The Rural Voice, 1999-06, Page 54moisture. 1999 has the potential to be
a great crop year.
In some areas we have seen some
moisture stress on spring grains, and
winter wheat that was planted
shallow last fall. If you have been out
scouting your fields, you will have
noticed that the annual grasses and
annual broadleafs are ahead of
schedule. This in some crops, may
present a challenge for spraying for
weed control.
Underseeded spring cereals may
become a real problem to spray. At
this time. the soil moisture has the
grain growing along with the weeds,
but the grass and legumes seeds are
dormant in the dry surface soil. Most
of our sprays should be applied when
the grain is in the two to five -leaf
stage, and when the legumes are in
the one to four -trifoliate, and grasses
in the two -leaf stage of development.
With current growing patterns, this
may not happen at the same time, so
you should double check your field
before spraying. Livestock producers
may want to consider the option of
taking the crop as grainlage as an
alternate method of weed control.
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50 THE RURAL VOICE
Advice
In some crops, changing the weed
control program from one of pre -
emergence to post -emergence, will
also have timing issues to stage of
crop and weed development. Here it
will become more important to read
product labels, and make sure it is a
recent label not one from a year ago.
Crop sensitivity is one thing to
always be careful of, especially if the
crop is under moisture or heat stress.
A new caution we have had to deal
with in the past few years is the
differences in herbicide tolerance
between different varieties. Gone are
the days that you can spray the same
product on all varieties, and only
have to worry about the weed kill.
The Guide to Weed Control 1999
is available at your local OMAFRA
office for $10 plus GST, and will be a
good resource for handling your
weed control for 1999.0
Critical weed control
period in corn
By Greg Stewart
OMAFRA Corn Specialist
Survey results indicate that in
1998, close to 90 per cent of the corn
acreage in Ontario was treated with a
weed control program that employed
at least some postemergent
applications.
It is with this trend in mind that a
review of the importance of timing in
herbicide applications becomes
appropriate. The critical period of
weed control is an important concept
to understand when employing
postemergent herbicides. Dr.
Clarence Swanton, University of
Guelph, has done extensive research
in order to illuminate this concept.
Swanton defines this "critical period"
as an interval in the life cycle of the
crop when it must be kept weed free
to prevent yield loss.
The critical weed control period in
corn is from the fourth to the tenth
leaf stage. This means that weeds
which emerge with the crop have
little effect on corn yields until the
corn gets to the fourth leaf stage.
However, these weeds, if not brought
under control by the fourth leaf stage
start to seriously affect yield. This
weed free period in corn must extend
to the tenth leaf stage. Weeds that
emerge after the corn passes the tenth
leaf stage will generally have little
effect on corn yields given near
normal conditions.
In reviewing the postemergent
herbicides that are, or will be
available for use, keep in mind that a
large application window (i.e. first to
eighth leaf) for applying the herbicide
without damaging the corn plant
should not lead you to believe that
you have that same window before
weed pressure reduces yields.
For example, a producer may
delay applying Roundup to his
Roundup Ready corn because: (a) he
wants to kill more of the later
emerging weeds and (b) because he
realizes that the herbicide presents
little crop damage risk. The result
may in fact be a very clean crop come
harvest time but one which may have
suffered yield losses due to weed
competition in the critical period.
Similarly, having a few weeds
hanging around at a harvest time may
be having no effect on yields if they
emerged after the tenth leaf stage of
the corn crop.
The moral of the story: Know your
critical weed free period and scout
fields to ensure proper timing for
postemergent herbicides.0
Scout crops for profit
By Peter Johnson
OMAFRA Soil & Crop Specialist
Designing a scouting schedule,
and making the right management
and input decisions can help to put
wheat back on the profit map.
The scouting process for wheat is
the most intensive after May 15. This
is the time frame that disease control
decisions come into play, and
decisions must be made quickly.
Scout every two or three days,
especially under warm, moist, humid
conditions that can allow diseases to
explode. Learn to recognize the flag
leaf and "penultimate" or second leaf
down. Keeping these two leaves
essentially free of disease is essential
to maximum yields.
Spray when thresholds of one per
cent disease on the flag leaf, or three
to five per cent disease on the
penultimate leaf are reached. Early
control is essential — spraying when
ten per cent of the flag leaf is
diseased is akin to closing the barn
door after the cattle have all escaped!0