Yield variability is determined by variety (genotype), variability and level of inputs (such as fertilizers, irrigation and pesticides), and variability in pests and diseases and in climatic factors (such as rainfall, frosts, and temperature). Interactions between these factors are important, although difficult to analyze. As national food-grain production has increased, especially through the introduction of improved varieties and the increased use of irrigation and fertilizers, so often has variability in yields from year to year. This leads to the perception of increased risk, which may make new technologies appear less attractive to farmers and hence slow agricultural development. It also increases instability in national and world food supplies, which may act to destabilize domestic prices, national income, and the food consumption of the poor, especially in poor agrarian countries.
The world's total production of wheat and rice increased linearly by some 150 percent from 1961 to 1993, although some variation occurred. The upward trend is attributed to technological development, and the variation is the result of climatic variability and political instability. Annual fluctuations in wheat production are larger than those for rice. This may indicate that the abundant use of water in rice-growing tempers the variations resulting from climatic fluctuations.
The changes in variability have presented challenges for policy-makers but they are not statistically significant. The decade of the 1980s has provided an example of unfortunate chance rather than a forecast that fundamental structural changes in production will cause equal or increasing instability at the global level in future years.
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