Agriculture Reference
In-Depth Information
this is released when the crop dies. Some forage crops produce a large amount of green
matter that serves as a green manure. These nutrients do not require transport or appli-
cation. They are already in the soil where the crop is to be planted. The disadvantage is
that an entire growing season is needed to form this extra reserve of nutrients.
10.13
WEED, INSECT, AND DISEASE CONTROL
Weeds, insects, and diseases are three dangers that can rob a farmer of all the work and
investment needed to plant a crop. Typically, soil preparation and planting require
about 60 percent of the total investment in a crop. All of this can be completely lost
by poor weed management or by unkind winds bringing insects or a disease to the
field. There are three types of response a farmer may have to these threats: do
nothing, preventive treatments, or curative treatments.
10.13.1 Do Nothing
This may seem foolish at first glance but for most insect and disease problems there is
an “economic threshold.” There are always insects and diseases present in any given
field. It is impractical, and sometimes even unwise, to attempt to eradicate the
problem insects or disease. For example, any attempt to control the European corn
borer will involve some cost and/or labor. If chemical insecticides are used, the chemi-
cals must be purchased and applied to the field. If traditional methods are used, they still
involve the time to prepare the treatment and to apply it. With very light infestations
(below the economic threshold), the loss of yield will be less than the cost of chemicals
to prevent the loss. It is more economical not to control a few borers than to treat the
entire field.
Chemical treatments applied to leaves also have the disadvantage of being
indiscriminate: They kill beneficial and damaging insects. Many insects in a field are
predators on damaging insects. When the predators are killed, other insects they had
controlled will multiply without checks and may cause other problems. So a farmer
may be economically better off not to control a light infestation of a damaging insect
with chemical insecticides.
Treatment of diseases is handled in a manner similar to that for insects. Diseases
are very dependent on a source of spores, wind to spread the spores, and humidity
for the spores to infect susceptible plants. If these conditions are not met, very little
disease will develop. There are normally spores, or sources of infection, of many dis-
eases present in a field. A disease becomes economically important when the weather
conditions are right for its spread and multiplication. As with insects, farmers should
only worry about most diseases when the economic threshold of infection is reached.
Doing nothing for a weed problem is normally not a solution. Once weeds have
become established they will continue to grow and reduce crop yields. Weeds
become less of a problem after the crop has grown large enough so that the leaves
completely shade the soil. Once the leaf canopy has closed over weeds, vital sunlight
is reduced and most weeds slowly die. In the critical stage between germination and
