Ecological intensification

The production of rice-based systems is expected to further intensify to meet the increasing global demand for rice, maize, vegetables, fruits, and other agricultural produce. The necessary ecological intensification aims to satisfy the anticipated increase in food demand while meeting acceptable standards of environmental quality (Cassman, 1999). An ecological intensification requires the identification of limiting constraints, a basket of management options from various disciplines, and an integration and participatory on-farm evaluation of the most promising practices at the site of concern. There is a need for relevant and robust performance indicators in the evaluation of interventions including standardized protocols for their measurement to arrive at evidence-based recommendations.

Integration of technologies

The simultaneous improvement in productivity, profitability, and input use efficiency - to name one indicator of environmental performance - will require sound decision support. There is a temptation to promote blanket discipline based recommendations or integrated packages for vast areas but more knowledge-intensive and site-specific solutions will be required as yield gaps between actual and potential yield diminish. Matching the most limiting constraints to system productivity with adequate changes in management practices at the right scale remains a challenge. Great care is required in the development and promotion of integrated packages. Most problematic is the integration of technologies such as the 'System of Rice Intensification' or SRI (Dobermann, 2004; Sheehy et al., 2004). SRI is not a science-based, integrated concept; it is a management package that was developed in adaptation to local needs and specific soil problems in Madagascar. The extrapolation of such locally adapted approaches to other locations without rigorous scientific evaluation bears the risk of ignoring local needs and is exposed to erratic validation.

The selection and integration of individual practices as and where required allows fine tuning of management systems at the local level in contrast to a promotion of one integrated water-weedspest- nutrient-crop management practice at the national level. In the latter case, a selection is made for the stakeholder regardless of whether one discipline needs to be considered at a location or not. It is a matter of transparency in the training of trainers versus delivering a blend of practices in a black box. However, we do need a better understanding and documentation of best management practices including expected interactions and synergies. The Irrigated Rice Research Consortium (IRRC, www.irri.org/irrc) promotes the identification, integration, and evaluation of disciplinebased management options at the field level, before the most promising combination of management practices is promoted in suitable domains with similar socio-economic and bio-physical conditions.

Once a robust set of technologies is identified for a particular site, a local brand name could be developed to jointly promote the set of selected management practices with all stakeholders involved. It is important that each component of a local initiative that integrates more than one technology can be traced back to its original roots. Thus, one needs to go back to the original basket of management options when developing an integrated solution for a new location.

Framework for the evaluation of Site-Specific Nutrient Management

The concept of ecological intensification is accompanied by a framework for the evaluation of interventions with clear guidelines on what parameters to measure and how to interpret them. The Southeast Asia Program (SEAP) of IPI and PPI/PPIC has developed such a framework for the evaluation of Site-Specific Nutrient Management (SSNM) in collaboration with the IRRC. Nutrient management strategies are evaluated considering productivity, profitability, sustainability, and environmental risks. The following performance indicators are used in the evaluation of SSNM:

Agronomic efficiency (AE) is the increase in grain yield per unit fertilizer nutrient applied. The AE is mainly used for N and ranges from 18 to 25 kg grain per kg fertilizer N applied with optimal management. At AE < 18 kg/kg, N could be managed inefficiently because of overuse, mismatch of plant N demand and supply, or other constraints to yield. At AE > 25 kg/kg, N supply could probably be too low to reach optimal yields.

Internal efficiency (IE) is the grain yield produced per unit plant nutrient. IE is an indicator of the efficiency with which the plant translates plant nutrients into grain yield. At optimal nutritional balance, about 68 kg grain is produced per kg plant N, 385 kg grain per kg plant P, and 69 kg grain per kg plant K (Witt et al., 1999). This translates into plant (not fertilizer!) nutrient requirements of about 15 kg N, 2.6 kg P, and 15 kg K per 1000 kg grain.

