Agriculture Reference
In-Depth Information
maize landraces adapted to the lowland tropics are typically tall, leafy, and prone to lodging, and
have low harvest indices (Goldsworthy et al., 1974). During the initial stages of maize improvement
at CIMMYT, reduction in the plant height was a priority (Fischer and Palmer, 1984), and reduced
plant height has continued only as a secondary trait in breeding activity. Johnson et al. (1986)
reported that 15 cycles of recurrent selection for reduced height in the lowland tropical maize popu-
lation reduced plant stature by 37%, crop duration by 7%, and increased the proportional allocation
of total biomass to husks, ears, and silks at the 50% silking stage. At the same time, researchers
observed that grain yield, harvest index, and optimum plant density for grain yield each increased by
50-70%. Lodging was also substantially reduced. Similarly, Edmeades and Lafitte (1993) reported
that lodging in maize declined from 39% to 10% with 18 cycles of recurrent selection.
Duvick (2005a,b) reviewed the literature regarding the contribution of breeding to yield advances
in corn. He concluded that corn yields have increased continually wherever hybrid corn has been
adopted, starting in the U.S. corn belt in the early 1930s. Plant breeding and improved management
practices have produced this gain jointly. On average, about 50% of the increase is due to manage-
ment and 50% due to breeding (Duvick, 2005b). The two factors interact so closely that neither of
them could have produced such progress alone. However, genetic gains may have to bear a larger
share of the load in future years (Duvick, 2005b). Trait changes that increase resistance to a wide
variety of biotic and abiotic stresses are the most numerous, but morphological and physiological
changes that promote efficiency in growth, development, and partitioning are also recorded. Newer
hybrids yield more than their predecessors in unfavorable as well as favorable growing conditions
(Duvick, 2005a,b). Table 1.21 shows data related to yield increase from 1961 to 2002 in different
countries or continents.
Duvick (2005b) reported that phenotypic changes may indicate improvements in the efficiency
of grain production (i.e., smaller tassels may release more energy for grain production). Stress toler-
ance is greatly improved, newer hybrids outyield the older ones not only in high-yield environments
but also when trials are subjected to abiotic stress (heat and drought), or to biotic stress (i.e., insect
pests). In addition, newer hybrids yield more than older hybrids because of continuing improvement
in the ability of the hybrids to withstand the stress of higher plant density, which in turn is owed to
their greater tolerance to locally important biotic and abiotic stresses (Duvick, 2005b).
In corn, changes in cultural practices such as weed and pest control, timelines of planting, and
increased efficiency of harvest equipment have helped to raise yield over the years. Application of N
fertilizers increased following World War II. They increased significantly from 60 kg ha
−1
in 1964 to
TABLE 1.21
Corn Yield Increase in Different Regions/Continents from 1961 to 2002
Average of 1961
(Mg ha
−
1
)
Average of 2002
(Mg ha
−
1
)
Annual Gain
(kg ha
−
1
year
−
1
)
Country/Continent
European Union (15)
2.5
9.1
169
USA
3.9
8.2
109
Canada
4.6
7.6
69
China
1.2
5.0
103
South America
1.4
3.4
48
South Asia
1.0
1.7
20
Eastern Europe
1.8
4.2
42
Southern Africa
0.7
1.3
8
World
1.9
4.3
61
Source:
Adapted from FAO 2004. Statistical Databases available at http://apps.fao.org/default.htm;
Duvick, D. N. 2005a.
Adv. Agron
. 86:83-145.
