Maize - Bioenergy Crop Plants - page 411

allowing increased photosynthesis per unit land area, and reduction in tassel size to free up carbon
for investment in other productive plant parts.
A change in grain composition has also contributed to grain yield over time. An increased grain
starch/protein ratio meant channeling of more photosynthate toward starch formation and thus grain
yield. Starch requires half the energy required for its formation than protein (also see section 16.6:
Bioenergetic Considerations for Biomass Interconversion). When compared to total biomass pro-
ductivity per unit land area, however, the contributions of reduced tassel size and starch/protein
ratio toward increased grain yield is minor.
The highest recorded grain yield has stayed around 22 Mg ha -1 (~350 bu acre -1 ) for several decades,
suggesting that this is likely the potential yield of maize. Environmental stresses are apparently
responsible for the gap between the potential and the actual grain yield. No wonder then that the
modern hybrids perform better under environmental stresses in comparison to the older ones. With
the current average US corn grain yield of 10 Mg ha -1 , significant room exists for yield improvement
assuming that genetic variation for tolerance to various stresses is not exhausted.
Beginning in 1996, seed companies started marketing maize hybrids containing a transgene
that imparted resistance to European Corn Borer (ECB), a major pest in the Corn Belt to which a
corn plant is susceptible from early vegetative stage all the way to physiological maturity. These
hybrids are referred to as Bt hybrids after a gene from the bacterium, Bacillus thuringiensis ( Bt ).
The first generation of Bt hybrids contained the gene that produced the insecticidal protein Cry1Ab
(Cryptochrome 1Ab), followed by stacking of additional insecticidal genes for corn rootworm resis-
tance later on. Aside from a substantial reduction in the application of insecticides, the yield advan-
tage of the transgenic hybrids containing the Bt genes over the non-transgenic controls can exceed
10% depending upon the insect pressure.
16.2.4 B iomaSS p roduction and h arvESt i ndEx
From the time hybrids were first introduced in the 1930s, grain yield in maize has maintained an
upward trend (Figure 16.3) (Tollenaar and Lee 2002; Duvick 2005a, 2005b; Anonymous 2007).
Linear regression coefficients of grain yield for 2-decade windows starting in 1931 are 58, 158,
99, and 152 kg/ha per year, respectively. The regression coefficients as a percentage of yield in the
beginning of each period are 4.6, 7.7, 1.9, and 2.3, respectively (Figure 16.4). The rate of yield gain
appears to be high in the beginning but then waned over time (Figure 16.4). However, the actual rate
of gain, which is approximately 150 kg/ha per year, has not changed since the 1950s. The average
yield in the beginning of each of the respective 2-decade windows was 1.2, 2.0, 5.2, and 7 Mg/ha.
The current average yield of corn is approximately 10 Mg/ha. Because of lower starting yields in
the 1930s and 1950s, the apparent rates of gain were relatively higher. The dip in the third decade
could be accounted for by a high frequency of low-yielding years because of environmental stresses.
Introduction of dwarfing genes in small grain cereals, such as rice and wheat, increased the harvest
index (HI), or the ratio of grain yield to total aboveground biomass (Hay 1995; Sinclair 1998). The
resulting dwarf varieties could be grown under extensive inputs but with significantly reduced lodging,
which was a recurrent problem with the older, taller varieties that limited their yield potential (Hay
1995; Sinclair 1998). Partitioning of a greater amount of biomass to grain and increased total biomass
production under intensive agricultural practices were the key factors that led to the green revolution.
In contrast to small grain cereals, HI in maize has remained unchanged at approximately 50%
over the last century (Hay 1995; Sinclair 1998; Tollenaar and Wu 1999). Grain yield improvement
has thus resulted from increased total biomass production per unit land area, that has been achieved
through selecting modern hybrids to be productive at increasing planting densities (Tollenaar and
Lee 2002; Duvick 2005a, 2005b). An unchanged HI as the grain yield increased severalfold implies
that the sink/source ratio, unlike in small-grain cereals, was already optimized in maize before the
era of modern breeding (Dhugga 2007).