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a CSIRO Division of Plant Industry, G.P.O. Box 1600, Canberra, ACT 2601 Australia
b Australian Center for International Agricultural Research, Canberra, A.C.T. Australia
l.evans{at}pi.csiro.au
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Abbreviations: CER, CO2 exchange rate • CIMMYT, Centro Internacional de Mejoramiento de Maiz y Trigo • IRRI, International Rice Research Institute
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONDefinitionMeasurementSignificanceConclusionREFERENCES |
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has been a much used but elusive concept which has proved difficult to define in a rigorous way. Estimates of it, obtained by several methods, have been used to assess progress in plant breeding programs and to analyze the relative contributions of plant breeding and agronomic advances to past increases in crop yields. Interpretations of the physiological basis of increases in yield potential have also been used to assess the scope for further increases in yield and to guide selection. However, the absence of agreement on its definition and measurement can lead to misunderstanding as well as to misleading claims.
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Definition |
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TOPABSTRACTINTRODUCTIONDefinitionMeasurementSignificanceConclusionREFERENCES |
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Plant breeders have long made the distinction between genes conferring resistance to the various stresses, which Frankel (1947) called "observable characters," and those increasing yield potential, the "productivity genes." It is the progressive assembling of these productivity genes that yield potential trials are intended to assess, as against simultaneous progress in assembling genes for quality and for resistance to contemporary biotypes of pests and diseases and to the prevailing environmental stresses such as drought, heat, cold, salinity, and so forth. Thus, yield potential has been defined as the yield of a cultivar when grown in environments to which it is adapted; with nutrients and water non-limiting; and with pests, diseases, weeds, lodging, and other stresses effectively controlled (Evans, 1993). We adopt this definition here, with the following glosses.
It should be apparent that this definition of yield potential builds upon the notion that there are yield genes and stress-resistance genes, and that a yield potential measurement attempts to measure only the effects of the former. This inference may have limitations, which are discussed later.
The term "potential yield" is often used synonymously for yield potential (e.g., Sinclair, 1993). We suggest that the term be kept for the maximum yield that could be reached by a crop in given environments, as determined, for example, by simulation models with plausible physiological and agronomic assumptions. "Yield potential" would then be used mainly for measured comparisons of cultivars, and "potential yield" for comparisons between different crops and different environments, as well as for estimating plausible future limits to crop yields.
Many other yields are referred to in the crop literature. We see reference to experiment station yields, which may not reach potential yields but imply levels of soil fertility, inputs, and technology sometimes superior to those used by farmers. Scaling up and moving off the experiment station, we can have on-farm trial yields, resulting when some factors are controlled experimentally in the general farm context. Finally there are actual farm or district average yields, reflecting farmers' natural resource endowment, their access to technology, and their skill and exposure to real market economics. Record yields and winning yields from grower competitions are another part of the lexicon of yield literature. Assuming that these yields are genuine, they represent potential yield at some very favorable confluence of genotype, management, radiation, and temperature. Unless these weather variables are carefully measured and related to average expected weather, for example by simulation modeling, such yields are difficult to interpret.
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Measurement |
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Figure 1a represents the simplest interaction between cultivar and agronomic management, where A is the level of agronomy when the old cultivar was released and B that at the time of the new cultivar's release. Yield progress between Point 1 and Point 4 is made up of progress in cultivar and in agronomy, and in a positive cultivar x agronomy interaction, as shown, which cannot be separated into breeding and agronomy parts. Many studies measure yield potential progress under modern agronomy (the distance from 2 to 4, usually given as a percentage of the value at 2), effectively including the interaction term in with the cultivar component to determine the overall progress in breeding for genetic yield potential. In real examples showing the interaction between cultivar and nitrogen level in wheat (Triticum aestivum L.) (Fig. 1b) and between hybrid and planting density in maize (Fig. 1c), the interaction is very obvious. Other examples include the adaptation of modern rices to direct seeding and, in many crops, adaptation to the use of herbicides which can not only permit earlier sowing and/or reduced tillage, but can also open up the possibility of more yield-efficient cultivar morphologies.
