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The Canadian Prairies and the U.S. Northern Great Plains have long, cold win-
ters and short, warm summers. In this area, spring wheat is the most commonly
produced crop. The traditional rotation of spring wheat-summer fallow has largely
been replaced with rotations that substitute fallow with cool season, annual crops
that mature early in summer and are well adapted for the region. For instance, in NE
Montana and NW North Dakota fallow land area decreased by nearly fourfold (1.2
to 0.3 million ha) from 1990 to 2015, with only 12 % of the total cropped area in
fallow during 2015. Pulses, such as lentil (Lens culinaris Medik.), field pea, chick-
pea (Cicer arietinum L.), and faba-bean (Vicia faba L.), and oilseed crops, such as
canola (Brassica napus L.) and sunflower (Helianthus annuus L.), are commonly
grown in rotation with spring wheat. No-till is practiced on more than half of the
dryland area (Hansen et al. 2012 ). Compared to the Central and Southern Great
Plains, the Northern areas have more diverse crop rotations and higher adoption of
no-till, largely due to lower potential evapotranspiration in summer months.
In the central Great Plains, the prevailing cropping system is a two-year rotation
of winter wheat and summer fallow. In this traditional practice, shallow tillage is
used during fallow periods to control weeds and help store moisture in the soil.
Sustainability of this practice is limited by soil degradation and erosion and poor
water use efficiency. The adoption of no-till practices on more than 25 % of the dry-
lands has resulted in greater precipitation storage and more diverse crop rotations.
For example, a no-till rotation of winter wheat-maize-fallow increased total annual-
ized grain yield by 75 % compared to winter wheat – summer fallow. Soil erosion
was reduced to just 25 % of that from a conventional tillage wheat-summer fallow
system (Peterson et al. 1996 , 1998 ). Other crops rotated with wheat in the Central
Great Plains include sorghum, sunflower, and proso millet (Panicum mileaceum L.).
In the Southern Great Plains, the hotter climate and longer growing seasons com-
pared to the Northern and Central Great Plains result in much greater ETP and more
arid conditions. Winter wheat is commonly grown in rotation with grain sorghum
(Baumhardt and Salines-Garcia 2006 ). Annual small grains are often produced as
dual purpose crops for both grazing and grain production. Other dryland crops
include maize, cotton (Gossypium hirsutum L.), and proso millet. No-till adoption
is limited due to lack of residue accumulation and residue removal from grazing.
The dryland farming area of the Inland Pacific Northwest (IPNW) is distinct
from the Great Plains. It has a semi-arid Mediterranean-like climate with cool wet
winters and warm dry summers. Three quarters of the precipitation in this region is
received from September to April. Inland PNW is broadly classified into three pre-
cipitation zones and dryland winter-wheat (Triticum aestivum L.)-based production
systems are the predominant cropping systems (Schillinger and Papendick 2008 ).
Low precipitation zone (< 300 mm annual precipitation) covers 1.56 × 10^6 ha and
more than 90 % of this zone is grown mostly to winter wheat under a winter wheat-
summer fallow system in which one crop is grown every 2 years. In the intermediate
precipitation zone with 300–450 mm annual precipitation (0.97 × 10^6 ha) the tradi-
tional wheat-summer fallow system and 3-year rotation of winter wheat – spring
barley (Hordeum vulgare L.) – fallow in drier and wetter regions, respectively, are
prevalent in this zone. In addition, several new crops such as spring legume crops
N.C. Hansen et al.