Agriculture Reference
In-Depth Information
Corn is an important grain crop worldwide. The United States is the highest corn-producing
country. Corn production in the United States during the 2013 cropping season was about 350 mil-
lion metric tons. China is the second largest corn-producing country in the world, with an annual
sowing area of about 25 million hectares, and production of 130 million metric tons (Ci et al., 2013).
Six cycles of cultivar replacement have occurred in China since the 1950s. Among corn cultivars
released from the 1950s to the 2000s in China, the newer hybrids possess increased grain yield and
relatively greater tolerance to compound stress (Ci et al., 2011). Many agronomic traits have been
improved and kernel number per year and kernel weight from the 1950s to the 2000s are important
advances in the Chinese corn improvement program (Ci et al., 2013).
The continuous increase in corn grain yield in the world's primary growing areas during the
past few decades was mainly driven by the development of crowding stress-tolerant hybrids that
allowed for dramatic increase in plant population and therefore in production per unit area (Duvick,
2005a; Robles et al., 2013). Differences in grain yield between older and newer corn hybrids have
been shown to be a function of plant population, rather than plant yield potential (Tollenaar and
Lee, 2002; Duvick, 2005a). This characteristic usually renders modern hybrids as density depen-
dent (Berzsenyi and Tokatlidis, 2012). High densities are required for maximum yield potential, but
optimum densities differ among hybrids and across seasons (Fasoula and Fasoula, 2000; Tokatlidis
and Koutrouba, 2004; Fasoula and Tollenaar, 2005).
Increased plant population leads to greater LAI and increased interception of PAR from the
midvegetative to early grain fill stages (Tollenaar and Aguilera, 1992; Maddonni and Otegui, 2004;
Novacek et al., 2013). Increased plant population has been shown to decrease the number of ears
per plant (Tollenaar et al., 1992) and number of kernels per ear (Westgate et al., 1997; Maddonni
and Otegui, 2006), but has less influence on kernel weight (Begna et al., 1997; Westgate et al., 1997;
Maddonni and Otegui, 2006). Corn grain yield is also influenced by other crop management prac-
tices, including reduced row spacing to obtain a more equidistant plant spacing, thereby reducing
interplant competition (Bullock et al., 1988), increasing the interception of solar radiation (Andrade
et al., 2002), and decreasing weed competition (Teasdale, 1995, 1998; Begna et al., 2001). In addi-
tion, transgenic insect resistance has complemented plant population increases and improved grain
yield (Novacek et al., 2013).
Assefa et al. (2012) reported that corn yield has increased from about 1.5 Mg ha
−1
in the early
1900s to 8.5 Mg ha
−1
in the beginning of the 2000s in the United States. These authors analyzed
data from corn trials conducted in Kansas from 1939 to 2009. On average, corn yields increased
at a rate of 90 kg ha
−1
year
−1
in dryland and 120 kg ha
−1
year
−1
in irrigated trials. Changes in hybrid
technology and changes in crop management factors, such as a decrease in planting and harvesting
date by about a quarter of a day per year increased planting density at the rate of 597 plants ha
−1
year
−1
, and increased N and P fertilizer rates by 2.6 and 0.40 kg ha
−1
year
−1
, respectively, were found
for the same time period in dryland corn (Assefa et al., 2012). These authors further reported that, in
addition, climate changes contributed to yield increases in the past through increased total rainfall,
average monthly minimum and maximum temperature in March, and decreased maximum temper-
ature from July through September. Increases in plant density generally have a large positive impact
on the incident solar radiation intercepted and as a consequence, on CGR around silking as well as
a final yield (Tollenaar and Aguilera, 1992). Corn yield in the United States has also increased due
to earlier planting dates (Kucharik, 2008) and more extensive use of irrigation (Cassman, 1999).
Genetic improvement in the grain yield of maize hybrids in North America and Europe during
the past three to five decades has been extensively documented (Tollenaar and Aguilera, 1992).
Grain yield improvement of maize hybrids appeared to be the result of increased dry matter accu-
mulation. Increased dry matter may be attributable to the increased absorption of incident PAR
and/or improved efficiency of converting absorbed PAR into dry matter. Some evidence indicates
that modern hybrids absorb more of the seasonal incidence PAR than the older hybrids. Maximum
LAI for modern hybrids was larger than for older ones, and leaves of modern hybrids stay green
longer during the final phase of the life growth cycle (Tollenaar and Aguilera, 1992). Full season
