The paper focuses on conservation agriculture (CA), defined as minimal soil disturbance (no-till, NT) and permanent soil cover (mulch) combined with rotations, as a more sustainable cultivation system for the future. Cultivation and tillage play an important role in agriculture. The benefits of tillage in agriculture are explored before introducing conservation tillage (CT), a practice that was borne out of the American dust bowl of the 1930s. The paper then describes the benefits of CA, a suggested improvement on CT, where NT, mulch and rotations significantly improve soil properties and other biotic factors. The paper concludes that CA is a more sustainable and environmentally friendly management system for cultivating crops. Case studies from the rice-wheat areas of the Indo-Gangetic Plains of South Asia and the irrigated maize-wheat systems of Northwest Mexico are used to describe how CA practices have been used in these two environments to raise production sustainably and profitably. Benefits in terms of greenhouse gas emissions and their effect on global warming are also discussed. The paper concludes that agriculture in the next decade will have to sustainably produce more food from less land through more efficient use of natural resources and with minimal impact on the environment in order to meet growing population demands. Promoting and adopting CA management systems can help meet this goal.

Germplasm from the spring wheat (Triticum aestivura L.) breeding program at the International Center for Improvement of Maize and Wheat (CIMMYT) has had a major impact on the yield of irrigated spring wheats in most developing countries in the past 30 yr. The rate and nature of yield potential progress in this germplasm was measured comparing eight outstanding short cultivars released in northwest Mexico between 1962 and 1988. They were grown under irrigation and optimal management, including disease and lodging protection, in each of six winter growing seasons (1989-1990 to 1994-1995) at the CIANO (Centro de Investigaciones Agricolas del Noroeste) experiment station in Sonora, Mexico. There were highly significant effects of cultivar on grain yield, and, although cultivar × year interaction was significant, there were few significant crossover interactions between pairs of genotypes and years in the grain yield data set. Yield averaged across the 6 yr increased linearly from 6680 kg ha-~ for the earliest cultivar, Pitic 62, to 8475 kg ha-~ for Bacanora 88, the latest. The rate of progress against year of release was 67 kg ha-~ yr-1 (r = 0.99, P < 0.001), or 0.88% per year. Grain yield progress was correlated with kernel number per square meter (r = 0.84, P < 0.01) and harvest index (r = 0.81, P < 0.02), but not with total biomass production, kernel weight, days to anthesis, spikes per square meter, or kernels per spike. Thus linear progress in yield within short germplasm has continued at least until the late 1980s, and the yield components studied did not indicate any clear direction for future progress, apart from that suggested by the strong relationships between grain yield and harvest index and grain yield and kernels per square meter, as has been seen in most studies of yield progress in cereals.

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Theory (Craig and Gordon 1965; Dongmann et al. 1974; Sternberg et al. 1986; Farquhar and Lloyd 1993) suggests that the oxygen isotope ratio (d 18 O) of plant material should reflect the evaporative conditions under which the material was formed, so that differences in stomatal conductance should show up in plant d 18 O. To test this theory we measured the oxygen isotope ratio of organic matter from flag leaves at anthesis and grain at harvest from eight cultivars of spring wheat (Triticum aestivum L.) grown under irrigation in each of three seasons in Mexico. The cultivars ranged widely in stomatal conductance and in average grain yield, with which conductance was positively correlated. Supporting theory, the oxygen isotope ratio of flag leaves (d 18 Ol) was found to correlate negatively with stomatal conductance for two of the three seasons. The significant correlations are consistent with high conductance cultivars having lower leaf temperatures and kinetic fractionation factors, and higher vapour pressure fractionation factors and Péclet numbers, all of which combine to result in less enriched d 18 Ol. Yield (grain weight per square meter) was also found to be significantly negatively correlated with d 18Ol in two of the three seasons. d 18 Ol was as good a predictor of yield as stomatal conductance, and significantly better than carbon isotope discrimination. Correlations between grain d 18 O (d 18 Og) and physiological parameters were less clear. Significant negative correlations between d 18 Og and stomatal conductance, leaf temperature and yield were found only during the first season. By measuring the oxygen isotope ratio of cellulose extracted from leaf samples, the difference in fractionation factors (ecp) for cellulose and whole leaf tissue was assessed. ecp was found to be variable, and more negative when d 18 Oc and d 18 Ol were lower. Cultivar means for d 13 C and d 18 O of whole leaf material were found to be significantly positively related, and the factors required to produce such a relationship are discussed.

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