The yield of durum wheats released in Mexico between 1960 and 1984

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Abstract
Summary: Two trials designed to measure progress in the yield of durum wheat cultivars released in Mexico by the Institute Nacional de Investigaciones Agrícolas over the period 1960–84 were grown in the Yaqui Valley, Sonora, Mexico, during the 1983–4 and 1984–5 cropping seasons. The trials compared grain yield, above-ground biomass, harvest index (ratio of dry grain yield to dry above-ground biomass), yield components, grain-growth rates and phenological characters for eight key cultivars and the modern advanced line, Carcomun ‘S’, when grown at a high level of agronomic inputs and management.The grain yield of durum wheat was estimated to have risen for 25 years of breeding from 3·70 to 8·40 t/ha. The estimated average annual rates of increase in grain yield for the periods 1960–71 and 1971–85 were 251 and 121 kg/ha respectively. Grain yield improvements were based on a linear increase in the number of grains/m2 over the 25-year period, the result of more grains per spikelet. An improved above-ground biomass at maturity was a feature of the two modern genotypes, Altar 84 and Carcomun ‘S’. Harvest index increased with each new cultivar up to the release of Mexicali 75 in 1975, but thereafter the higher grain yields achieved with the modern genotypes were not associated with a higher harvest index. Thousand-grain weight remained steady for the released cultivars but fell slightly for the advanced line Carcomun ‘S’. Improvements in yield were not associated with a longer cropping cycle.It is concluded that a breeding strategy combining selection for morphological characters thought to confer high yield potential, such as a more erect leaf posture and high number of grains per spikelet, with selection for grain yield per se has been successful in improving the grain yield of durum wheats adapted to north-west Mexico. Improvements have come not only in the size of the grain sink and the efficiency of assimilate partition to grain but also in the biomass produced above ground.