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This paper examines spring wheat breeding in Mexico from the point of view of the biological factors associated with recent yield increases and discusses the likelihood of further progress. Studies of old and new varieties, planted together at various levels of soil nitrogen, suggest that the dramatic yield jump of the 1960s in north-west Mexico can be attributed almost equally to increased soil fertility, greater resistance to lodging and, finally, other desirable characteristics closely associated with the Norin 10 dwarfing genes which are universally found in the newer short-statured wheats. Future progress in yield potential is likely to be much slower. International experiments, while demonstrating the broad adaptability of the Mexican wheats, indicate slower progress in the recent past, probably because of the generally less favourable test environments involved. Nevertheless, there have been sharp rises in national productivity as a result of the spread of a few key Norin 10-derived semidwarf genotypes, derive.d directly or indirectly from Mexico. The implications for future progress in breeding as a result of the major restraints on productivity found in such situations, namely water, disease, nitrogen and weeds, are discussed.

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