It is the nature of wheat breeding that new varieties should exceed, or at least equal, older varietes with regard to yield levels. However, an increase in the yield potential can be more or less directly pursued in the breeding process, in the way that primary selection criteria will often be focussed on qualitative characters such as resistance to important diseases. Breeding directly for yield requires a focus on a range of yield components in order to optimize the combined effects of these. Sylvester-Bradley et al. (2005) estimated the theoretical maximal wheat yield under British conditions to approximate 19.2 t/ha, which indicates the maximal yield also under Danish conditions. Thus, targeted approaches to increase wheat yields sub-stantially above the current levels should still be plausible.
The yield increases observed over the last 50 years are mostly attributed to increases in harv-est index that in modern varieties approximates an apparent maximum around 0.6 (Fisher 2007). However, Fisher (2007) also noted that many of the best north European winter wheats, with a harvest index at around 0.45-0.50, are still well below this limit, leaving lots of space for improvement. Future increases in yield need to rely on increases in total biomass while maintaining an efficient re-allocation of photosynthates to the grain, i.e. a high harvest index. Foulkes et al. (2007) suggested that this can be achieved through different targeted optimizations: i) a better rooting system for efficient water and nutrient uptake; ii) extension of the stem-elongation phase in order to increase the accumulation of pre-anthesis water so-luble carbohydrates in the stems; iii) optimization of photosynthesis and radiation use efficen-cy either via changes in plant architecture or directly via higher specificity for carboxylation by Rubisco, regulation of Rubisco activation, or ribulose bisphosphate regeneration (see also Parry et al. 2007); and iv) development of strong sinks to efficiently absorb photosynthates during the grain filling period, i.e. setting of a high number of kernels at anthesis. Overall, an important element for the combination of high biomass and high harvest index is the delaying of senescence processes (Gregersen et al. 2008) that should secure the re-allocation of photo-synthates and nutrients to the grain.
certified sowing seeds sold, as reported by the Danish Plant Directorate every year
(www.pdir.dk), there is a lag of at least four years from their first appearance in the National Field Trials until new high-yielding varieties get their break-throughs as commonly grown varieties. Furthermore, the choice of varieties can be based on other reasons than pure yield, for example quality or disease resistance. For instance, the sowing seed data shows that in the year 2002 the share of sowing seeds from varieties that in previous years had been part of the group of high-yielding varieties, as defined in this analysis, had fallen to a minimum of around 50 %. After 2006, the high-yielding varieties have comprised more than 90 % of the sowing seed sold, and it seems that the new and taller winter wheat varieties with a presuma-bly better ability to take up nitrogen are gaining importance in common agricultural practice.
4.4 Future winter wheat yields – what to expect?
It is the nature of wheat breeding that new varieties should exceed, or at least equal, older varietes with regard to yield levels. However, an increase in the yield potential can be more or less directly pursued in the breeding process, in the way that primary selection criteria will often be focussed on qualitative characters such as resistance to important diseases. Breeding directly for yield requires a focus on a range of yield components in order to optimize the combined effects of these. Sylvester-Bradley et al. (2005) estimated the theoretical maximal wheat yield under British conditions to approximate 19.2 t/ha, which indicates the maximal yield also under Danish conditions. Thus, targeted approaches to increase wheat yields sub-stantially above the current levels should still be plausible.
The yield increases observed over the last 50 years are mostly attributed to increases in harv-est index that in modern varieties approximates an apparent maximum around 0.6 (Fisher 2007). However, Fisher (2007) also noted that many of the best north European winter wheats, with a harvest index at around 0.45-0.50, are still well below this limit, leaving lots of space for improvement. Future increases in yield need to rely on increases in total biomass while maintaining an efficient re-allocation of photosynthates to the grain, i.e. a high harvest index. Foulkes et al. (2007) suggested that this can be achieved through different targeted optimizations: i) a better rooting system for efficient water and nutrient uptake; ii) extension of the stem-elongation phase in order to increase the accumulation of pre-anthesis water so-luble carbohydrates in the stems; iii) optimization of photosynthesis and radiation use efficen-cy either via changes in plant architecture or directly via higher specificity for carboxylation by Rubisco, regulation of Rubisco activation, or ribulose bisphosphate regeneration (see also Parry et al. 2007); and iv) development of strong sinks to efficiently absorb photosynthates during the grain filling period, i.e. setting of a high number of kernels at anthesis. Overall, an important element for the combination of high biomass and high harvest index is the delaying of senescence processes (Gregersen et al. 2008) that should secure the re-allocation of photo-synthates and nutrients to the grain.
