The present paper focuses on the physiology of yield potential in wheat (Triticum aestivum L.), because breeding progress in yield potential has overtaken farm yield progress. The paper examines developments largely in the last 10 years seeking routes to higher yield potential. Lately this subject has come under pressure from two new imperatives: perceived slowing of genetic progress and ambitious functional genomics. Analysis of trials between 1996 and 2005 at the CIANO research centre in northwest Mexico suggests that yield potential progress in CIMMYT spring wheats has slowed to around 0·50% per year, but has not ceased there nor in winter wheats elsewhere. Meanwhile, in the last 10 years or so, physiological understanding has advanced somewhat. Increased kernel number/m2 remains strongly associated with genetic progress in grain yield, and new research reinforces the importance of spike dry weight (g/m2) at anthesis in its determination. Lengthening the spike growth period through manipulation of sensitivity to photoperiod looks promising, but more attention to kernels per unit of spike weight is also urged. With respect to plant height, an optimum somewhere between 0·7 and 1·0 m is accepted and we are moving away from infatuation with the Norin 10 dwarfing genes as a way of reaching that. What has not been achieved is good lodging resistance in all short spring wheats, nor a complete understanding of its physiological basis. New information is coming to light on the possible role of stored stem reserves at anthesis, for these reserves appear to have increased as yield potential has increased. Part of the benefit may be related to assimilate supply per kernel around anthesis, which new understanding suggests is important for maximum potential kernel weight. Nevertheless, results continue to suggest that despite more kernels/m2, the most recent wheats are still largely sink-limited during grain filling. Growing evidence from spring and winter wheat (and from rice and maize) now points to the importance of increased photosynthetic activity before and around flowering for recent genetic increases in yield potential. This opens up new possibilities for selection in field plots. Finally, attention is given to effects of weather on yield potential and recent advances in techniques for elucidating the physiological basis of genotype by year interactions. From physiological understanding such as described, traits can be suggested as possible selection criteria for yield potential. However, apart from the ACIAR/CIMMYT project looking at stomatal aperture-related traits as well as source and sink traits (Condon et al., in press; Reynolds et al., in press; van Ginkel et al., in press), there appear to have been few attempts to validate physiological (or morphological) selection criteria for wheat yield potential in the last decade, but recent promising results with spectral reflection indices could foreshadow more validation work. This contrasts with efforts to improve the performance of wheat (and maize) under water-limited conditions, and with the new plant type and super rice approaches of IRRI and China, respectively. Such research could be mapped out for wheat yield potential improvement, and could lead to more efficient breeding for yield potential and/or faster progress, but it requires a multidisciplinary team, including, nowadays, molecular biologists. It also needs suitable controlled and field environments and substantial long-term support. All this may no longer be available in the public sector, at least at a single location.

CIMMYT (International Maize and Wheat Improvement Centre) and other research groups within the Consultative Group for International Agricultural Research (CGIAR) have made major contributions to agricultural development, e.g. underpinning the ‘green revolution’, but it is unlikely they will continue making such far-reaching contributions without the ability to collect, analyse and assimilate large amounts of spatially orientated agronomic and climatic data. Increasingly, application of modern tools and technologies are crucial elements in order to support and enhance the effectiveness of international agricultural research. Bread and durum wheats (Triticum aestivum and Triticum durum) occupy an estimated 200 million ha globally, are grown from sea level to over 3500 m asl, and from the equator to latitudes above 60 ° N in Canada, Europe, and Asia. For organizations like CIMMYT, which seek to improve wheat production in the developing world, understanding the geographic context of wheat production is crucial for priority setting, promoting collaboration, and targeting germplasm or management practices to specific environments. Increasingly important is forecasting how the environments, and their associated biotic and abiotic stress patterns, shift with changing climate patterns. There is also a growing need to classify production environments by combining biophysical criteria with socio-economic factors. Geospatial technologies, especially geographic information systems (GIS), are playing a role in each of these areas, and spatial analysis provides unique insights. Use of GIS to characterize wheat production environments is described, drawing from examples at CIMMYT. Since the 1980s, the CIMMYT wheat programme has classified production regions into mega-environments (MEs) based on climatic, edaphic, and biotic constraints. Advances in spatially disaggregated datasets and GIS tools allow MEs to be characterized and mapped in a much more quantitative manner. Parallel advances are improving characterizations of the actual (v. potential) distribution of major crops, including wheat. The combination of improved crop distribution data and key biophysical data at high spatial resolutions also permits exploring scenarios for disease epidemics, as illustrated for the stem rust race Ug99. Availability of spatial data describing future climate conditions may provide insights into potential changes in wheat production environments in the coming decades. There is a pressing need to advance beyond static definitions of environments and incorporate temporal aspects to define locations or regions in terms of probability or frequency of occurrence of different environment types. Increased availability of near real-time daily weather data derived from remote sensing should further improve characterization of environments, as well as permit regional-scale modelling of dynamic processes such as disease progression or crop water status.

