The rate of crop varietal replacement on farms: measures and empirical results for wheat

The rate o[ crop varietal replacement on [arms is an important measure o[ the impacts o[ plant breeding programmes, the degree o[ varietal diversification over time, and the potential exposure to disease epidemics due to breakdown in disease resistance o[ older varieties. This paper proposes a simple measure, the weighted average age o[ varieties, [or determining the rate o[ varietal replacement. This measure is used to compare the rate o[ wheat varietal replacement across countries/regions and over time in several developing and developed countries. The results indicate an average age o[ varieties o[ about 7 years, a figure close to the expected longevity o[ disease resistance o[ wheat varieties in many environments. However, the average age varies irom less than 4 years in the Yaqui Valley in Mexico to over 10 years in the Punjab o[ Pakistan. Finally, the weighted average age is divided into the lag between varietal release and adoption initiation, and the speed oidiffusion, once adoption is initiated. Differences in the relative sizes oithese lags were noted between countries/regions, implying different policy interventions to increase the rate oi varietal replacement.