Recovery efficiency (RE) is the increase in plant nutrient per unit fertilizer nutrient applied. About 45-55% of applied fertilizer N is recovered by the plant under optimal management and growing conditions. If other factors are not limiting, RE largely depends on the amount of N applied, the timing of N application, the expected yield response to N application, and climatic factors such as temperature and wind speed.

Nutrient losses are calculated from the plant RE and changes in soil nutrient pools. Nutrient losses through percolation and run-off are usually small because irrigated rice is grown on heavy soils with levees. Nitrogen losses through volatilization can be minimized through best management practices and SSNM. The leaf color chart (IRRI 2006) is a management tool that farmers use to increase the RE of fertilizer N and reduce N losses to the environment.

Yield potential (Ymax) is the maximum theoretical yield determined by climate and variety. Ymax is used as guidance in the selection of a yield goal and to estimate the exploitable yield gap between Ymax and actual yield in farmers' fields. Several crop models are available to estimate Ymax. The average yield potential in Asia is about 8.5 t ha^-1 but varies depending on site and season. The physiological yield barrier for rice in the tropics appears to be near 12 t ha^-1 and higher yields can only be reached in more temperate climates with cooler night temperatures and high solar radiation.

Yield target is the yield attainable by farmers with good crop and nutrient management and average climatic conditions. The selected yield goal should not be more than 75-80% of Ymax to avoid excessive fertilizer inputs and increased risk of crop failure and profit losses.

Gross return over fertilizer cost (GRF) is the revenue (rice yield x rice price) minus the total fertilizer cost. GRF is used to compare fertilizer programs and any new practice attractive to farmers is likely to require an increase in GRF. Recent surveys have shown that only 7% of farmers mentioned that the availability of cash or credit influenced their fertilizer management decisions in the irrigated lowlands. Fertilizer cost is only about 10-20% of GRF. The profitability of rice farming is therefore largely linked to the yield achieved by farmers.

Nutrient balances and yield stability. Preventive, long-term fertilizer P and K strategies are required to avoid nutrientdepletion and ensure high N use efficiencies and yields. Negative P and K balances have to be expected when nutrient inputs from other sources such as straw or farmyard manure are lacking, and fertilizer is not applied because expected yield gains are small. This would lead to yield reduction in the long-term because of mining of soil nutrient supplies, particularly at elevated yield levels where nutrient removal with grain and straw is high. If only a small or no yield response is expected, fertilizer P and K rates need to be calculated based on a nutrient balance approach to meet the long-term P and K need and avoid soil nutrient depletion. Yield trends and changes in soil organic matter and nutrient stocks provide essential information in the assessment of management practices particularly when other crops are included in rice-based systems.

References

• Cassman, K.G. 1999. Ecological intensification of cereal production systems: Yield potential, soil quality, and precision agriculture. In: National Academy of Sciences colloquium; "Plants and Population: Is There Time?", Irvine, CA., December 5-6, 1998. Proc. Natl. Acad. Sci. USA: p 5952-5959.
• Dobermann, A. 2004. A critical assessment of the system of rice intensification (SRI). Agric. Syst. 79:261-281.
• International Rice Research Institute (IRRI). 2006. Site-specific nutrient management. www.irri.org/irrc/ssnm/. Accessed 23 Oct 2006.
• Sheehy, J.E., Peng, S., Dobermann, A., Mitchell, P.P., Ferrer, A.B., Yang, J., Zoue, Y., Zhong X. and J. Huang. 2004. Fantastic yields in the system of rice intensification: fact or fallacy? Field Crops Res. 88:1-8.
• Witt, C., Dobermann, A., Abdulrachman, S., Gines, G.C., Wang, G.H., Nagarajan, R., Satawathananont, S., Son, T.T., Tan, P.S., Tiem, L.V., Simbahan, G.C. and D.C. Olk. 1999. Internal nutrient efficiencies of irrigated lowland rice in tropical and subtropical Asia. Field Crops Res. 63: 113-138.