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C ereals, dominated by wheat, rice, and maize, provide approximately 50% of human food calories directly and considerably more indirectly via feed grains (Tweeten and Thompson, 2008). Over the last 20 yr, a period chosen to best estimate current rates of progress without infl uence of earlier periods, the linear rates of yield change for the world (Fig. 1) Even if these relative rates could be maintained, various studies suggest they would not prevent real price rises for the three cereals, in the face of projected demand growth to 2050 (Tweeten and Thompson, 2008). Thus there is little doubt that the world needs to continue increasing cereal yields.In this paper we focus on factors determining current rates of yield progress in several key situations (or case studies) and consider Breeding and Cereal Yield Progress ABSTRACTThis paper reviews recent progress in wheat (Triticum aestivum L.), rice (Oryza sativa L.), and maize (Zea mays L.) yields resulting from substantial breeding efforts in mostly favorable environments and examines its physiological basis. Breeding and improved agronomy lift potential yield (PY), namely yield with the best variety and management in the absence of manageable abiotic and biotic stresses, and PY increase is a key component of progress in farm yield (FY), the other component being closure of the PY to FY gap. Changes in PY and FY are reviewed for several key production regions, namely the United Kingdom and the Yaqui Valley of Mexico for wheat, Japan and Central Luzon in the Philippines for rice, and Iowa and briefl y sub-Saharan Africa for maize. The PY growth rates have fallen and are currently generally no more than 1% per annum and usually much less. The trajectory of FY with time often closely parallels PY, but, especially in developing countries, there remain large yield gaps. In at least one instance (maize in Iowa) the gap between PY and FY appears to be closing rapidly. Current genetic progress is linked to increased biomass accumulation, and this will remain the way forward in the future given the limits to increased harvest index (HI). There is evidence that recent progress is related to increased photosynthesis (e.g., greater radiation use effi ciency (RUE) at the canopy level and/or maximum photosynthetic rate P max at saturating irradiance at the leaf level) before and around anthesis. There is no theoretical reason why this trend cannot continue, especially given the vast genetic resources already found within each crop species. However, it will not be easily or cheaply accomplished, so prospects for higher rates of potential yield growth appear to be limited, notwithstanding new molecular tools and claims to the contrary. Closing the yield gap, therefore, becomes more important. Many factors are involved, but breeding can also help farmers achieve this through, for example, improved host plant resistance.

In most maize-growing areas yield reductions due to drought have been observed. Drought at flowering time is, in some cases, the most damaging. In the experiment reported here, trials with F families, derived from a segregating F population, were conducted in the field under well-watered conditions (WW) and two other water-stress regimes affecting flowering (intermediate stress, IS, and severe stress, SS). Several yield components were measured on equal numbers of plants per family: grain yield (GY), ear number (ENO), kernel number (KNO), and 100-kernel weight (HKWT). Correlation analysis of these traits showed that they were not independent of each other. Drought resulted in a 60% decrease of GY under SS conditions. By comparing yield under WW and SS conditions, the families that performed best under WW conditions were found to be proportionately more affected by stress, and the yield reductions due to SS conditions were inversely proportional to the performance under drought. Moreover, no positive correlation was observed between a drought-tolerance index (DTI) and yield under WW conditions. The correlation between GY under WW and SS conditions was 0.31. Therefore, in this experiment, selection for yield improvement under WW conditions only, would not be very effective for yield improvement under drought. Quantitative trait loci (QTLs) were identified for GY, ENO and KNO using composite interval mapping (CIM). No major QTLs, expressing more then 13% of the phenotypic variance, were detected for any of these traits, and there were inconsistencies in their genomic positions across water regimes. The use of CIM allowed the evaluation of QTL-by-environment interactions (Q;E) and could thus identify ''stable'' QTLs Communicated by G. E. Hart

Average maize yields have increased steadily over the years in the USA and yet the variations in harvestable yield have also markedly increased. Much of the increase in yield variability can be attributed to (1) varying environmental stress conditions; (2) improved nitrogen inputs and better weed control; and (3) continuing sensitivity of different maize lines to the variation in input supply, especially rainfall. Drought stress alone can account for a significant percentage of average yield losses. Yet despite variable environments, new commercially available maize hybrids continue to be produced each year with ever-increasing harvestable yield. Since many factors contribute to high plant performance under water deficits, efforts are being made to elucidate the nature of water-stress tolerance in an attempt to improve maize hybrids further. Such factors include better partitioning of biomass to the developing ear resulting in faster spikelet growth and improved reproductive success. An emphasis on faster spikelet growth rate may result in a reduction in the number of spikelets formed on the ear that facilitates overall seed set by reducing water and carbon constraints per spikelet. To understand the molecular mechanisms for drought tolerance in improved maize lines better, a variety of genomic tools are being used. Newer molecular markers and comprehensive gene expression profiling methods provide opportunities to direct the continued breeding of genotypes that provide stable grain yield under widely varied environmental conditions.

