425study in the Northern Great Plains, Allen et al. (2011a) concluded that a convention-
ally tilled wheat-fallow cropping system did not restore soils damaged by erosion
even if yields were maintained with fertilizer application. In a different 30-year
study, Sainju et al. (2015a) reported 18 % lower soil organic C and 20 % greater
bulk density in the surface 7.5 cm soil of a spring wheat-tilled fallow rotation com-
pared to continuous wheat in spring tilled or no-till treatments. In the same experi-
ment, annualized crop yield was 23–30 % lower, and P, K, Zn and CEC were
23–60 % lower in a spring wheat-tilled fallow rotation than for continuously
cropped treatments with or without tillage (Sainju et al. 2015b). These studies high-
light the need for diversifying and intensifying the traditional wheat-summer fallow
rotation in North America dryland cropping. Improved precipitation storage effi-
ciency during fallow through adoption of reduced-till or no-till systems is key to
successfully diversifying and intensifying crop rotations (Black and Power 1965 ;
Tanaka and Aase 1987 ; Peterson et al. 1996 ).
Among the dryland cropping areas of North America, the Northern Great Plains
has been the most successful in adoption of diversified crop rotations. Cropping
systems in the Northern Great Plains have evolved to include fallow replacement
crops ranging from green manure crops to continuously cropped systems that
include forage, oilseed, and pulse crops. Obtaining the advantage of diversified crop
rotations can require time for the system to evolve and for the producer to adapt. In
a simple example, the fallow phase of a 2-year spring wheat-summer fallow rotation
was replaced with lentil grown as a green manure crop in a 12-year study (Allen
et al. 2011b). In that study, low soil nitrate in the green manure treatment during the
first five years resulted in 33 % reduction in spring wheat yield compared to wheat
grown after fallow. However, beginning with the sixth year, wheat yield after the
lentil green manure crop differed by less than 2 % compared to wheat after summer
fallow due to 26 % greater spring soil nitrate and a two-fold increase in overall
nitrogen cycling. The study demonstrates that over time, adopting the diversified
rotation increased sustainability of the system, as evidenced by enhanced nitrogen
cycling in the soil. The study also illustrated the importance of careful management
to achieve success in the diversified rotation. Soil water at the time of wheat plant-
ing was similar for fallow and green manure cropping systems if the lentil green
manure crop was terminated at full bloom. However, if the lentil green manure crop
was allowed to mature, there was a 20 % reduction in soil water at the time of plant-
ing the subsequent wheat crop.
Several other studies document the advantages of crop diversification in the
NGP. Inclusion of an annual forage crop in rotation with wheat has been shown to
improve sustainability. For example, Lenssen et al. ( 2010 ) showed that replacing
fallow in a wheat-fallow rotation with forage barley enhanced water use efficiency
and led to greater profit. Annual forages can be beneficial in dryland systems
because they require less water than grain crops, compete with weeds, and protect
the soil from erosion. A drawback to fallow replacement crops is the possibility that
the subsequent wheat crop will have reduced soil water at planting and reduced
yield. Reduced yields, however, are offset by the overall benefit to the system (Lyon
et al. 2004 ; Lenssen et al. 2010 ). Management practices becomes more critical
Dryland Agriculture in North America