The Rural Voice, 2001-06, Page 50resistant green pigweed growing in
an Ontario carrot field. At the same
time. Karmex resistant Amaranth (a
close relative of pigweed) was
identified in a Michigan asparagus
field. Unfortunately. pigweed
resistance in herbicides like Sencor
and Simazine is nothing new.
These products represent the
backbone of weed control in
asparagus. Should widespread
resistance develop. we do not have a
lot of options to fall back on.
Rotating between chemical
families is the best way to avoid
developing resistance. However, due
to the limited selection of herbicides
in asparagus, this can be challenging.
Karmex and Linuron (Lorox, Afolan)
are from the same herbicide family
(group 7.) Avoid continuous use of
these two products.
Sencor (Lexone), Simazine
(Princep Nine -T, Simadex) and
Sinbar are all group 5 herbicides.
Repeated use of these products
should also be avoided.
Just to make things more difficult,
researchers at Michigan State have
found that weeds with resistance to
group 5 herbicides often show
moderate resistance to Linuron as
well. For this reason. it is a good idea
to have a weed control strategy that
includes the occasional use of a
herbicide that is not from these two
families.
2,4-D provides good control of the
annual broadleaf weeds, and it will
break the dependence on the triazine
and linuron products. Devrinol, while
more expensive than the above-
mentioned products, is also a good
choice for resistance management. It
is strong on the annual grasses, and it
gives good control of both pigweed
and lambsquarters.0
Trying to grow
alfalfa after alfalfa
yields Tess
By Joel Bagg
Forage Specialist, OMAFRA,
Lindsay
Reseeding alfalfa following alfalfa
is not a recommended practice!
Autotoxicity, seedling diseases
46 THE RURAL VOICE
Advice
and insect pests can wreak havoc,
seriously reducing yields. Established
alfalfa plants produce •toxins
(medicarpin) which reduce
establishment and growth of new
alfalfa plants. This autotoxicity
occurs when alfalfa is reseeded into
an old alfalfa field, or when alfalfa is
seeded into an existing alfalfa stand
to thicken it.
Germination & Root Growth
While seed germination and plant
density are reduced by alfalfa
autotoxicity, root growth impacts are
even more severe. Roots are swollen,
curled, discoloured, and lack root
hairs. Mature plants appear to be
more "branch -rooted", rather than
"tap -rooted". These effects on root
growth can significantly impact
yields.
Factors Affecting Autotoxicity
Responses to alfalfa autotoxicity
are variable, depending on many
factors. The water soluble
autotoxicity chemicals are more
concentrated in leaves and stems than
in the roots. Thus removing
topgrowth reduces injury. Tillage can
help reduce the autotoxic effect.
Higher temperatures speed break-
down of the toxic compound, and
rains leach them from the root zone.
Autoxicity may initially be more
severe on light soils, but lasts longer
on heavier soils.
Poor Establishment
There is disagreement on what
minimum period of time is required
betvrcen killing the old stand (either
by plowing or glyphosate
application) and reseeding. Time is
required for the toxins to degrade or
move out of the root zone. While
research has shown that only two or
three weeks is required to eliminate
the effects on germination, the effects
on root development and forage
yields can persist much longer.
Reduced Yields
Studies have shown that yields are
reduced when alfalfa is seeded after
alfalfa, regardless of the waiting
period. This yield reduction persists
for the life of the stand. Stands
affected by autotoxicity regrow
slower after each cutting. Even when
alfalfa was killed in the fall and
reseeded in the spring, yields were
reduced when compared to alfalfa
following corn. At the farm level,
yield reductions from autotoxicity are
difficult to determine, and are often
attributed to other causes. For
maximum yield, if the alfalfa is two
or more years old, an intervening
year of an alternate crop is required
before reseeding to alfalfa.
Reseeding New Seeding Failures
The toxins are not present in new
seedings, so seeding failures can be
reseeded without an autotoxicity
effect. This would include a summer
seeding after an unsuccessful spring
seeding, or a spring seeding after an
unsuccessful summer seeding.
Thickening Old Stands?
This is rarely successful. New
seedlings germinate, look acceptable
initially, and then die out over the
summer. Research shows the area of
autotoxicity influence is a 16 -inch
radius from established plants. A
field with plant density of less than
0.2 plants per square foot (almost
nonexistent) would be required
before interseeding could avoid
autotoxicity. Any stand with more
than 1.3 plants per square foot has a
high risk of failure. In an emergency
situation, thin spots can be
interseeded with red clover. Of
course, interseeding a stand that is
less than one year old is acceptable,
because there is no alfalfa
autotoxicity effect.
One more example of the benefits
of a good crop rotation!O
Fine tune your
nitrogen rates
for best results
By Keith Reid
Soil Fertility Specialist, OMAFRA
How much nitrogen is enough?
Rather than trying to identify an
exact optimum rate, a "partial delta
yield" plot can get you into the right
range. The key to this approach is to
gather the information needed with a
minimum of inconvenience,
complexity and expense from lost
yield.
Laying out the strips:
• Strips need to be repeated at least
three times in each field-tested.
• Strips need to be representative of
the field._