Agriculture Reference
In-Depth Information
Conventional management. This stands in contrast to soybean yields, for which
the Biologically Based system is equivalent to the Conventional system (Fig. 2.1).
Rotational diversity clearly matters to the delivery of ecosystem services, includ-
ing yield (Smith et al. 2008). A characteristic of intensive row-crop agriculture is its
severe reduction of plant diversity of both crops and weeds. The conventional norm
for most grain and other major commodity crops in the United States is weed-free
monocultures or simple two-crop rotations. In the U.S. Midwest, corn is grown in
a corn-soybean rotation on ~60% of corn acreage and in a continuous corn-only
rotation on ~25% (Osteen et al. 2012). Simplified rotations date from the onset of
highly mechanized agriculture in the 1940s. Until 1996 U.S. farm subsidies were
linked to the area planted in selected crops (notably, wheat, corn, and other feed
grains), which tended to encourage simplified rotations. Today, there are two fed-
eral programs that favor simpler rotations. The most important one is the 2007 leg-
islative mandate to blend grain-based ethanol—made entirely from corn—into the
national gasoline supply. This raises demand for corn and therefore its price, creat-
ing an incentive to increase its presence in crop rotations. The second is crop insur-
ance subsidies that reduce farmer incentives to manage risk through crop diversity.
Simplified rotations and larger fields lead to simplified landscapes, because
total cropland becomes constrained to two or three dominant species in ever-larger
patches (Meehan et al. 2011, Wright and Wimberly 2013). Plant diversity is fur-
ther constrained by increasingly effective weed control, with chemical technologies
dating from the 1950s and genomic technologies dating from the 1990s. In 2011,
94% of U.S. soybean acreage and 70% of U.S. corn acreage were planted with
herbicide-resistant varieties (Osteen et al. 2012).
Reduced plant diversity at both the field and the landscape scales can have neg-
ative consequences for many other taxa—most notably, arthropods; vertebrates;
and, possibly, microbes and other soil organisms. The loss of these taxa can have
important effects on community structure and dynamics—most notably on species
extinctions and changes in trophic structure that can affect pest suppression—and
on ecosystem processes, such as carbon flow and nitrogen cycling. To what extent
might greater rotational complexity provide these important ecosystem services?
That continuous monocultures suffer a yield penalty that persists even in the
presence of modern chemicals is well known. For millennia, agriculturalists have
used multispecies rotations to improve yields by advancing soil fertility and sup-
pressing pests and pathogens (Karlen et al. 1994, Bennett et al. 2012). Since the
1950s, monoculture penalties in grain crops have been largely ameliorated with
chemical fertilizers and pesticides; the remaining penalties, which appear mainly
from soil pathogens or other microbial factors (Bennett et al. 2012), are largely
addressable with simple two-species rotations, such as corn and soybean.
To what extent might the restoration of rotational complexity in row crops sub-
stitute for today's use of external inputs? This is a fundamental question that under-
pins the success of low chemical input farming. As was noted above, the inclusion
of legume cover crops plus mechanical weed control in our Reduced Input corn-
soybean-wheat rotation alleviated the need for two-thirds of the synthetic nitrogen
and herbicide inputs otherwise required for high yields (Fig. 2.1). Can rotational
complexity substitute for the provision of all synthetic inputs? In our Biologically
