Agriculture Reference
In-Depth Information
or postharvest seed dormancy. These two terms
are often used interchangeably, and some ques-
tion remains as to whether a true distinction exists
between the two. Whatever the terminology used,
it has long been known that some cultivars require
a greater period of after-ripening than others and
as a result do not germinate well when sown early.
The degree of postharvest dormancy is under
both genetic and environmental controls. Inhibi-
tory substances in the red seed coat of some
cultivars, for example, prolong dormancy (Ching
and Foote 1961). Environmental conditions
during both seed maturation and germination
impact seed dormancy. For example, dormancy is
strengthened with decreasing air temperature
during grain-fi ll (Reddy et al., 1985). Likewise
soil temperatures greater than 20 ºC after sowing
will strengthen dormancy, lengthen the after-
ripening requirement, and delay emergence.
While most seed will germinate once soil condi-
tions cool, delayed emergence due to dormancy
will result in less cumulative light interception by
the crop canopy and reduced total forage produc-
tion. For this reason, cultivars with strong after-
ripening requirements are frequently avoided for
dual-purpose production systems.
A semierect growth habit will allow wheat to
be easily grazed by cattle without extensive
damage to the wheat plant. Cultivars that are too
erect generally do not tolerate grazing very well,
as much of the green leaf area and photosynthetic
capacity of the plant is removed by cattle during
the grazing process, which can reduce subsequent
grain yield (Arzadun et al., 2003). Conversely,
cultivars that are too prostrate frequently have
less available forage than more erect cultivars.
While these cultivars recover from grazing well,
they do not typically provide adequate forage pro-
duction. So, an intermediate or semierect to semi-
prostrate growth habit is desirable for dual-purpose
wheat cultivars.
Characterizing a suitable
dual-purpose cultivar
The requirements for a dual-purpose wheat cul-
tivar are different than for grain-only cultivars.
A suitable dual-purpose wheat cultivar needs to
have the ability to germinate well under hot soil
conditions, a semierect growth habit, prolifi c
tillering ability, and nonprecocious arrival of
fi rst-hollow-stem stage. Dual-purpose wheat
production requires rapid accumulation of plant
biomass in a narrow window of time. Plant
biomass accumulation is a function of cumulative
light interception by the crop canopy, so wheat
producers using the dual-purpose wheat produc-
tion system place great emphasis on rapid emer-
gence and vigorous growth early in the fall. To
aid in rapid canopy closure and establishment of
leaf area, seeding densities are generally much
higher in dual-purpose than in grain-only systems.
In fact, seeding densities of 4-5 times the normal
recommended seeding density have been shown
profi table in dual-purpose systems (unpublished
data).
While important, rapid plant emergence is
sometimes diffi cult to obtain in dual-purpose
systems. Soil temperatures during September can
often exceed 35 ºC in geographic areas where
dual-purpose production is common. There is an
inverse relationship between soil temperature
during germination and the coleoptile length of
wheat. As the coleoptile of modern semidwarf
wheat cultivars is already shortened relative to
older, taller cultivars, further shortening of the
coleoptile due to hot soil conditions can dictate
optimal sowing depth. If the coleoptile does
not protrude through the soil surface and the
fi rst true leaf emerges below the soil surface,
complete stand loss frequently occurs (see Color
Plate 6). For this reason, producers sowing wheat
early into hot soil conditions often choose to sow
wheat at a depth of 2.5 cm or less with anticipa-
tion for rainfall, rather than sow deeper
to reach suffi cient moisture to immediately
induce germination.
Another issue associated with hot soil condi-
tions is high-temperature germination sensitivity
Fertility management
Fertilization practices in dual-purpose wheat are
much more reliant on preplant applications of
nitrogen fertilizer than grain-only systems. There
