Environmental Engineering Reference
In-Depth Information
his experiments at the Connecticut Agricultural Experiment Station also observed that repeated
selfing caused a reduction in performance but was of the opinion that, given the poor performance
of inbreds, it would be impractical to produce enough seed for commercial use. A solution was
provided by Donald Jones, a graduate student of East, in 1922, in the form of double cross hybrids,
that is, a cross between two hybrids, to overcome the seed production bottleneck. Soon thereafter,
Henry A. Wallace introduced the first commercial hybrid, Copper Cross, a single cross hybrid, in
1924 in the Corn Belt. He went onto found a corn seed company, Hi-Bred Corn Company, which
sold 650 bu of corn seed in its first year but eventually went on to become the largest and premier
seed company in the world and is currently known as Pioneer Hi-Bred International, Inc., a DuPont
Business. It was in the 1930s, however, that farmers adopted the practice of growing hybrids instead
of OPV. By 1943, all of the corn acreage in Iowa was planted to hybrids, which contrasted with that
of 90% of the Corn Belt and 60% of the area in the entire U.S.
Initial inbred lines developed from landraces were not very productive so the breeders started
developing inbred lines from the crosses between two elite lines. Pedigree selection for develop-
ment of inbred lines from biparental crosses constituted less than 40% of the effort up until the
1970s, with the rest of the inbred lines coming from populations of various types, that is, geneti-
cally broad or narrow, or those generated from a mixture of elite inbred lines. The shift to the
use of biparental crosses for inbred line development over the last forty years is exemplified by
the fact that they account for nearly 80% of the current inbred parents of hybrids across the seed
industry.
Coincident with the introduction and acceptance of hybrids in the 1930s, grain yield started to
increase at a rate of ~5% year
-1
(Figures 16.3 and 16.4). Increased corn production resulted both
from an increase in grain yield per unit area as well as increased land area planted to hybrids.
Application of synthetic N as a fertilizer picked up after World War II, which further boosted
grain yield (Figure 16.5). Whereas fertilizer application peaked in the 1980s, grain yield kept an
upward trend (Figure 16.5). N use efficiency is defined as grain yield as a function of soil N. It has
been increasing linearly since the time N fertilizer application peaked, having improved by more
than 50% over twenty five years (from ~40 kg kg
-1
N in 1980 to 60 kg kg
-1
N in 2005) (Figure 16.6).
A combination of improved N acquisition from the soil and its utilization by modern hybrids as well
as improved cultural practices apparently contribute to improved N use efficiency.
9
8
7
6
5
4
3
2
1
0
1930-1950
1951-1970
1971-1990
1990-2008
Year range
FIGure 16.4
Rate of gain for grain yield during different eras as measured over 2-decade windows start-
ing in 1930. (From U.S. Department of Agriculture/National Agricultural Statistics Service,
Agricultural
Statistics Database
, 2009. Available at http://www.fas.usda.gov/wap/current/toc.asp)
