A24-Lancast»r Farming, Saturday, Fabruary 17,1998
Crops Day Discussions
(Continued from Pago A 1)
Speakers during the morning
portion of the all-day meeting were
John Yocum, manager of the
Landisville Penn State Research
Station. Dr. Elwood Hatley, a Penn
State agronomist, and Bob Ander
son, a Lancaster County extension
agent
Yocum discussed weed herbi
cide resistance, and strategies far
mers can use to avoid propogating
fields of herbicide-resistant weed
strains.
He also talked about new herbi
cides for ttye year, labeling
changes, and difficult weeds.
Hatley discussed com variety
selection, especially selecting
hybrids for silage. Basically, he
said that there are some hybrid
advantages for making silage, but
farmers who use them most likely
find they have to get used to raising
the plants, as traditional eyeballing
clues some farmers used to deter
mine readiness for harvest aren’t
dependable with the hybrids.
Anderson announced some
county winners of the state S-acre
com club contest, though awards
were originally announced during
the Pennsylvania Crops Confer
ence and reported in the Feb. 3
issue of Lancaster Farming.
Yocum said that several crop
ping practices can set up an ideal
setting for developing herbicide
resistance in weeds, and those are
the ones that farmers should strive
to avoid in order to prevent that
from occuring in their fields.
He said that rotating chemistry
and using strategic tank mixes can
reduce the potential to create
resistance.
Using a slide presentation he
helped develop, Yocum said that a
growing layman opinion is that
herbicides are causing mutations
in weeds and thus creating resis
tant strains.
That is not the case, he said. The
resistance is showing up in plant
species that already have the gene
tic potential to be resistant
He said that the way resistance
is developed, a plant that repro
duces using a high number of
seeds, such as most annuals, is
treated with an herbicide that has a
specific killing strategy.
What he meant by a specific
killing strategy is that some herbi
cides are created to interfere with a
specific metabolic function in a
plant, such as blocking photosyn
thesis (photosynthesis is the basic
function of a plant converting solar
energy into stored energy).
A photosynthetic-blocking
chemical would bind with a chemi
cal in the plant integral to photo
synthesis, thereby blocking photo
synthesis and basically robbing the
plant of its ability to make its own
food.
Within the plant population
there arc subtle variations in some
of the photosynthetic chemicals,
and while the majority of plants
representative of that species are
affected by the herbicide, those
individual plants with an inherent
chemical variation that is not
affected by the herbicide are the
ones that survive and provide the
genetic material for the next
population.
What happens is that the farmer
who does not change his herbicide
chemistry and continues the same
cropping and herbicide practices in
same fields year after year, eventu
ally will (given that the plant has
the genetic variability to resist and
enough time) foster an herbicide
resistant strain.
The herbicide chemicals them-
selves aren’t causing the resis
tance, Yocum said, that ability is
already existant in die genetic pool
of the plant species, it just needs a
farmer to create a situation that
rewards that genetic variation.
It has been the observation of
some that nature fills a vacuum.
Others have described it more as
filling an empty niche.
When herbicides are used exten
sively and repeatedly in the same
area against the same plants, with
out variation, it creates an empty
niche. The plant able to resist the
herbicide grows in number and can
quickly take over and cause a high
population of resistant plants.
The way to get the most effect
out of an herbicide that works is to
work it into the program so that it
kills effectively one year, then
allow a reprieve.^
While it might sound perverse,
the idea is to not destroy the parent
weed stock that is susceptible to
the herbicide. If the susceptible
parent stock-is all killed off, then
only the resistant stock is left to
take over, and it will.
If susceptible stock is allowed to
remain, it’s more probable that
since it is already the dominant
stock, it would help suppress the
resistant variation.
According to Yocum, there are
severl conditions that lead to an
high risk of developing resistant
weed stock.
• The seed of the weed can’t
have a long dormancy. It can’t be
able to survive more than a couple
seasons without germinating. If it
would, then susceptible stock
would mix in with the resistant
stock and probably limit the
expression of resistance.
• The weed must have a high
susceptibility to the herbicide.
Again, this kills off the suscepti
ble, dominant variety and allows
the resistant strain to populate.
Also it must a high frequency of
resistance. In other words, the
chances of an individual weed
plant carrying a trait for resistance
should be more like 1 million,
rather than 1; trill ion.
• The herbicide must be used at
the same site for a number years
without change.
• The herbicide must have a
long residual efficacy, meaning
that it doesn’t break down quickly
and allow susceptible plants to
survive.
• The amount of herbicide used
must be high relative to the amount
needed to do the job. This sounds
as though farmers might be pur
posefully using herbicides way
beyond recommended levels, but
frequently when using an herbi
cide to kill a variety of susceptible
weed species, a minimum rate of
application needed to kill one spe
cies may actually be twice what is
needed for another species.
If all the above conditions
describe current farming practices,
they should be changed
Those herbicides that attack
plants by interfetring with several
plant functions are considered bet
ter at not developing resistance,
because total resistance would
require selection for several diffe
rent resistant traits.
By using a combination of crop
rotations (not only fields, but crops
with different seasonal life cycles),
herbicide rotations, and care when
introducing machinery into fields
so as not to introduce resistant
weed seed there is good chance of
not developing herbicide
resistance.
