Agriculture Reference
In-Depth Information
moisture in the top 1.8 m of the soil profi le at
wheat planting was 9% and 20% less for wheat
following soybean or sunfl ower than for wheat
following maize or grain sorghum. Baumhardt
et al. (1993), however, found no differences in
water infi ltration when comparing wheat tillage
treatments. Similarly, Unger (1994) found that
tillage had no effect on wheat yield or water
storage. In areas that frequently experience
extreme water-defi cit stress, such as the Middle
East, no-till has resulted in greater soil water
content in the top 30 cm of soil, which resulted
in signifi cantly greater seedling survival in this
harsh environment (Klein et al., 2002).
Long-term experiments
As time progresses more critical data is emerging
from long-term no-till experiments. In a 41-year
study Tarkalson et al. (2006) found that wheat
yields were 12% higher in no-till than conven-
tional till systems, and grain sorghum (
Sorghum
bicolor
L.) yields were 35% higher. They attrib-
uted most of this effect to increased soil moisture
retention. Rooting characteristics also play a part
in the success of no-till systems. In Switzerland,
Qin et al. (2004) found that wheat root density
was actually higher in the top 5 cm of soil in no-
till systems than in conventional-till systems, but
lower from 10 to 30 cm soil depth. Overall the
root density was lower in no-till systems, but root
diameter was larger and root density in the planted
row was greater than in the conventional-till
system. Increased root diameter and the resulting
root channels after decomposition of old roots
likely play a leading role in improved water infi l-
tration and bulk density of soils in long-term
no-till systems and the amount of carbon they
sequester.
Much of the no-till wheat research over the
past 20 years has emphasized crop rotation.
Studies have shown that crop rotations generally
create synergistic effects on yield among the
crops, but occasional antagonistic effects have
been observed (Tanaka et al., 2007). Kelley and
Sweeney (2007), for example, observed that crop
sequence signifi cantly infl uenced wheat yield. In
their 4-year experiment, wheat following soybean
or maize produced greater yield than wheat fol-
lowing grain sorghum. Increased nitrogen fertil-
ization, however, decreased the yield differential
among the rotational sequences. Their work indi-
cated greater nitrogen effi ciency can be obtained
in no-till systems by subsurface banding of nitro-
gen compared to broadcast applications, leading
them to conclude that nitrogen immobilization
probably had more negative effect on wheat yield
than any allelopathic effects associated with grain
sorghum.
There is some debate about the infl uence of
no-till production systems on water infi ltration
and storage. Norwood (2000) found that soil
FUTURE PERSPECTIVES
Although we now know much more about wheat
production systems and how they interact than
we did just 20 years ago, considerable research
and extension work lies ahead. For example, there
is a need to continue to focus on improved
wheat cultivars that respond to increased inputs
in intensive management systems. Increases in
wheat yield through intensive wheat management
systems were fi rst made possible through increased
harvest index and then through increased total
biomass (Shearman et al., 2005). Most recently,
improvements in postanthesis radiation-use
effi ciency have been observed in some cultivars
(Shearman et al., 2005). Further advances in
breeding for increased radiation-use effi ciency
and more partitioning of assimilate to grain would
likely result in advances in grain yield. Those
increases, however, would likely require even
more nitrogen fertilizer. For long-term sustain-
ability, advances in crop nitrogen-use effi ciency
will also be needed to sustain productivity.
Future intensifi cation might arise as the result
of intensifi cation of the cropping system as a
whole rather than intensifi cation of wheat pro-
duction alone. Shorter-season wheat cultivars
offer opportunity for double-cropping in areas
previously not considered to be a viable option.
When combined with shorter-season soybean
cultivars and sorghum and maize hybrids, these
