Environmental Engineering Reference
In-Depth Information
200
11
10
180
9
160
8
140
7
120
6
100
5
N (kg/ha)
Yield (t/ha)
80
4
60
3
1964
1969
1974
1979
1984
1989
1994
1999
2004
Year
FIGure 16.5 N application rates and grain yield of maize in the United States since 1964. (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; U.S. Department of Agriculture/Economic
Research Service, Economic Research Service Database , 2011. Available at http://www.ers.usda.gov/Data/
FertilizerUse/).
70
y = 0.82 x + 40.9
R 2 = 0.59
65
60
55
50
45
40
35
30
1980
1985
1990
1995
2000
2005
2010
Year
FIGure 16.6 N use efficiency (NUE) in the United States since 1981 when N fertilizer application peaked.
Figure derived from data in Figure 16.5.
Planting density (number of plants per unit land area) also increased linearly, matching more or
less an increase in N application up until the mid-1980s. The average density, which was less than
12,000 plants acre -1 in the 1930s, reached 16,000 plants acre -1 in the 1960s, 24,000 plants acre -1 in
the 1980s, and currently stands at approximately 34,000 plants acre -1 . Planting density has kept an
upward trend, however, even after the N application peaked, and still keeps increasing at an average
density of 400 plants acre -1 year -1 , a rate that has been maintained over the last 55 years.
Selection of modern hybrids for adaptation to higher density planting has been accompanied
by several favorable agronomic traits, for example, resistance to stresses such as drought, disease
and insects, and low soil fertility, improving their relative stability to perform consistently in vari-
able and unpredictable environments. Reductions in barrenness and late-season stalk lodging fur-
ther contributed to yield improvement. A number of other adaptive changes accompanied selection
of modern hybrids at higher planting densities: a more acute leaf angle to reduce shading, thus
 
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