There is considerable evidence that biomass increases have been associated with specific in-troductions of alien genes into wheat germplasm in the last decades, which has been exten-sively used in wheat breeding programmes across the world (Foulkes et al. 2007; Reynolds et al. 2001). However, the complexity of the yield traits makes targeted selection difficult. An-ticipated future insights into the genetic and molecular regulation of these complex traits will presumably provide means for a more efficient selection. Studies in rice (e.g. Xue et al. 2007;
Wang et al. 2008) have shown that several yield components, although quantitative of nature, are relatively simply governed by single genes, for example genes that have large effects on grain size and weight, plant height, and grains per panicle. This holds promises as well for wheat breeding. When the wheat genome sequence eventually becomes available
(www.wheatgenome.org), the characterization of important yield-determining genes could form the basis for development of molecular markers that can assist their efficient exploitation in breeding programmes.
Detailed characterization of yield-determining genes could also, in the longer term, form the basis for genetic modification, by up- or down-regulation of gene activities in order to achieve yield increases. This could be exploited in e.g. a cisgenesis approach (Rommens et al. 2007), where only the plant’s own genes are used during the genetic engineering.
The introduction and use of wheat hybrids will possibly increase yield, as hybrids can achieve a higher harvest index, biomass and yields through combining yield components from their parents (Evans, 1993). Grain yield data from 1975 to 1995 from four North American loca-tions, selected and analyzed by relative yield indices, indicated an 11% advantage of hybrids over pure lines. In European studies the levels of heterosis for grain yield averaged 5% to more than 10% (Kindred & Gooding, 2005). In 2006 the wheat hybrid Hysun was the highest yielding variety in the National Field Trials. The hybrid variety was, however, excluded the following year because of 40 percent mother plants in the plots. In 2008 another hybrid vari-ety, Hymack, did not exceed the trial reference mixture in the National Field Trials
(www.sortinfo.dk).
4.5 Conclusion
For the period 1980 to 2008, the analysis indicated that breeding and genetics have contri-buted with an annual yield increase of up to 1.1 %. The results might indicate, that the Danish cereal breeders have adapted to the new conditions very quickly, e.g. with new and taller win-ter wheat varieties with presumably betwin-ter abilities to take up nitrogen, as an answer to the reduced nitrogen quotas in Denmark. The estimation is that there are still possibilities for im-proving winter wheat yields considerably, but that this will rely on targeted breeding efforts on the genetic combination of yield components, possibly by the help of new technologies such as the development of molecular markers.
There is considerable evidence that biomass increases have been associated with specific in-troductions of alien genes into wheat germplasm in the last decades, which has been exten-sively used in wheat breeding programmes across the world (Foulkes et al. 2007; Reynolds et al. 2001). However, the complexity of the yield traits makes targeted selection difficult. An-ticipated future insights into the genetic and molecular regulation of these complex traits will presumably provide means for a more efficient selection. Studies in rice (e.g. Xue et al. 2007;
Wang et al. 2008) have shown that several yield components, although quantitative of nature, are relatively simply governed by single genes, for example genes that have large effects on grain size and weight, plant height, and grains per panicle. This holds promises as well for wheat breeding. When the wheat genome sequence eventually becomes available
(www.wheatgenome.org), the characterization of important yield-determining genes could form the basis for development of molecular markers that can assist their efficient exploitation in breeding programmes.
Detailed characterization of yield-determining genes could also, in the longer term, form the basis for genetic modification, by up- or down-regulation of gene activities in order to achieve yield increases. This could be exploited in e.g. a cisgenesis approach (Rommens et al. 2007), where only the plant’s own genes are used during the genetic engineering.
