PESTS AND PEST CONTROL - Agrochemical and Pesticide Safety - page 93

Herbicide selectivity may vary according to the application rate. High
rates of selective herbicides usually will injure all plants at the application
site. Some nonselective herbicides can be used selectively by applying them
at a lower rate. Other factors that affect selectivity include the time and
method of application, environmental conditions, and the stage of plant
growth.

Several factors affect a plant's susceptibility to herbicides:

Growi ng Point s —Thos e that are sheat hed or locat ed below the soil
surface are not reached by contact herbicide sprays.

Leaf Shape —Herbicides tend to bounce or run off narrow, upright
leaves. Broad, flat leaves tend to hold the herbicide longer.

Wax and Cuticle —Foliar sprays may be prevented from entering the
leaf by a thick wax and cuticle layer. The waxy surface also tends to cause a
spray solution to form droplets and run off the leaves.

Leaf Hairs —A dense layer of leaf hairs holds the herbi cide dropl ets
away from the leaf surface, allowing less chemical to be absorbed into the
plant. A thin layer of leaf hairs causes the chemical to stay on the leaf surface
longer than normal, allowing more chemicals to be absorbed into the plant.

Size and Age— Young, rapidly growing plants are more susceptible to
herbicides than are larger, more mature plants.

Deactivation— Certain plants can deactivate herbicides and are less sus-
ceptible to injury from these chemicals. Such plants may become dominant
over a period of time if similar herbicides are used repeatedly.

Stage in life Cycle — S eedl ings are very susceptible to herbi cides and to
most other weed control practices. Plants in the vegetative and early bud
stages are very susceptible to translocated herbicides. Plants with seeds or in
the maturity stage are the least susceptible to weed control practices.

Timing of Stages in the Life Cycle— Plants that germinate and develop
at different times than the crop species may he susceptible to carefully timed
herbicide applications.

2. Chemicals Which Change Plant Processes

Plant growth regulators, defoliants, and desiccants are classified as pesti-
cides in federal laws. These chemicals are used on plants to alter normal plant
processes in some way. They must be measured carefully, because they usu-
ally are effective in very small amounts. Overdosing will kill or seriously
damage the plants.