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Likewise, to the extent that chemical protection against pests and diseases is unlikely to be completely effective, apparent gains in yield potential will be inflated by those in genetic resistance to biotic stresses. This arises not only because more recent cultivars are more likely to have better resistance (often a condition of cultivar release, reinforced by higher pressure from some pests and diseases under modern agronomy), but also because when an historic series of cultivars is being compared, the previously effective resistance genes of the older cultivars may have broken down. The results of Sayre et al. (1998) comparing CIMMYT wheat cultivars released between 1966 and 1988, both with and without fungicide protection, illustrate this point. The apparent progress without fungicide was much greater than the true progress in yield potential revealed with it. The difference highlights the importance of "maintenance breeding," and is a reminder of the likely cost of neglecting it. Indeed, in tropical environments, particularly in the wet season, pest and disease pressures may be so high that it is difficult to protect the older cultivars enough for a valid estimate of the improvement in yield potential to be obtained by direct comparison.
We need also to consider the possibility of cultivar interaction with long-term environmental change, as distinct from changes in agronomic management or pathogen virulence mentioned above. Gradual changes could have occurred in the physical, chemical, or biological status of the soil, arising, for example, from alterations in crop rotation or tillage, or from prolonged fertilizer use or misuse. The pattern of biotic stress is continually evolving. The concentrations of atmospheric gases such as CO2 and of pollutants like ozone are changing steadily, and climate may also have altered significantly. Empirical selection could well have adapted newer cultivars to gradual changes in the environment. Even if significant environmental changes at a location were well understood, it would be no easy task to measure the confounding effect of cultivar interaction with these changes.
Indirect Assessment by Sequential Comparisons
There are many published results of well-managed yield trials at a number of locations over many years comparing new cultivars with the several older standard ones. Such comparisons can be compiled serially to provide widely replicated estimates of the relative yield potential of long series of major cultivars when grown in the environments and with the agronomy to which they were adapted. However, it is important to restrict such comparisons to those trials made before the pest and disease resistances of the standard cultivar (against which the performance of the new cultivars is measured) began to fail, otherwise the progress in yield potential may be substantially overestimated. For example, compilation of the relative yield data for British winter wheats from `Little Joss' (1908) to `Norman' (1981) gave an overall rise in yield potential of 94% (Silvey, 1986). However, when the compilation was restricted to the years before the resistance of the several standard cultivars began to fail, the overall increase was reduced to 50% (Evans, 1981), comparable with the 45% improvement determined by direct comparisons with effective disease control (Austin et al., 1980, 1989). Comparable "variety improvement indices" and "vintage yield functions" from such data sets have been used by agricultural economists (e.g., Godden and Brennan, 1994) to compare progress in various breeding programs. Their models treat year of cultivar release as one of usually several independent variates explaining yield variation.
Direct Comparison of Historical Series of Cultivars
Progress in yield potential has been assessed for many crops in many environments by growing historical series of leading cultivars side by side (Evans, 1993). Plot size and arrangement must be adequate, e.g., to avoid all edge effects including shading by older, taller cultivars (e.g., Clarke et al., 1998). The planting density should be representative, which is not always the case (e.g., Perera et al., 1998). Relative yields may vary to some extent from year to year, as with CIMMYT wheats (Sayre et al., 1998) and U.S. potato (Solanum tuberosum L.) cultivars (Douches et al., 1996), so comparisons over several years are desirable. Those studies which incorporate several levels of agronomy (e.g., Austin et al., 1980; Duvick, 1997; Ortiz-Monasterio et al., 1997) are particularly useful.
Linear regression is usually applied to yield x year of release, the slope giving the annual rate of progress as kilograms per hectare per year or percentage per annum. In fact, these are simple, not compound, rates of increase and should be stated to be so, along with the base denominator used. Compound rates are appropriate for an exponential increase in yield potential, which has sometimes been found (see Voldeng et al., 1997).
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Significance |
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TOPABSTRACTINTRODUCTIONDefinitionMeasurementSignificanceConclusionREFERENCES |
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Relevance to Farm Yields and Less Favorable Environments
It is widely assumed that the cultivars with which progress in yield potential has been measured have been adopted by farmers, and that this progress is a significant component of progress in actual farm yields. The first assumption is undisputed: indeed many of the cultivars used in historical variety trials are those already widely adopted by farmers. The second assumption deserves more attention, because it is often assumed in estimates of the impact of plant breeding that the relative rates of progress found in yield potential trials also apply in farmers fields (e.g., Silvey, 1986; Duvick, 1977; Bell et al., 1995; Godden and Brennan, 1994), although frequently challenged especially in the popular media.
