice damages soil and leaves it exposed to erosion, particularly by wind and water. To overcome these limitations the concept of conservation agriculture has been propounded. Conservation agriculture which involves zero tillage prepares the land for farming without mechanically disturbing the soil. The farmer only slightly punctures the soil to plant the seed. There are several benefits to the land, the farmer and the environment from adopting a zero tillage system. Primarily, leaving the soil undisturbed and leaving high levels of crop residues behind, soil erosion is almost eliminated by zero tillage. The utilization of crop residues in zero tillage farming also increases water infiltration and therefore retention (i.e. less evaporation) by the soil. The farmer also significantly benefits through a reduction in labour.
Zero tillage farming goes hand in hand with the direct seeding of rice. Traditional rice farming involves raising rice seedlings in a nursery and transplanting the seedlings into standing water. This involves lot of labour and water consumption. In recent years, direct seeding of rice (DSR) has received much attention because of its low-input demand. It involves sowing pre-germinated seed into a puddled or wet soil or dry seeding into a prepared seedbed. The development of early-maturing varieties and improved nutrient management techniques along with increased availability of chemical weed control methods will encourage farmers to switch from transplanted rice to DSR cultivation. This shift would substantially reduce crop water requirement, soil organic-matter turnover, nutrient relations, carbon sequestering and greenhouse-gas emissions. Scientists have developed low-cost drum seeders to facilitate direct seeding. Three persons are needed to plant the seed in one acre in two hours where as transplanted rice needs twenty persons. The seed requirement per acre is also low as compared to conventional planting.
Weed control is a serious challenge for the DSR approach. Weeds are one of the main reasons rice is traditionally sprouted in nurseries and transplanted because standing water prevents germination of competing weed species. Development of herbicide tolerant rice varieties will solve this problem. Several eco-friendly and rapidly degraded herbicides are widely used in agriculture. However, their use is limited because the crop plants are also susceptible to the chemicals. Genetic engineering of elite genotypes of rice which can withstand herbicide applications fits into the direct seeded, aerobic and zero tillage cultivation. The foreign gene confers tolerance to the herbicide and makes the crop plants unaffected by the sprays. Herbicide tolerant varieties can also be developed by inducing mutations in the crop. Eighteen years long global experience of cultivating herbicide tolerant soybean, maize, cotton and canola has demonstrated enormous benefits that resulted from efficient weed management, conservation agriculture, reducing costs of farming and protecting biodiversity and the environment. Availability of herbicide tolerant rice varieties will help farmer to follow zero tillage practices, direct seeding and cultivating rice in aerobic conditions as is the practice with other crops. Enormous amount of water is saved if such varieties are cultivated in future by minimal irrigation to keep the soil moist. Other practices such as drip irrigation and use of sprinklers will make the whole process much more efficient, especially if the water being supplied is laced with nutrients (fertigation). This also helps in significantly reducing the fertilizer consumption and preventing green house gas emissions. Experiments have shown that rice can be grown with subsurface drippers and can save 57% of irrigation water without any effects on crop productivity. Although the scenario of irrigating rice crop with sprinklers appears little far-fetched now, the future will force us to resort to such water-saving mechanisms.
Rice is a crop species that has adapted itself to standing water conditions that reduce oxygen availability to the roots. In addition, anaerobic conditions in the soil lead to preponderance of ammonia form of nitrogen. Rice is better equipped to take up ammonium from the water logged soils. Research is needed to investigate if the elite rice varieties would perform equally well under aerobic conditions. Breeding for rice varieties that are nitrogen use efficient in aerobic conditions is an important endeavour for future. Such efforts will be supported by an understanding of the gene expression in root tissues of rice with respect to nitrate and ammonium uptake. Under aerobic conditions of rice cultivation, disease problems such as Blast caused by a fungus may become more prevalent. Development of disease resistant rice varieties is already underway. Similarly, nematode infestation and micronutrient (Iron and Zinc) deficiencies may affect the crop under aerobic conditions. Scientists at National Rice Research Institute, Cuttack and University of Agricultural Sciences, Bangalore have developed varieties suitable for aerobic cultivation. Research directed towards developing efficient agronomic and soil and water management systems is needed to integrate the old and new concepts for better harvest of rice.
Our group at IIRR has been working on genomics of rice grown under aerobic conditions, especially the expression of genes in the root system.
- Phule, AS., Barbadikar, KM., Madhav, MS., Senguttuvel, P., Prasad Babu, MBB, and Kumar PA. 2018. Genes encoding membrane proteins showed stable expression in rice under aerobic condition: novel set of reference genes for expression studies. 3 Biotech 8:383-395.
- Phule, AS., Barbadikar, KM., Madhav, MS., Subrahmanyam, D., Senguttuvel, P., Prasad Babu, MBB, and Kumar PA. 2018. Studies on root anatomy, morphology and physiology of rice grown under aerobic and anaerobic conditions. Physiol. Mol. Biol. Plants. https://doi.org/10.1007/s12298-018-0599-z