Agriculture Reference
In-Depth Information
observed in soybean or wheat. The modest effect of biodiversity on corn produc-
tivity may have been influenced by N fertilization: species diversity is known to
enhance overall productivity in infertile plant communities (Tilman et al., 2001),
but would have a minimal effect in a nutrient-enriched environment such as fertil-
ized corn. Also surprisingly, as for corn in the MCSE, diversity had no effect on the
variability of grain yield over time: the coefficient of variation over 12 years was
37% in both continuous corn and in the diversified systems for both the Organic and
Integrated Conventional systems (Snapp et al. 2010a).
Management other than rotational diversity also influences grain yield. The
average grain yield in monoculture corn in the Organic system was 5.0 Mg ha
−1
yr
−1
,
22% lower than monoculture corn in the Integrated Conventional system (Snapp
et al. 2010a). Similarly, a 22-year European trial showed that organic management
was associated with yields ~20% lower than conventional across a range of crop
species (Maeder et al. 2002). In the Mid-Atlantic region of the U. S., Cavigelli et al.
(2008) documented >30% yield reductions in organic vs. conventionally managed
crops. This yield reduction is not surprising, as management intensity and reliance
on external inputs are generally associated with high crop yields. In the adjacent
MCSE systems, corn grain yields in the Conventional and No-till systems were
higher than in the Reduced Input and Biologically Based systems over the period
1989-2007 (Table 15.2), although overall grain yield of the corn-soybean-wheat
rotation in the Reduced Input system equaled that of the Conventional system when
averaged over 1996-2007 (Table 15.3).
Moderate yield reductions under organic management are typically compensated
for by market premiums. A profitability analysis of a long-term trial in Wisconsin
showed an 85 to 110% increase in profit for organically managed grain crop sys-
tems when organic price premiums were included (Chavas et al. 2009). Jolejole
(2009) found a similar result in an analysis of the MCSE systems: the Biologically
Based system was more profitable than the Conventional system when assigned
premium prices; otherwise, lower yields and higher labor and cover crop costs off-
set savings in chemical use.
Grain yield in LFL systems varied markedly with year (Snapp et al. 2010a).
Under organic management, where N supply is often limiting, the highest corn
yield was obtained >58% of the time in the six-species system (with the legumes
red clover and crimson clover). Dry summer conditions in lower-yielding years and
the well-drained nature of the site may explain why diversity did not support high
corn yields in water-deficient years (Snapp et al. 2010a). Weed competition has
also been markedly variable over time and may have contributed to a low corn yield
response in some years, despite management designed to control weeds (see below;
Smith and Gross 2006a).
Soil Carbon and Phosphorus
Soil organic matter and fertility are key supporting services in agriculture.
Management and diversity both affected LFL soil resources. Organic management
maintained inorganic phosphorus and enhanced soil organic C by 52%, compared
to initial values at the onset of the experiment (Table 15.4). Integrated conventional