Sayre (see Bell et al., 1995) has compared the relative yields of 24 wheat cultivars grown for 3 yr on an experiment station in north west Mexico with those of the same cultivars over the same years when grown on five or six farms throughout the surrounding Yaqui Valley, irrigated and managed by the farmers. Although on-farm yields were consistently lower (by an average of 18%), no significant cultivar x location interaction was found. Bell et al. (1995) then constructed an index of cultivar improvement relevant for the Valley, since the cultivar composition in farmer's fields is surveyed annually. This increased in yield by 29 kg ha-1 yr-1 over the 1968-1990 post-green revolution study period. It was previously determined by a simulation model that farmers' yields had increased 103 kg ha-1 yr-1 over the same period, this value having been derived after actual yield increase in the Valley was corrected upwards for significant warming in the weather over the period. Thus breeding progress accounted for 28% of the overall farm yield progress, compared with 48% ascribed to the increase in use of N fertilizer from 80 to 230 kg ha-1 in the period. Because N levels were already moderate in 1968, there was no evidence for a cultivar x N interaction over the study period.
Not all examples of breeding and farm yield progress are as clear cut as the above. The case of cotton in USA is interesting in that relative progress in yield potential exceeded that at the farm level during a period of restrictions on pesticide use by farmers (Bridge and Meredith, 1983). The case of potato in USA is the opposite: there has been a six-fold increase in national potato yields since 1920 without any measurable increase in yield potential, but significant increases in earliness and tuber quality (Douches et al., 1996).
For many reasons, one might expect less favorable growing conditions in farmers' fields than, by definition, those under which yield potential progress has been measured. However, the percentage increase in yield in historical variety trials is as great in unfavorable years as in favorable ones (e.g., Duvick, 1997; Sayre et al., 1998). Similarly, relative progress appears to be as great at lower as at optimal levels of nitrogen fertilization for both winter and spring wheat cultivars (Austin et al., 1980; Ortiz-Monasterio et al., 1997) and in temperate maize (Castleberry et al., 1984). Modern cultivars have higher nitrogen-use efficiency associated, in the case of wheat, with both greater uptake efficiency and greater utilization efficiency (Ortiz-Monasterio et al., 1997).
However, it is the question of relative performance when water availability is less than potential evapotranspiration that generates most controversy. Such comparisons require that the degree of water shortage be specified, and are also more complicated because of subtle interactions between maturity, the timing of the water shortage, and soil type. Notwithstanding such complications, in general, one would expect that the greater the degree of water shortage, the less likely that relative performances will reflect relative differences in yield potential. There have been few explicit studies of this expectation. In sets of cultivars of wheat including older tall ones and newer short ones, mean yield levels must be reduced to about 30% of potential before the yield potential advantage of the modern wheats was lost (Laing and Fischer, 1977; Fischer and Wood, 1979). In fact, Siddique et al. (1990) found greater water-use efficiency in the higher yielding modern wheat cultivars. Bolanõs and Edmeades (1993) found some recently selected tropical maize lines to maintain their absolute advantage in yield across an eight-fold range in site mean yields largely determined by water availability. Duvick (1997) found the overall trend towards higher yield potential in maize to be maintained even with annual fluctuations from adverse to nearly ideal years for the crop. In sharp contrast to the findings above, Ceccarelli and Grando (1991), using unselected barley (Hordeum vulgare L.) lines, found that performance at a high yielding site was not associated with performance when lack of water reduced yields below 3 Mg ha-1, about half of the potential yield, and was negatively associated with yields of only 1 Mg ha-1 level.
The question arises whether the higher yields of modern cultivars on farm reflect concurrent selection (deliberate or otherwise) for stress tolerance as well as for yield potential, rather than being a consequence of selection for greater yield potential alone. The latter strategy might be expected to lead to a correlated loss of abiotic stress tolerance, and thus rapid loss of yield advantage as stress increases. As we have seen, this does not generally happen; thus, the former situation seems more likely, as most breeders would affirm. Even so, there is no evidence that relative progress is greater under abiotic stress, except perhaps with the effect of heterosis seen with hybrids.
Certain attributes of the most modern cultivars may make them more susceptible to poor farm management, such as inadequate weed control or seedbed preparation, or uncontrolled grazing. Such attributes would include shorter stature, smaller and more erect leaves, earliness, more determinate growth habit, and lower tillering. On the other hand, the generally greater disease, pest, and lodging resistance of these cultivars would add to their relative advantage in farmers' fields, and in one case at least, the modern maize hybrids are more competitive with weeds (Tollenaar et al., 1997). It is notable, after more than almost three decades or so of interest in on-farm research, how few studies report inferior performance by modern cultivars.