Conservation agriculture (CA), defined as minimal soil disturbance (no-till) and permanent soil cover (mulch) combined with rotations, is a more sustainable cultivation system for the future than those presently practised. The present paper first introduces the reasons for tillage in agriculture and discusses how this age-old agricultural practice is responsible for the degradation of natural resources and soils. The paper goes on to introduce conservation tillage (CT), a minimum tillage and surface mulch practice that was developed in response to the severe wind erosion caused by mouldboard tillage of grasslands and referred to as the American dust bowl of the 1930s. CT is then compared with CA, a suggested improvement on CT, where no-till, mulch, and rotations significantly improve soil properties (physical, biological, and chemical) and other biotic factors, enabling more efficient use of natural resources. CA can improve agriculture through improvement in water infiltration and reducing erosion, improving soil surface aggregates, reducing compaction through promotion of biological tillage, increasing surface soil organic matter and carbon content, moderating soil temperatures, and suppressing weeds. CA also helps reduce costs of production, saves time, increases yield through more timely planting, reduces diseases and pests through stimulation of biological diversity, and reduces greenhouse gas emissions. Availability of suitable equipment is a major constraint to successful CA, but advances in design and manufacture of seed drills by local manufacturers are enabling farmers to experiment and accept this technology in many parts of the world. Estimates of farmer adoption of CA are close to 100 million ha in 2005, indicating that farmers are convinced of the benefits of this technology. The paper concludes that agriculture in the next decade will have to produce more food, sustainably, from less land through more efficient use of natural resources and with minimal impact on the environment in order to meet growing population demands. This will be a significant challenge for agricultural scientists, extension personnel, and farmers. Promoting and adopting CA management systems can help meet this complex goal.

Further genetic gains in wheat yield are required to match expected increases in demand. This may require the identification of physiological attributes able to produce such improvement, as well as the genetic bases controlling those traits in order to facilitate their manipulation. In the present paper, a theoretical framework of source and sink limitation to wheat yield is presented and the fine-tuning of crop development as an alternative for increasing yield potential is discussed. Following a top-down approach, most crop physiologists have agreed that the main attribute explaining past genetic gains in yield was harvest index (HI). By virtue of previous success, no further gains may be expected in HI and an alternative must be found. Using a bottom-up approach, the present paper firstly provides evidence on the generalized sink-limited condition of grain growth, determining that for further increases in yield potential, sink strength during grain filling has to be increased. The focus should be on further increasing grain number per m2, through fine-tuning pre-anthesis developmental patterns. The phase of rapid spike growth period (RSGP) is critical for grain number determination and increasing spike growth during pre-anthesis would result in an increased number of grains. This might be achieved by lengthening the duration of the phase (though without altering flowering time), as there is genotypic variation in the proportion of pre-anthesis time elapsed either before or after the onset of the stem elongation phase. Photoperiod sensitivity during RSGP could be then used as a genetic tool to further increase grain number, since slower development results in smoother floret development and more floret primordia achieve the fertile floret stage, able to produce a grain. Far less progress has been achieved on the genetic control of this attribute. None of the well-known major Ppd alleles seems to be consistently responsible for RSGP sensitivity. Alternatives for identifying the genetic factors responsible for this sensitivity (e.g. quantitative trait locus (QTL) identification in mapping populations) are being considered.

In plant physiology and breeding, it is important to understand the causes of genotype×environment interactions (GEIs) of complex traits such as grain yield. It is difficult to study the underlying sequential biological processes of such traits, their components and other intermediate traits, as well as the main environmental factors affecting those processes. The structural equation models (SEMs) used in the present study allow the external and internal factors affecting GEI of various traits and their interrelations to be accounted for. The study included 86 wheat genotypes derived from three different crosses and evaluated over 3 years. Several attributes, as well as grain yield and yield components, were measured during five crop development stages. Environmental data for the five development stages were averaged. The SEM approach facilitated comprehensive understanding of GEI effects among the different traits, and decomposed the total effects of grain yield components and cross-product covariates on grain yield GEI into direct and indirect effects. External climatic variables were related mostly to main final yield components, and only more intermediate endogenous variables, such as spikes/m2, were affected by minimum temperature and radiation in the early stages of plant development.

