Experiments were carried out to assess the increase in yield potential of winter wheat in the U.K. due to variety improvement since the early years of this century. The effects of other genetic changes were minimized by applying fungicide to control eyespot and foliar diseases, and by using nets to prevent lodging. The experiments were carried out in 1978 at Cambridge. One, on soil of high fertility in Camp Field, received 104 kg N/ha and the other, on soil of lower fertility in Paternoster Field, received 38 kg N/ha. Twelve genotypes were tested. Eight were varieties which formed a chronological series beginning with Little Joss, introduced in 1908. The remaining genotypes were recently developed selections from the Plant Breeding Institute and a line bred by the French breeders, Benoist.
The average yield of the 12 varieties and lines tested was 3·96 t/ha in Paternoster Field and 6·40 t/ha in Camp Field. In both fields the two highest yielding entries, Hobbit and the advanced breeding line 989/10, outyielded Little Joss by close to 40%. Benoist 10483 was the only entry for which the percentage yield advantage depended on high soil fertility.
The newer, high yielding, varieties were shorter and reached anthesis earlier than the older varieties. They had lower stem weights/m2 than the older varieties but similar maximum leaf area indices and leaf weights/m2. Within each experiment the total dry-matter production of the varieties was similar, the increase in grain yield due to variety improvement being associated mainly with greater harvest index (ratio of grain yield to grain + straw yield).
It is argued that by a continuation of the trend towards reduced stem length, with no change in above-ground biomass, breeders may be able to increase harvest index, from the present value of about 50% to about 60%, achieving a genetic gain in yield of some 25%. As the limit to harvest index is approached, genetic gain in yield will depend on detecting and exploiting genetic variation in biomass production.
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