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A Just-Pope production function is used to estimate the effect of genetic improvement on the first two moments of wheat yield for the period 1950–86. The model characterizes the green revolution as the culmination of an era in which wheat breeders achieved rapid increases in yield potential accompanied by higher yield variances. To date, the post-green revolution has been an era of slower mean yield growth, but of relatively rapid improvement in yield stability. Overall, the analysis indicates steady progress in producing "better" varieties; successive releases have improved either stability, mean yield, or both.

Date of sowing experiments were established under irrigation in Ludhiana, in the Indian state of Punjab, from 1985 to 1992, using a total of 66 spring wheat ( Triticum aestivum L.) cultivars and 10 sowing dates. To study interactions among cultivars, sowing dates and years and to estimate losses with delayed sowing for grains m −2 and grain yield, an orthogonal subset of four years, three cultivars (PBW 34, PBW 154 and PBW 226) and seven sowing dates was used. To study the relationship between temperature and radiation, and yield and yield components, the same three cultivars were used with all the available years and sowing dates for each. The optimum sowing dates for yield were 5 November for PBW 34 (long-season) and 15 November for cultivar PBW 154 (medium-season) and PBW 226 (short-season). The combined effect of pre-heading radiation and temperature, referred to as the photothermal quotient (PTQ), was a better predictor of grains m −2 than was either variable alone. The sole effect of post-heading temperature was the best preditor of individual kernel weight. Yield and grains m −2 were maximized for all cultivars when exposed to the highest PTQ between 20 days before to 10 days after heading. Hence, the optimum heading date was the same for all three cultivars and, for the years studied, was in mid-February. After the optimum sowing date, there was a 0.8, 0.7 and 0.7% day − yield reduction for PBW 34, PBW 154 and PBW 226, respectively. The optimum heading date is earlier than observed for several other important areas of irrigated spring wheat. This may be explained by the rapid increase in temperature from February onwards at Ludhiana. We discuss the use of PTQ and temperature to determine optimum heading dates and, therefore, sowing dates for different cultivars.