Cassava (Manihot esculenta Crantz) is one of the two most important food crops in sub-Saharan Africa. This area accounts for most of the root harvest worldwide, followed by Asia and Latin America – the centre of origin for Manihot species. In Africa and Latin America, cassava is mostly used for human consumption, while in Asia and parts of Latin America it is also used commercially for the production of animal feed and starch-based products. Cassava is regarded as a crop adapted to drought-prone environments, where cereals and other crops do not thrive, and it also grows well in poor soil. There are about 100 wild Manihot species, which provide an important genetic endowment for cassava breeding. Professional cassava breeding started in the 20th century and was spurred on by increasing population demands. The main breeding goals are high yield per unit area, particularly in marginal or pest-prone environments. The most notable results from cassava breeding are seen today in sub-Saharan Africa, where it has been transformed from a poor man's crop to an urban food, and in Southeast Asia, where it has changed from a subsistence crop to an industrial cash crop. Long-term research by many international and national partners has led to breeding high-yielding cassava cultivars that increased crop yields up to 40%. Manipulation of genes from wild species has led to new cultivars that resist prevailing diseases and pests, allowing the avoidance of large-scale famine in sub-Saharan Africa. Cassava improvement continues to tap genetic variation through conventional breeding (including the use of wild species) and biotechnology, because many pathogens still take their toll and occasionally epidemics affect farmer fields significantly. However, new sources of variation are needed to genetically enhance the nutritional quality of this important food crop in Africa and other areas in the tropics of the developing world.

The present study was conducted to evaluate the effects of long-term administration of recombinant bovine somatotropin (rbST) on the regulation of body fluids, mammary blood flow (MBF) and other variables relevant to milk synthesis, in crossbred dairy cattle (0·875 of Holstein–Friesian (HF) genes and 0·125 Red Shindi (RS) genes. Ten first lactation, non-pregnant, animals were chosen and divided into the control and experimental groups of five animals each. Four consecutive measurements were carried out in each group beginning on days 45 (pre-treatment), 105, 165 and 225 of lactation. Animals that had completed 60 days of lactation were injected subcutaneously every 14 days with 500 mg of rbST (POSILAC, Monsanto, USA) in the experimental group, while animals in the control group were injected subcutaneously every 14 days with 800 mg of sterile sesame oil, without rbST, as a control. All animals were fed with rice straw treated with 50 g urea/l as the source of roughage in combination with a similar concentrate throughout the experiments. During the treatment periods, the daily dry matter intake (DMI) was numerically greater for rbST-treated animals than for control animals, while the relative values of DMI per kg body weight and water intake showed no differences.

Animals in both groups gained weight throughout the experiment with no significant differences between the groups. Animals receiving rbST for 45 days increased their peak milk yield from 13·4 kg/day per animal during pre-treatment to 15·9 kg/day per animal (18·7% increase) on day 105 of measurement and this peak yield was higher (19·5%) than those of control animals in the same period. Milk yields on days 225 in late lactation of both groups significantly decreased (P<0·05) in comparison with the early and mid-lactating periods. Over the course of the experiment, milk yield of the rbST-treated animals was significantly higher than those of the control animals (P<0·01). The administration of rbST significantly increased MBF (P<0·05) and mammary plasma flow (MPF) (P<0·01). The ratio of MBF to milk yield slightly increased as lactation advanced in both controls and rbST-treated animals. The administration of rbST significantly increased the absolute values of both plasma volume (P<0·01) and blood volume (P<0·05) when compared with the control animals. The control animals showed no significant changes in values of extracellular water (ECW) throughout the course of treatment periods. The rbST-treated animals increased in both the absolute values and the relative (proportion of body weight) values of ECW throughout the experiment (P<0·05). The estimated values of intracellular water (ICW) in both groups showed no significant changes during the course of treatments. There were no significant changes in the water turnover rate (WTO) and the biological half-life of tritiated water in different periods of lactation in both groups. The absolute values of total body water space (TOH) and total body water (TBW) were significantly greater in rbST-treated than control animals (P<0·05). The relative values of both TOH and TBW as a proportion of body weight of control animals decreased, while no alteration was apparent in rbST-treated animals during the course of treatments. These differences were statistically significant (P<0·05). The absolute values of empty body water (EBW) of the control animals showed no significant changes, while animals treated with rbST tended to increase in absolute values of EBW throughout the course of treatments. The effects of these differences were statistically significant (P<0·05). There were no significant changes in relative values of EBW during the course of treatments in both controls and rbST-treated animals. These data demonstrated that the rbST exerts its galactopoietic action in part through increases in TBW, EBW and ECW in association with an increase in MBF, which partitions the distribution of nutrients to the mammary gland for milk synthesis. During this long-term administration of rbST, the stimulant effect for milk yield was less in late lactation despite a higher MBF. The decline in milk yield during rbST treatment without fall in MBF in late lactation must be attributed to a local change within the mammary gland.