Agriculture Reference
In-Depth Information
levels, suppression of grain nitrogen content
increased as the size of the competing weed
species increased (Thompson and Woodward
1994). Mason and Madin (1996) proposed that
the effect of weeds on wheat grain protein is
complex and depends on the interspecifi c compe-
tition for nitrogen and water at the end of the
season and the relative importance of the compe-
tition for nitrogen and water in a particular
situation.
Test weight is a critical wheat trade factor and
serves as an indicator of milling quality. Wheat
grain from plots infested with wild mustard [
Bras-
sica kaber
(DC.) L.C. Wheeler] had higher test
weight than from plots where wild mustard was
controlled with herbicides or by hand weeding.
The wheat kernels from weedy plots were also
more uniform in size than those from plots where
weeds were controlled (Burrows and Olson 1955).
These unexpected fi ndings likely refl ected
reduced tiller density of wheat in the presence of
wild mustard, which resulted in fewer kernels per
unit area but more uniform kernel size.
Weed seeds frequently contribute to grain
dockage levels (Zimdahl 1990; Donald and Ogg
1991; Justice et al., 1994; Koscelny and Peeper
1997). Dockage consists of nonmillable material
such as weed seeds, chaff, stems, and stones that
can be removed from grain because it differs from
grain in weight, size, or both (Webb et al., 1995).
Dockage increases the chance of heating and dete-
rioration of stored grain by supporting infesta-
tions of insects, microbes, or both (Jian et al.,
2005). Populations of
Liposcelis bostrychophilus
Badonel, a cosmopolitan insect pest of stored
grain, increased the most (10- to 16-fold) on
cracked wheat grain, screenings that included
various weed seeds, and wild buckwheat (
Polygo-
num convolvulus
L.) seed (Mills et al., 1992).
Insects damage wheat grain by direct feeding,
which reduces grain quality through reduction in
grain weight, nutritional value, or germination.
They also spread and encourage mold germina-
tion and increase grain rancidity. The presence of
live insects or insect-damaged kernels can lower
grain grade, which reduces the value of the grain
in the marketplace.
Some weeds and volunteer or feral small grains
contribute to foreign material levels in wheat
(Dexter et al., 1984; Wray 1993; White et al.,
2006). Foreign material is more diffi cult and
costly to remove from grain than dockage material
because it is similar to wheat in weight, size, and
shape (Webb et al., 1995). The milling and/or
baking performance of spring wheat was compro-
mised by the presence of barley (
Hordeum vulgare
L.), wild oat, and domestic oat (
Avena sativa
L.)
(Dexter et al., 1984). Foreign material in wheat
quickly affects grain grade and consequently the
value of the grain in the marketplace.
Weed seed contamination in wheat products
can result in loss of export market access (P.
Berglund, pers. comm.). For example, wild buck-
wheat seed can result in discolored specks in
durum wheat products and has resulted in loss of
market access to Japan for US growers. Wild oat
in spring wheat fl our can resemble insects in
puffed wheat products and has caused loss of
market access to South Korea.
Rainfall that occurs as wheat is ripening in the
fi eld often encourages weed growth that can chal-
lenge harvest operations. Weeds can increase the
moisture content of harvested grain, and they
reduce harvest speed and grain cleaning effi ciency
that may result in increased dockage and foreign
material. Herbicides may be used prior to harvest
to desiccate weeds and facilitate harvest; however,
some herbicides used for this purpose can affect
grain, fl our, and breadmaking quality of hard red
spring wheat (Manthey et al., 2004). Glyphosate
applied at the hard dough stage can affect rheo-
logical properties and breadmaking quality of
hard red spring wheat.
Another example of weeds indirectly causing a
loss in wheat grain quality occurred in 1991, when
teliospores of
Sporisorium neglecta
from infected
heads of yellow foxtail [
Setaria glauca
(L.) Beauv.]
growing in a Manitoba wheat fi eld resulted in
grain discoloration and a lower grain grade because
more than 5% of the kernels were “naturally
stained” (Thomas et al., 1998). Smut spores from
weeds may also be a problem if they are confused
with quarantined species such as karnal bunt
(
Tilletia indica
Mitra).