The introduction and use of wheat hybrids will possibly increase yield, as hybrids can achieve a higher harvest index, biomass and yields through combining yield components from their parents (Evans, 1993). Grain yield data from 1975 to 1995 from four North American loca-tions, selected and analyzed by relative yield indices, indicated an 11% advantage of hybrids over pure lines. In European studies the levels of heterosis for grain yield averaged 5% to more than 10% (Kindred & Gooding, 2005). In 2006 the wheat hybrid Hysun was the highest yielding variety in the National Field Trials. The hybrid variety was, however, excluded the following year because of 40 percent mother plants in the plots. In 2008 another hybrid vari-ety, Hymack, did not exceed the trial reference mixture in the National Field Trials
(www.sortinfo.dk).
4.5 Conclusion
For the period 1980 to 2008, the analysis indicated that breeding and genetics have contri-buted with an annual yield increase of up to 1.1 %. The results might indicate, that the Danish cereal breeders have adapted to the new conditions very quickly, e.g. with new and taller win-ter wheat varieties with presumably betwin-ter abilities to take up nitrogen, as an answer to the reduced nitrogen quotas in Denmark. The estimation is that there are still possibilities for im-proving winter wheat yields considerably, but that this will rely on targeted breeding efforts on the genetic combination of yield components, possibly by the help of new technologies such as the development of molecular markers.
Acknowledgement: Many thanks to Lars B. Eriksen, Sejet Plant Breeding, and Erik Tybirk, Nordic Seed, for contributing to the analysis.
4.6 References
Evans, L.T. (1993) Crop Evolution, Adaptation, and Yield. Cambridge University Press, Cambridge.
Fischer, R. A. (2007) Understanding the physiological basis of yield potential in wheat. Jour-nal of Agricultural Science 145, 99-113.
Foulkes, M.J., Snape, J.W., Shearman, V.J., Reynolds, M.P., Gaju, O. & Sylvester-Bradley, R. (2007) Genetic progress in yield potential in wheat: recent advances and future pros-pects. Journal of Agricultural Science, Cambridge 145, 17-29.
Gregersen, P.L., Holm, P.B. & Krupinska, K. (2008) Leaf senescence and nutrient remobilisa-tion in barley and wheat. Plant Biology 10, 37-49.
Kindred, D.R. & Gooding, M.J. (2005) Heterosis for yield and its physiological determinants in wheat. Euphytica 142, 149-159.
Kjærsgaard, J. (2002) Planteavlsberetning 2002, LandboCentrum, Møllevej 15, DK-4140 Bo-rup.
Kjærsgaard, J. (2001) Planteavlsberetning 2001, LandboCentrum, Møllevej 15, DK-4140 Bo-rup.
Parry, M.A.J., Madgwick, P.J., Carvahlo, J.F.C. & Andralojc, P. (2007) Prospects for increa-sing photosynthesis by overcoming the limitations of Rubisco. Journal of Agricultural Science 145, 31-43.
Reynolds, M.P., Calderini, D.F., Condon, A.G. & Rajaram, S. (2001) Physiological basis of yield gains in wheat associated with the LR19 translocation from Agropyron elongatum.
Euphytica 119, 137-141.
Rommens, C.M., Haring, M.A., Swords, K., Davies, H.V. & Belknap, W.R. (2007) The in-tragenic approach as a new extension to traditional plant breeding. Trends in Plant Sci-ence 12, 397-403.
Sylvester-Bradley, R., Faulkes, J. & Reynolds, M. (2005) Future wheat yields: Evidence, theory and conjecture. Yields of farmed species, constraints and opportunities in the 21st century. In: R. Sylvester-Bradley & J. Wiseman (Eds.) Nottingham University Press, UK. 651 pp.
Wang, E., Wang, J., Zhu, X., Hao, W., Wang, L., Li, Q., Zhang, L., He, W., Lu, B., Lin, H., Ma, H., Zhang, G. & He, Z. (2008) Control of rice grain-filling and yield by a gene with a potential signature of domestication. Nature Genetics 40, 1370-1374.
Xue, W., Xing, Y., Weng, X., Zhao, Y., Tang, W., Wang, L., Zhou, H., Yu, S., Xu, C., Li, X.
& Zhang, Q. (2008) Natural variation in Ghd7 is an important regulator of heading date and yield potential in rice. Nat Genet 40, 761-767.
Öfversten, J., Jauhiainen, L. & Kangas, A. (2004) Contribution of new varieties to cereal yields in Finland between 1973 and 2003. The Journal of Agricultural Science (2004) ), 142:3:281-287 - Cambridge Univ Press.
