Yield Constraint Analysis of Lowland Organic Rice at Annapurna Farm, South India: Field Measurements and Modelling

The experiment was conducted at Annapurna farm, Tamilnadu, South India with medium straw varieties in which farmer experienced decline in yield. The overall objective of research was to identify causes of low yield and contribute to possible yield solutions in organic lowland rice. Experimental results showed that the response to nitrogen differed depending on cultivar. CP showed response to high application of nitrogen for LAI, total above ground dry matter, nitrogen content and nitrogen uptake at 85 and 120 DAT (Days After Transplanting). However, cultivar ASD did not show response to high application of nitrogen. The application of high nitrogen did not produce a strong effect on the growth parameters for both the cultivars until 60 DAT. APSIM simulated yields with a wide range of C: N ratios of soil and green manure found that the current range of C: N ratios in the farmer's field are not so high that they result in N-limited production through immobilization. The effect of the larger amount of water applied did not result in major effects on growth. Effect of AWD (Alternate wetting and Drying) was found on LAI at 85 DAT whereas farmers practice treatment did not show any effect. The difference was not so considerable for total dry matter production for both the cultivars between two irrigation regimes. APSIM simulation with double application of water did not produce any effect on growth, suggesting that the production was not water limited. The effect of zinc was different for the two cultivars. CP produced higher LAI, dry matter production and nitrogen uptake at later phase of crop when zinc had been applied. The positive effect of zinc application was seen at later phase of the crop for cultivar CP which should have shown at an early phase as per the hypothesis. Since early effects were not noticed and the results were not consistence between the cultivars, it was concluded that zinc had no major effect on crop growth. Amongst the two cultivars CP was found to be producing higher LAI, more dry matter, taking up more nitrogen and having higher yield. Varietal differences in terms of nutrient uptake capacity, resistance to anaerobic soil condition, and root structure might be accountable for the observed difference in growth parameters. The pest attack was comparatively more severe in ASD than in CP which contributed to less growth and dry matter production for ASD. Water and nitrogen limited yields simulated with APSIM were markedly higher than obtained in the experiment. One of the factors not included in APSIM and observed in the field experiment was the effect of pests and diseases on root and leaf growth. Browning of root was visually observed especially at an early stage of the crop may explain the reason for lower observed crop growth than simulated at an early stage despite ample available water and nitrogen. In this study the results did not confirm any of the hypotheses formulated. The possibilities of simulating effect of root browning on crop production specifically during the early stages of growth were not well possible. The hypothesized problem with root structure and root colour especially at initial phase of the crop needs to be investigated further. High soil organic matter content coupled with anaerobic condition leads to formation of toxic substances and root pest and diseases are some of the possible causes of root browning. Cultivar CP can be recommended if the results are consistent over the years and if not other objectives than yield is taken in to account. Further research can be conducted by using classical design mainly focusing on early growth of the crop. Nitrogen and zinc can be included and irrigation can be excluded in further research. It is recommended to use compost and consider early incorporation of fresh green manure in further experiment to observe the effect on root browning.