Agriculture Reference
In-Depth Information
in the region being studied. Some investigators
used different N fertilizer levels, fungicide treat-
ments, and planting dates to create different yield
levels.
Generally, the highest rates of gain were mea-
sured at the highest level of productivity. Shear-
man et al. (2005) reported gains of 119 kg ha
−1
yr
−1
in the UK during the period from 1972 through
1995. This increase was found at a very high yield
level, in which the 2-year mean of the three
most recently released cultivars exceeded
11,000 kg ha
−1
. This contrasted sharply with data
from the US Great Plains winter wheat produc-
tion areas, where yields of the newest cultivars
varied from 2,000 to 6,000 kg
−1
ha
−1
depending on
the environment (Cox et al., 1988; Donmez et al.,
2001). Where N rates were varied, yield gains
were higher at the higher N rates, but signifi cant
gains were also detected at the low N rates.
Although the absolute gains (in kilograms per
hectare per year) were higher under the highest
production environments, the rate of gain on a
percentage basis was similar across environments,
averaging about 1% per year. Rajaram and Braun
(2008) reviewed yield gains within the CIMMYT
program and demonstrated that absolute genetic
yield gains were higher under irrigated environ-
ments, but the percentage gains were similar.
Some genetic gain estimations employed fun-
gicides and insecticides as necessary to control
diseases, while others involved natural conditions
(see Environment in Table 17.2). In the unpro-
tected environments, yield was reduced by leaf
rust (caused by
Puccinia triticina
Eriks.) (Cox
et al., 1988; Donmez et al., 2001; Underdahl
et al., 2008), stem rust (
P. graminis
Pers.:Pers.
f. sp.
tritici
Eriks. & E. Henn.) (Cox et al., 1988),
Fusarium head blight (
Fusarium graminearum
Schwabe) (Underdahl et al., 2008), and tan spot
(
Pyrenophora tritici-repentis
Died.) (Cox et al.,
1988). Genetic improvement in grain yield was
positively related with resistance to diseases tar-
geted by the breeding programs (Fusarium head
blight, leaf rust, and stem rust). The highest per-
centage gain reported in Table 17.2 was 2.5% per
year in the presence of a Fusarium head blight
epidemic in North Dakota, where the most
recently released cultivars offered improved
resistance to Fusarium head blight (Underdahl et
al., 2008). On the other hand, no genetic gain for
grain yield was realized under a heavy tan spot
epidemic in Kansas (Cox et al., 1988).
Yield components
Parallel with these changes in wheat yield has
been a consistent reduction in height, increase in
harvest index (HI), and increase in kernel number
per unit area. These are all documented to be
phenotypical expressions of the semidwarf (
Rht
)
genes that were introduced into wheat cultivars
beginning around 1960 (Borlaug 2007; Fischer
2007; Ortiz et al., 2007). The increases in HI and
kernel number are not entirely related to the
introduction of semidwarf genes. Several studies
have shown that these two yield components have
continued to increase among semidwarf cultivars
(Sayre et al., 1997; Abbate et al., 1998; Sankaran
et al., 2000; Shearman et al., 2005; White and
Wilson 2006).
Greater kernel number per unit area can be the
result of more spikes per unit area and/or more
kernels per spike. There was more often a trend
for newer cultivars to produce more kernels per
spike, but evidence was found for an increase in
the number of spikes. De Vita et al. (2007) and
Giunta et al. (2007) showed that newer Italian
durum wheat cultivars produced more spikes per
unit area and more kernels per spike. Some studies
showed no increase with year of release for kernels
per spike or the number of spikes. Hence genetic
variability may be present, where some cultivars
have relatively more spikes and some have rela-
tively more kernels per spike, but there was no
defi nitive trend over time of cultivar release.
Kernel weight generally has not changed over
time, although Calderini et al. (1995) observed an
increase among newer cultivars in Argentina.
Zhou et al. (2007b) found that kernel weight
increased in southern China, where large seed was
a common selection criterion among breeding
programs (He et al., 2001). Kernel weight also
increased among North Dakota spring wheat cul-
tivars, where a consistent and larger kernel size is
desired for milling yield (Underdahl et al., 2008).
They attributed some of the increase in kernel
