Growth and yield responses of soybean in Madhya Pradesh, India to climate variability and change

Abstract

This study is aimed at assessing the impact of thermal and moisture stresses associated with observed intraseasonal and interannual variability in key climatic elements on the nature and extent of losses in growth and yield of soybean crop in central India through the use of CROPGRO model. The crops are found to be more sensitive to higher cumulative heat units during cropping season. The yields respond substantially to temporal variations in rainfall (associated with observed swings in the continuity of monsoon). Prolonged dry spells at critical life stages of the soybean crop are found to adversely affect crop development and growth and hence the yields at selected sites. We have also examined the plausible effects of future climate change on soybean yields in the selected region based on simulations carried out for doubled atmospheric CO₂ level and with modified weather variables using the available seasonal projections for the future. Our findings on the response of elevated CO₂ concentrations in the atmosphere suggest higher yields (50% increase) for soybean crop for a doubling of CO₂. However, a 3°C rise in surface air temperature almost cancels out the positive effects of elevated CO₂ on the yield. Soybean crops at selected site are more vulnerable to increases in maximum temperature than in minimum temperature. The combined effect of doubled CO₂ and anticipated thermal stress (likely by middle of the next century) on soybean crop is about 36% increase in yield at the selected sites. A decline in daily rainfall amount by 10% restricts this yield gain to about 32%. Deficient rainfall with uneven distribution during the monsoon season could be a critical factor for the soybean productivity even under the positive effects of elevated CO₂ in the future.

Introduction

Indian agriculture has made great strides over the last 50 years. The foodgrains production in India has increased from 54.92 million tonnes in the 1949–50 to 198.96 million tonnes in 1996–97 with the per capita availability improving from a low 395 g in 1951 to about 500 g per day despite the population increase from 361 millions to 950 millions today. However, in recent years, the pace of the green revolution seems to have started slowing down due to immense pressure on India's land resources and indiscriminate addition of restorer inputs such as inorganic fertilizers, pesticides etc. and their inefficient use. To achieve long-term sustainability in food production in the coming decade, higher growth rates in crop yields must be attained from diminishing per capita arable land and scarce irrigation water resources.

The strong links between vagaries of the Indian summer monsoon and agricultural productivity are well known. Even with minor deviations from the `normal' weather, the efficiency of externally applied inputs and food production is seriously impaired. Moisture stress due to prolonged dry spells or thermal stress due to heat wave conditions significantly affect the agricultural productivity when they occur in critical life stages of the crop. Lack of our understanding on the links between the climate variability and crop productivity together with global warming and its likely impact could seriously endanger sustained agricultural production in the coming decades.

Recent studies on plausible changes in climate explored by global climate models (GCMs) suggest that, in addition to the thermal stress due to global warming, stress on water availability in tropical Asia is likely to be exacerbated in the future (IPCC, 1996a). Studies also suggest a marked reduction in crop yields in the arid and subhumid tropical regions (IPCC, 1996b). While these are long-term assessments focussing on average effects over space and time, at regional and local scales the effects of climate change could be more adversely felt particularly in developing countries like India which shares only 2% of the world's geographical area but supports around 18% of the world's population and over 15% of world's livestock. In India, about 65% of gross cropped area corresponds to the summer monsoon season (about 70% of the total annual rainfall in India occurs during June–September) indicating its heavy dependence on the monsoon rainfall. In order to ensure a balanced growth and development in agriculture (during 1990–1996 the growth rate is 2.37% per annum), a comprehensive understanding and assessment of the likely impact of climate variability/change on our agricultural productivity is warranted.

There have been a few studies both in India and elsewhere aimed at understanding the nature and magnitude of gains and/or losses in yield of particular crops at selected sites under elevated atmospheric CO₂ conditions and associated climatic change (e.g., Abrol et al., 1991; Sinha and Swaminathan, 1991; Aggarwal and Sinha, 1993; Aggarwal and Kalra, 1994; Gangadhar Rao and Sinha, 1994; Mearns et al., 1996; Riha et al., 1996; Lal et al., 1998). These studies have been mainly confined to cereal crops namely wheat and rice. In this study, an attempt has been made to assess the effects of climate variability and change on the productivity of soybean, a leguminous crop in the state of Madhya Pradesh, India using CROPGRO–soybean simulation model (IBSNAT, 1989).

In recent years, soybean [Glycine max (L.) Merrill] has emerged as one of the major rainy season cash crops in central India. The state of Madhya Pradesh has distinguished itself as a `Soya State' on account of its largest share in area (77%) and production (72%) of soybean in India. The growth in area, production and productivity of soybean in Madhya Pradesh has been from 1.21 mha in 1986–1987 to 3.70 mha in 1995–1996, from 0.67 million tonnes in 1986–1987 to 2.90 million tonnes in 1995–1996 and 560 kg ha⁻¹ in 1986–1987 to 784 kg ha⁻¹ in 1995–1996, respectively (SOPA, 1996). This trend of fast adoption of soybean by the farmers is indicative of its potential to emerge as a leading commercial crop in future. Soybean is also ideal for intercropping as well as crop sequences as it is a short duration (85–130 days) crop and is comparatively tolerant to drought (Lawn, 1982) and excessive soil moisture conditions (Wright et al., 1988). Its better ability to fix nitrogen, low phosphorous requirement and tolerance to low pH and high levels of aluminium (Tanaka, 1983) make it a suitable choice for adoption in a wider area. Compared to sorghum and corn, soybean – an edible oil generating legume – has been reported to fetch higher price and net returns (Soni et al., 1990).

The ability of the CROPGRO–soybean model to simulate realistically the observed soybean yields in the region during the past decade has been established here using the daily weather data for four stations viz., Indore, Gwalior, Jabalpur and Raipur located in the state of Madhya Pradesh. The geographical location of these stations is depicted in Fig. 1. The responses of the crop growth and yield in the region to thermal and moisture stresses due to observed intraseasonal and interannual variability in key weather parameters have been examined in this paper. We also report here our findings on the possible impacts of climate change on soybean yields in the selected region based on simulations carried out using doubled atmospheric CO₂ level and modifying the baseline weather variables with the future regional projections as inferred from recent GCM results.