Principles of Plant Genetics and Breeding

References
Concept of yield plateau
It is important for food production to keep pace with
population growth. Even though global total crop
yields are continually rising, the rate of yield growth is
slowing. This trend is termed yield stagnation or yield
plateau, and has been observed in many of the crops
that feed the world, especially cereals. For example, yield
growth rates for wheat declined from 2.92% (between
1961 and 1979) to 1.78% (between 1980 and 1997).
The decline in corn yield for the same periods was from
2.8% to 1.29%.
Several key factors may be responsible, at least in part,
for yield stagnation. From the research perspective,
many agricultural research programs do not focus on
yield per se increases but rather on improving specific
traits such as drought tolerance, insect resistance, and
disease resistance. A major factor is a shifting away from
cereal production to the production of more profitable
crops, with the decline in world cereal prices. There are
no incentives for producers to pump production inputs
into an enterprise that could raise field yields, only to
take a loss at harvest time. Another factor is crop
intensification whereby multiple crops are being grown
where previously only one was grown. This practice is
suspected to cause a rapid decline in soil fertility.
The genetic potential of many important crops
remains untapped. For example, the global average yield
of wheat is 2 metric tons/ha compared to the record
yield of 14 metric tons/ha, and even a possible 21 metric
BREEDING FOR PHYSIOLOGICAL AND MORPHOLOGICAL TRAITS 361
Blackshaw, R.E., and K.N. Harker. 2003. Selective weed control with glyphosate in glyphosate-resistant spring wheat (Triticum
aestivum). Weed Technol. 16(4):885–892.
Context Network. 2004. Biotech traits commercialized, 2004. CD ROM.
Goodman, R.E., Bardina, L., Nemeth, M.A., et al. 2003. Comparison of IgE binding to water-soluble proteins in genetically
modified and traditionally bred varieties of hard red spring wheat. In: World Allergy Organization Congress – Vancouver,
September 2003. Abstract. World Allergy Organization.
Hu, T., S. Metz, C. Chay, et al. 2003. Agrobacterium-mediated large-scale transformation of wheat (Triticum aestivum L.) using
glyphosate selection. Plant Cell Rep. 21:1010–1019.
Hyun, Y., G.E. Bressner, R.L. Fischer, et al. 2005. Performance of growing-finishing pigs fed diets containing YieldGard
Rootworm corn (MON 863), a nontransgenic genetically similar corn, or conventional corn hybrids. J. Animal Sci.
83:1581–1590.
Kan, C.A., and G.F. Hartnell. 2004. Evaluation of broiler performance when fed Roundup Ready® wheat (event MON 71800),
control, and commercial wheat varieties. Poultry Sci. 83:1325–1334.
Obert, J., W. Ridley, R. Schneider, et al. 2004. The composition of grain and forage from glyphosate tolerant wheat, MON 71800,
is equivalent to that of conventional wheat (Triticum aestivum L.). J. Agric. Food Chem. 52(5):1375–1384.
Padgette, S.R., D.B. Re, G.F. Barry, et al. 1996. New weed control opportunities: Development of soybeans with a Roundup
Ready gene. In: Herbicide-resistant crops (Duke, S.O., ed.), pp. 54–84. CRC Press, Boca Raton, FL.
Zhou, H., J.D. Berg, S.E. Blank, et al. 2003. Field efficacy assessment. Crop Sci. 43:1072–1075.
tons/ha. Plant breeding is needed to increase crop
yields by developing high-yielding cultivars for various
ecotypes.
Yield stability
Plant breeders are not only interested in developing
high-yielding cultivars. They are interested in developing
cultivars with sustained or stable high performance
over seasons and years (yield stability). One of the key
decisions made by the plant breeder at the end of the
breeding program is the genotype to release as a cultivar.
This decision is arrived at after yield trials conducted
over locations, seasons, and years, as applicable. When
different genotypes exhibit differential responses to
different sets of environmental conditions, a genotype
× environment (G × E) interaction is said to occur.
This subject is best fully discussed elsewhere (see
Chapter 23). Genotypes that are more responsive to the
environment are less stable in performance, doing well
in a good production environment and poorly under
less optimal production conditions. On the contrary,
less responsive genotypes perform generally well across
varied environments.
Stability is rather difficult to determine or breed for in
a breeding program. While it might be desirable to set as
an objective to breed for either more or less environmental
responsiveness, it is more practical and realistic
to exploit whatever turns up during yield trails. Each