Agriculture Reference
In-Depth Information
140 kg ha
−1
in 1985 but leveled off in the 1980s (Duvick, 2005b). Plant density increase has contrib-
uted significantly in corn yield in the United States. Plant density averaged about 30,000 plants ha
−1
in the 1960s, 60,000 plants ha
−1
in the 1980s, and densities at present are typically 80,000 plants ha
−1
or higher in the major corn-producing regions of the country (Duvick, 2005a,b). Duvick (2005b)
concluded that compared with the older hybrids, today's hybrids produce approximately the same
amount of grain per plant, but on considerably more plant per unit area.
1.2.4.2.1.4 Soybean
Soybean is an important legume crop worldwide. There are several uses
of soybean such as oil for human consumption, feed for animals, and food for human consumption.
Brazil, the United States, and Argentina are the largest soybean-producing countries in the world.
These three countries are also the largest soybean-exporting countries in the world. The soybean
production area is second only to that of corn in U.S. agriculture (Villamil et al., 2012). Soybean
yield has improved in the United States through breeding as well as adopting improved management
practices. North American soybean breeding efforts began early in the twentieth century and have
resulted in a dramatic improvement in soybean yield (Carter et al., 2004). In 1924, the national esti-
mates of U.S. soybean yield was just 740 kg ha
−1
, but by 2010, the national yield estimate had almost
quadrupled to 2925 kg ha
−1
, translating into a linear yield of 23.4 kg ha
−1
year
−1
(USDA-NASS, 2012;
Fox et al., 2013). Though the U.S. soybean yield trends have shown a steady 1.5% annual increase
since the 1920s, some evidence indicates that yields of soybean and other crops may have reached
a plateau in highly productive regions worldwide (Egli 2008; Villamil et al., 2012). Results from
experimental sites within the Midwest region show that planting date (De Bruin and Pedersen,
2008a), seeding rate and row spacing (De Bruin and Pedersen, 2008a,b; Cox and Cherney, 2011),
and crop rotation and tillage practices (Pedersen and Lauer, 2004) are among the crucial agronomic
practices that farmers can manipulate to maximize soybean yields.
Contributions to this rise in soybean yield over time include improved crop genetics, increases in
atmospheric CO
2
, extended growing seasons as evidenced by the northern shift of USDA growing
zones (Kaplan, 2012), and the optimizing of the production environment through changes in cultural
practices, advances in planting and harvesting equipment, and improved herbicides and pesticides
(Specht et al., 1999; Ustun et al., 2001; Egli, 2008; Fox et al., 2013). Breeding also contributed to
lodging and disease resistance, improvements in photosynthetic efficiency, and higher nutrient use
efficiency (Wilcox, 2001; Cober et al., 2005; De Bruin and Pedersen, 2009). Kumudi (2002) reported
that the contribution of genetic improvement to overall soybean yield improvement has been esti-
mated to range from 0.5% to 0.7% per year in continental North America. Published estimates of the
annual gain in yield attributable to genetic improvement averaged about 15 kg ha
−1
year
−1
prior to the
1980s but now averaging about 30 kg ha
−1
year
−1
in both the public and private sectors.
In Canada, Morrison et al. (2000) tested 41 cultivars released over seven decades of breed-
ing and selection, and found that the yield improvement had an association with a decrease in
protein concentration and some reduction in lodging. Recent cultivars, compared with older cul-
tivars, had a lower maximum leaf area, and higher photosynthetic rate and stomatal conductance
per unit area (Morrison et al., 1999, 2000). Further studies on population density revealed that
modern cultivars were more tolerant of population stress than older ones (Cober et al., 2005).
Morrison et al. (2008) also reported that seed isoflavone concentration significantly increased
over 58 years of soybean breeding for yield in the short-season region, and modern cultivars were
more environmentally influenced for isoflavone concentration than older cultivars. Isoflavone
was positively associated with N fixation and disease resistance (Zhang and Smith, 1995; Dixon,
20 01).
Soybean yield increased significantly in China through breeding and adopting improved crop
management practices. Xue et al. (2006) reported that the average soybean yield per hectare in the
1990s increased by 71.4%, compared with that of the 1950s, and the annual yield increase aver-
aged 13.4 kg ha
−1
. Similarly, Jin et al. (2010) also investigated improvements in the soybean yield
in Northeast China from 1950 to 2006. These authors concluded that a positive correlation between