Physiological Basis
Analysis of the physiological basis of increases in yield potential is important as a guide to further gains, both from the changes that have already occurred and from those that have not. It also provides a basis for simulation modeling of crop yields and potential yields.
Different paths have been followed in different crops, even among the cereals, and many characteristics modified, but we confine our comments here to only a few. Among the small grain cereals, the introduction of the dwarfing genes immediately improved resistance to lodging to make the Green Revolution possible. It also freed assimilates from stem growth for investment in greater floret development and grain growth, leading to a progressive rise in harvest index and a parallel increase in yield potential. Although there is some scope for further increase in the harvest index, it is clearly limited, and other sources of increase in yield potential will have to be elicited.
Clearly, more than just the freeing of assimilates from reduced stem growth has been involved. The rise in harvest index in wheat has been mainly associated with the development of more of the distal florets to reach anthesis and set and fill more grains per square meter. One factor associated with this is slow initial growth of the first grains to set, which is apparent not only in modern wheat cultivars (Rawson and Evans, 1971) but even more so in maize and sorghum [Sorghum bicolor (L.) Moench] inflorescences. Given that the CO2 exchange rate (CER) is very responsive to demand and that it is often down-regulated in many crops, the development of more florets and the setting of more grains may well elicit the requisite increase in CER. However, Fischer and Hille Ris Lambers (1978) found the assimilate supply to be more limiting in modern than in old wheat cultivars.
In sharp contrast, the improvement of yield potential in Corn Belt maize hybrids has not involved any marked reduction in crop height or rise in harvest index (Tollenaar, 1989; Duvick, 1992). The increase in yield potential has come mainly from improved ability to withstand increase in the density of planting. Empirical selection has, for example, led to plants with smaller tassels and more inclined upper leaves, leading to improved floret development and grain set as does the shorter interval between anthesis and silking.
In maize there has also been an increase in the duration of photosynthetic activity by the leaves, manifest in their greater "stay green" (Duvick, 1997). A slower decline in the photosynthetic activity of their canopies also characterizes modern cultivars, as in soybean [Glycine max (L.) Merr.] (Wells et al., 1982) and rice (Sasaki and Ishii, 1992). Improved agronomy and crop protection have made such changes, and their further extension, possible.
Finally, there is the vexing question of whether potential rates of photosynthesis and growth have increased; it is too complex to be thoroughly reviewed here. There is no question that selection can, and has, improved photosynthesis under various stresses, e.g., of maize exposed to cool nights or mild drought (Dwyer et al., 1991; Nissanka et al., 1997), although leaf photosynthetic potential does not seem to have increased. In coming to this conclusion, we have to bear in mind the well-known trade-off between leaf area and CER per unit leaf area, which gives the wild relatives of many crop plants much higher CERs than even those of the recent cultivars (Evans, 1993). Moreover, the maximum CER is expressed only under conditions of maximum demand within the plant, and cultivar comparisons do not always ensure that. Consequently, some comparisons during grain growth may indicate a positive correlation between CER and yield potential which may reflect demand rather than photosynthetic capacity. For example, with an historical series of Japanese rice cultivars, Sasaki and Ishii (1992) found no differences in CER at its maximum, only as it fell.
Nevertheless, a few cultivar comparisons indicate changes in photosynthetic capacity that may have consequences for yield potential. For example, among Australian wheat cultivars, selected until recently for performance at moderate soil N levels, Watanabe et al. (1994) found a rise in maximum CER associated with the Rht dwarfing genes and with parallel increases in leaf N, rubisco activity, chlorophyll content, and electron transport capacity, along with a fall in chlorophyll a/b ratio. These changes could have resulted from selection for leaf greenness, made possible by greater use of nitrogenous fertilizers following the introduction of the dwarfing genes, yet another example of the synergism between agronomy and plant breeding. A rise in CER has also been found among recent wheat cultivars in Israel (Blum, 1990).
Comparisons of old and new cultivars of both wheat and cotton selected for yield under irrigated warm conditions have revealed a rise in stomatal conductance, particularly in the early afternoon, associated with lower canopy temperatures (Radin et al., 1994; Fischer et al., 1998; Lu et al., 1998). Greater stomatal conductance and leaf cooling are clearly associated with increased yield in these crops, but not with increased crop growth rate, even when CER has increased, as in the case of wheat. Indeed, among the many comparisons of old and new cultivars, there has been only occasional evidence of a possible increase in crop growth rate. Avoidance of heat stress and its adverse effects on boll set and early development may be the explanation of the yield advantage in cotton, but not necessarily in wheat. Either way, however, these are excellent examples of the power of empirical selection for yield.