tion in an elite lowland tropical maize population 'Tuxpeñ o Crema I' (Johnson et al., 1986(Johnson et al., ) in 1975 Drought is common in tropical environments, and selection for conditions of managed drought stress. This population, drought tolerance is one way of reducing the impacts of water deficit on crop yield. The primary objective of this study was to evaluate later renamed 'Tuxpeñ o Sequia', underwent eight cycles biomass, grain yield, and harvest index of maize (Zea mays L.) popula-G.O. Edmeades, Pioneer Hi-Bred International, Inc., 7431 Kaumualii

It is not known whether selection for improved tolerance to a specific abiotic stress leads to correlated changes in performance under other stresses. Drought and N deficiency are important constraints to production in the tropics. We examined the effect of selection for drought tolerance on performance of tropical maize (Zea mays L.) under a range of N levels. Original and advanced selections of four populations, improved for tolerance to midseason drought for two to eight recurrent selection cycles each, were evaluated in two experiments under severe N stress, one experiment under medium N stress, and two well-fertilized experiments. Nitrogen accumulated in the aboveground biomass at maturity averaged 52, 63,105,151, and 163 kg N ha" 1 in the five experiments, and grain yields of 3.0,2.9,5.2,6.0, and 6.5 Mg ha ' were obtained. Selection for tolerance to midseason drought stress increased grain yields by an average of 86 kg ha ' yr" 1 with nonsignificantly larger gains under severe N stress (100 kg ha" 1 yr" 1). Drought-tolerant selections had increased biomass and N accumulation at maturity, the changes being largest under severe N stress. Additionally, drought-tolerant selection cycles were associated with delayed leaf senescence and an increased or unchanged N harvest index, indicating that leaf N was used more efficiently for grain production. Selection for tolerance to midseason drought stress appears to increase grain yield across a range of N stress levels and may lead to morphological and physiological changes that are of particular advantage under N stress. M AIZE YIELDS in farmers' fields in many tropical countries average from 1 to 2 Mg ha" 1 (CIM-MYT, 1994), in stark contrast to yields ranging from 4 to 12 Mg ha" 1 reported on breeding stations in those same countries (e.g., CIMMYT, 1995, 1996). This indicates that farmers in the tropics are growing maize under conditions that differ from those used by many researchers during crop improvement. Abiotic stresses in farm

A shortened anthesis‐to‐silking interval (ASI) in maize (Zea mays L.) is associated with tolerance of stresses which occur around flowering. It is not known if this reflects differences in tassel and ear initiation dates, or in rates of development and growth. We examined these characteristics in selection cycles 0, 2, 4, 6, and 8 (designated Cx, where the subscript indicates the cycle number) of the lowland tropical maize population ‘Tuxpeño Sequía’, which was selected recurrently for drought tolerance and whose selection cycles differ for ASL Plants were grown at high plant density and under mild and severe drought stresses that occurred at flowering. Silking was delayed significantly, especially in C0 and C2 under severe drought. Stress treatments did not affect days to 50% tassel initiation, ear initiation, or anthesis. The interval between 50% tassel and ear initiation, the rate of spikelet initiation, and crop growth rate near flowering were unaffected by selection. Per cycle changes were −0.21 d in duration of ear spikelet initiation and −16.9 for spikelet number per ear at 50% anthesis. Selection changed relative growth rates at 50% anthesis by 0.005 d−1 cycle−1 for ears, 0.007 d−1 cycle−1 for spikelets and −0.009 d−1 cycle−1 for tassels. These changes were unaffected by stress level. Eight cycles of selection increased the mean ear and spikelet weights at 50% anthesis by 127 and 173%. The ASI increased rapidly when mean spikelet biomass at 50% anthesis was less than 0.8 rag. Results indicate that a decreased ASI arising from selection in this population is due largely to an increased rate of biomass accumulation per spikelet, and development of fewer spikelets per ear.