Acknowledgement: Many thanks to Lars B. Eriksen, Sejet Plant Breeding, and Erik Tybirk, Nordic Seed, for contributing to the analysis.
4.6 References
Evans, L.T. (1993) Crop Evolution, Adaptation, and Yield. Cambridge University Press, Cambridge.
Fischer, R. A. (2007) Understanding the physiological basis of yield potential in wheat. Jour-nal of Agricultural Science 145, 99-113.
Foulkes, M.J., Snape, J.W., Shearman, V.J., Reynolds, M.P., Gaju, O. & Sylvester-Bradley, R. (2007) Genetic progress in yield potential in wheat: recent advances and future pros-pects. Journal of Agricultural Science, Cambridge 145, 17-29.
Gregersen, P.L., Holm, P.B. & Krupinska, K. (2008) Leaf senescence and nutrient remobilisa-tion in barley and wheat. Plant Biology 10, 37-49.
Kindred, D.R. & Gooding, M.J. (2005) Heterosis for yield and its physiological determinants in wheat. Euphytica 142, 149-159.
Kjærsgaard, J. (2002) Planteavlsberetning 2002, LandboCentrum, Møllevej 15, DK-4140 Bo-rup.
Kjærsgaard, J. (2001) Planteavlsberetning 2001, LandboCentrum, Møllevej 15, DK-4140 Bo-rup.
Parry, M.A.J., Madgwick, P.J., Carvahlo, J.F.C. & Andralojc, P. (2007) Prospects for increa-sing photosynthesis by overcoming the limitations of Rubisco. Journal of Agricultural Science 145, 31-43.
Reynolds, M.P., Calderini, D.F., Condon, A.G. & Rajaram, S. (2001) Physiological basis of yield gains in wheat associated with the LR19 translocation from Agropyron elongatum.
Euphytica 119, 137-141.
Rommens, C.M., Haring, M.A., Swords, K., Davies, H.V. & Belknap, W.R. (2007) The in-tragenic approach as a new extension to traditional plant breeding. Trends in Plant Sci-ence 12, 397-403.
Sylvester-Bradley, R., Faulkes, J. & Reynolds, M. (2005) Future wheat yields: Evidence, theory and conjecture. Yields of farmed species, constraints and opportunities in the 21st century. In: R. Sylvester-Bradley & J. Wiseman (Eds.) Nottingham University Press, UK. 651 pp.
Wang, E., Wang, J., Zhu, X., Hao, W., Wang, L., Li, Q., Zhang, L., He, W., Lu, B., Lin, H., Ma, H., Zhang, G. & He, Z. (2008) Control of rice grain-filling and yield by a gene with a potential signature of domestication. Nature Genetics 40, 1370-1374.
Xue, W., Xing, Y., Weng, X., Zhao, Y., Tang, W., Wang, L., Zhou, H., Yu, S., Xu, C., Li, X.
& Zhang, Q. (2008) Natural variation in Ghd7 is an important regulator of heading date and yield potential in rice. Nat Genet 40, 761-767.
Öfversten, J., Jauhiainen, L. & Kangas, A. (2004) Contribution of new varieties to cereal yields in Finland between 1973 and 2003. The Journal of Agricultural Science (2004) ), 142:3:281-287 - Cambridge Univ Press.
5 Changes in fertilization practice and impact on yield of winter wheat Jens Petersen 1) & Leif Knudsen 2)
1) Aarhus University, Faculty of Agricultural Sciences (DJF), Department of Agroecology and Environment
2) Danish Agricultural Advisory Service (DAAS), Knowledge Centre for Agriculture
5.1 Introduction
The Danish Parliament passed Actions plans for the aquatic environment in 1987, 1998 and 2004, a Plan for Sustainable Agriculture in 1991, and an Action Plan to reduce Ammonia Emission in 2001. As a result of the plans, The Danish Plant Directorate was authorized by the Minister of Food, Agriculture and Fisheries to lay down statutory orders on the agricul-tural use of nitrogen (N) with the aim of reducing nitrate leaching from fields. The annual statutory order is divided into two parts for nitrogen: one setting out standard N rates for each crop, and the other concerning a substitution rate for N in animal manures that has to be taken into account in observing the standard N rate.