Yocum said that farmers really
need to pay attention to those
plants that do escape and survive
In a public meeting held in conjunction with the Lancaster County Crops and Soils
Day, Karl Brown, executive director of the State Conservation Commission, reads a
question from the audience about proposed regulations to implement the state’s
Nutrient Management Act. Sitting on stage in the background and fielding questions
are some of the members of the SCC Nutrient Management Advisory Board and offi- -
dais with the departments of Environmental Protection and Agriculture.
after V«ng treated. Without paying cany it through from season to sea- After a presentation explaining
attention to potentially low popu- son. the more the opportunity for the regulatory package proposed
lations of what may be an resistance to develop. and recommended by the advisory
herbicide-resistant weed, it can The goal of the former then is to board to implement the state Nutri
quickly spread and overwhelm the integrate cultural, chemical and ent Management Act, staff col
field before the former realizes he mechanical cropping techniques to lected question's that members of
has a resistant population. diminish the possibility of creat- the audience wrote down on index
Generally Yocum said, “You ing that scenario. cards about specifics of the
don’t realize resistance until about Yocum also reviewed some of regulations,
a third of the population survives.” the herbicide labeling changes and The meeting was the last in a
Yocum showed a slide of a table newly approved chemicals. series of educational public meet
that compared estimated years of The test of the meeting was ings the advisory board, SCC staff,
herbicide use until weed restis- devoted to a presentation by mem- PDA and DEP officials held
rnnn». would probably occur. bets of the State Conservation around the state.
The purpose of the table was to Commission Nutrient Manage- A series of public hearings to
illustrate and reemphasize the les- ment Advrsory Board, staffof the comments on the proposed
son that the simpler the function of SCC. and support suff from the moas about the begin. A
an herbicide and die more the state Department of Agriculture is scheduled for the* Farm
plant species depends on die pro- andthestateDepartmcntofEnvir- and f £ mc Cen|er for Mareh n
duction of large amounts of seed to onmental Protection.
Weed Knowledge Is Weed Control
VERNON ACHENBACH JR.
Lancaster Fanning Staff
LEBANON (Lebanon
Co.) If a weed can’t be identi
fied within days, perhaps hours of
emerging, then determining the
most efficient and least weed man
agement program is difficult if not
impossible.
While many people can identify
adult specimens of weed species,
that’s way too late to determine
what control program may be
needed to prevent losses.
The solution is to become adept
at identifying weeds soon after
they emerge, according to Del
Voight, a Penn State Extension
agronomic agent for Lebanon
County.
There are several reasons that
justify making the effort to learn
how to identify immature weeds
and they all have to do with design
ing a professional-quality weed
management program.
Seed identification can be help
ful in some instances, but mostly
seed knowledge only needs to be
limited to a general understanding
of how each weed species repro
duces, the hardiness and germina
tion of seeds.
There is no need to memorize all
the weed species and life cycles,
because there are good references
available, but it is important to use
them. The more those references
are used and weeds identified, the
faster and more efficient the far
mer’s management program can
be applied.
Voight said that to start a solid,
workable weed control program,
farmers should scout and map their
acres for weed species.
This can be done up to just
before com canopy, or during
combining. Preferably, scouting
and mapping should not be an iso
lated event, but should occur with
updating on every trip to the
field if something looks diffe
rent, a new plant, etc., write it
done.
Also map out nearby “weed
banks” areas on or off the farm
that have undesirable plants of
which the seeds could be moved by
wind, water or wildlife onto the
cropland.
The soil itself could be a prodi
gious weed seed bank, and some
common weed seeds remain viable
in the soil for 40 years or more,
which is why tilling can be a prob
lem in some areas.
There is a technique for deter
mining the weed seed-bank profile
of a soil. Taking a measured
sample of a known depth of soil,
putting it into a container and
adding water and a surfactant
(such as a soap, which prevents the
soil from adhering to die seeds), it
can be estimated how much of a
potential problem is in the soil
already.
Identifying the seeds can show
relative populations of seeds, but it
shouldn’t be used to plan a specific
control prbgram.
Since the emergence of weeds
from soil seed is unpredictable, it
is considered foolish to attempt to
treat a field with a pre-emergence
herbicide to control a weed sus
pected of being a problem based
soley on the soil seed population.
However, a soil seed analysis
can help determine what kinds of
weeds are potentially there, which
ones potentially pose a problem,
and whether or not tillage will help
or hurt control unwanted growth.
Voight said for example, one
plant of redroot pigweed produces
about 117,000 seeds. Not only
does this show an opportunity of
the plant to quickly spread out, but
also indicates a stronger opportun
ity to develop herbicide resistance.
Other seeds, such as the large,
heavily coated burcucumber seeds
may not be as prolific, but are easi
ly moved around by tilling, tough
er to kill with pre-emergent herbi
cide, and can germinate at a wide
variety of depths.
Further, burcucumber germi
nates all summer long and if tillage
distributes the seeds. throughout
the sod depth, plants can continue
to emerge even after an applied
post-emergence herbicide such as
atrazine has been applied and loses
effectiveness.
The goal of the weed control
program should be to control the
wild growth of plants in a field
intended for domestic plant
production.
A weed control program has to
take into account the types of
plants (crops) intended to be
grown, They also should be con
sidered according to plant life
cycle.
For example, there are summer
annuals such as com, winter annu
als such as winter wheat, and sum
mer and winter biennials and
perenials.
Each type of plant and its season
of growth should be listed accord
ing to category.
The weeds in a field should also
be identified and catagorized
according to life cycle and
seasonality. «
For example, pokeweed is a
perennial, wild mustard can be a
winter annual, wild carrot is a
biennial.
Voight cited two publications
which are low cost and can provide
Pennsylvanians with a good key to
weed identification.
He said a publication available
through Penn Slate Extension is
the “Common Weed Seedlings of
Michigan.”
Produced by Michigan State
University Extension, the 16-page
key provides color photographs of
33 of the most common problem
(Turn le Pag* A 25)