Quite apart from their value in the assessment of yield potential, comparisons of historical series of cultivars can provide leads for further selection, e.g., for canopy cooling in irrigated wheat, as suggested by Fischer et al. (1998). As agronomy and pest and disease control improve, there may be scope for earlier flowering or longer duration of photosynthesis and storage, raising the harvest index still further. Longer leaf life from better fertilizer application and crop protection may also permit fewer leaves to be produced, freeing assimilates for grain growth. There may also be scope for more effective use of surplus assimilates accumulating through the period from earing to the beginning of grain growth.
Genetic Basis and Implications for Selection
Notwithstanding yield potential jumps with the advent of the dwarfing genes in wheat and rice, it is generally considered that yield is under multigenic control and that yield potential progress is mostly gradual. Presumably, yield potential progress results from the progressive accumulation of genes conferring higher yield and/or the elimination of unfavorable genes through the breeding process. Breeders may argue about selection and generation advance methodologies, but mostly agree that extensive multilocational yield testing, made possible by mechanization and electronic computation and facilitated by advances in biometrics, has been vital in maintaining rates of progress.
The overall linear rise in yield potential, illustrated in Fig. 2, suggests a progressive assembling of "productivity genes." To some extent this may reflect the progressive broadening of the land race background of modern cultivars evident in CIMMYT wheats, IRRI rices and Pioneer maize hybrids (Fig. 3) . For both maize and wheat, yield potential is correlated with the number of background land races, but this may be fortuitous. Rasmusson and Phillips (1997) have suggested that sufficient genetic variation may continue to be found within quite limited elite gene pools. On the other hand, there may be considerable benefits to yield to be gained with alien chromosome segments (Singh et al., 1998), via synthetic wheats (Villareal et al., 1997), or from the genetic diversity of wild relatives with the aid of molecular marker-aided selection, as in the case of tomato (Lycopersicon esculentum Mill.) and rice (Tanksley and McCouch, 1997).
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Understanding the relationship between traits and yield is being facilitated by the identification and marking of more of the controlling genes and their alleles. Isogenic comparisons have their limitations, but as more are carried out, the importance of certain genes for yield is being confirmed. For example, in wheat, there was little doubt about the value of the major Norin 10 dwarfing genes, but new major gene alleles for dwarfing are now being tested (Worland and Sayers, 1995). The recent determination of the importance of the Ppd1 allele, conferring slight earliness, for high yield in winter wheats in southern Europe (Worland, 1996) further illustrates this point. Molecular markers will undoubtedly prove a powerful tool in this task.
The identification of yield-related physiological selection traits (as distinct from morphological or visual ones) has been of great interest to many physiologists and some breeders. Reasons for the generally frustrating results have been thoughtfully discussed by Jackson et al. (1996). As field instruments continue to improve, new opportunities arise, as with airborne infra red imagery to register canopy temperature and other remote sensing techniques (Araus, 1996). Such techniques could complement molecular-aided selection for yield in the future.
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Conclusion |
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TOPABSTRACTINTRODUCTIONDefinitionMeasurementSignificanceConclusionREFERENCES |
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As agronomy evolves, so do the opportunities to raise yield potential, as the use of dwarfing genes illustrates. Initially sought for resistance to lodging with heavier dressings of fertilizer N, their advantage for yield potential was then realized progressively through improved partitioning to the organs of yield. Moreover, the higher level of N availability also made selection for increased chlorophyll and rubisco content in wheat, and longer stay green in maize, possible, raising the yield potential still further. As agronomy and plant breeding techniques continue to evolve, opportunities for further increase in yield potential will arise.
There is no evidence yet that a ceiling on yield potential has been reached, but should this occur, average yields could still continue to rise as crop management improves and as plant breeders continue to improve resistance to pests, diseases, and environmental stresses. We have seen that advances in stress tolerance are often associated with and difficult to separate from those in yield potential, as among temperate maize hybrids or in the case of CIMMYT wheats. Nevertheless, despite the difficulties of meeting the requirements of its definition, greater yield potential remains a valid and by no means exhausted goal for plant breeding programs.
Received for publication December 28, 1998.
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). The maize data represent means for 2 yr, three locations per year and three densities per location 
), 37 IRRI rice cultivars grouped by decade of release (•), and 38 Pioneer maize hybrids also grouped by decade (
) and maize hybrids (