Introduction of standard N rates was decided in 1992 and took effect starting with the 1993/94 growing season. The standard N rates depend on soil type, irrigation, and preceding crop, and are defined as the N rate required to obtain economically optimal yields. To meet the Nitrate Directive (EEC, 1991) the second Action Plan for the Aquatic Environment from 1998 stipu-lates a suboptimal N rate for crops (Mikkelsen et al., 2005; Kronvang et al., 2008). Thus re-duced standard N rates corresponding to c. 90% of the N rate for economically optimal yields have been prescribed in Denmark since 1999 (see Textbox 1). The substitution rate for N in animal manures took effect starting with the 1993/94 growing season, and it has been tight-ened gradually ever since (Figure 5.1). The substitution rates include the first year and the residual fertilizer values. These regulations have, together with statutory orders determined by the Danish Environmental Protection Agency (DEPA) regarding application time and method, caused significant changes in the use of N in animal manure and mineral fertilizers. A detailed description of implemented regulations is given by Mikkelsen et al. (2005) and Kronvang et al. (2008), but the most important DEPA statutory order regarding winter wheat is the ban of slurry application in the autumn (from harvest to 1st February), taking effect from July 1993.
The overall question on how changes in fertilization practice may have affected winter wheat grain yields may be divided into two: The first question is how the N rate for winter wheat has changed. Here it is important to consider the N source and separate the discussion for animal manure and mineral fertilizer as several measures aim to improve the utilization of N in ani-mal manure. The second question is how the change in N rate has affected the grain yield.
Before this question may be answered, we have to discuss some uncertainties in estimation yield response including the first-year effect and the long-term effect of varying N rate.
5 Changes in fertilization practice and impact on yield of winter wheat Jens Petersen 1) & Leif Knudsen 2)
1) Aarhus University, Faculty of Agricultural Sciences (DJF), Department of Agroecology and Environment
2) Danish Agricultural Advisory Service (DAAS), Knowledge Centre for Agriculture
5.1 Introduction
The Danish Parliament passed Actions plans for the aquatic environment in 1987, 1998 and 2004, a Plan for Sustainable Agriculture in 1991, and an Action Plan to reduce Ammonia Emission in 2001. As a result of the plans, The Danish Plant Directorate was authorized by the Minister of Food, Agriculture and Fisheries to lay down statutory orders on the agricul-tural use of nitrogen (N) with the aim of reducing nitrate leaching from fields. The annual statutory order is divided into two parts for nitrogen: one setting out standard N rates for each crop, and the other concerning a substitution rate for N in animal manures that has to be taken into account in observing the standard N rate.
Introduction of standard N rates was decided in 1992 and took effect starting with the 1993/94 growing season. The standard N rates depend on soil type, irrigation, and preceding crop, and are defined as the N rate required to obtain economically optimal yields. To meet the Nitrate Directive (EEC, 1991) the second Action Plan for the Aquatic Environment from 1998 stipu-lates a suboptimal N rate for crops (Mikkelsen et al., 2005; Kronvang et al., 2008). Thus re-duced standard N rates corresponding to c. 90% of the N rate for economically optimal yields have been prescribed in Denmark since 1999 (see Textbox 1). The substitution rate for N in animal manures took effect starting with the 1993/94 growing season, and it has been tight-ened gradually ever since (Figure 5.1). The substitution rates include the first year and the residual fertilizer values. These regulations have, together with statutory orders determined by
Introduction of standard N rates was decided in 1992 and took effect starting with the 1993/94 growing season. The standard N rates depend on soil type, irrigation, and preceding crop, and are defined as the N rate required to obtain economically optimal yields. To meet the Nitrate Directive (EEC, 1991) the second Action Plan for the Aquatic Environment from 1998 stipu-lates a suboptimal N rate for crops (Mikkelsen et al., 2005; Kronvang et al., 2008). Thus re-duced standard N rates corresponding to c. 90% of the N rate for economically optimal yields have been prescribed in Denmark since 1999 (see Textbox 1). The substitution rate for N in animal manures took effect starting with the 1993/94 growing season, and it has been tight-ened gradually ever since (Figure 5.1). The substitution rates include the first year and the residual fertilizer values. These regulations have, together with statutory orders determined by