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BIOCHEMICAL 
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MORPHOLOGICAL 
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OBSTACLES THAT DETERMINE CONCENTRATION OF TOXIC MATERIAL AT SITE OF ACTION 
Figure 2,--(From Shaw et al, (6).) 
chemical weed control. In some instances, 
plants detoxify herbicides to inactive forms, 
The conversion of 2-chloro-4,6-bis (ethyl- 
amino)-s-triazine (simazine) to a nonphyto- 
toxic form in corn, but not in many weeds and 
other crop plants, is a classic example of 
herbicide selectivity dependent on differential 
metabolism of herbicides in plants, These are 
examples of widely effective herbicides in 
controlling weed populations but with high 
tolerance and selectivity of action on a few 
crops with few if any harmful effects. These 
contrast sharply with such a herbicide as 
pentachlorophenol (PCP), which has high tox- 
icity to nontarget organisms in the environ- 
ment. The current trend is clearly toward 
compounds with greater specificity and 
selectivity and low toxicity to nontarget or- 
ganisms, 
NEW APPROACHES IN DEVELOPMENT 
OF HERBICIDES 
In developing herbicides, more attention will 
be given to acquiring a better understanding of 
the life-cycle weaknesses in the growth of 
plants. Research emphasis will again be re- 
quired. Chemicals are needed to stimulate 
the uniform germination of weed seeds at 
27 
varying depths in the soil in a relatively short 
period of the growing season. This would per- 
mit their control with a single application of 
a nonpersistent herbicide. There is also a 
need for chemical inhibitors to induce long 
periods of weed-seed dormancy. This practice 
would cause a loss of viability in the seeds of 
many weeds resulting in their control. Chem- 
icals to induce or break bud dormancy of 
perennial weeds could be used to stimulate 
growth in winter. Such effects might result in 
control by winterkill or in an increase in the 
susceptibility of perennial weeds to control by 
present herbicides and cultural methods. 
Chemicals that prevent fertilization in weeds 
and weed-seed production need further con- 
sideration. These are but a few examples of 
approaches that may be explored in the future. 
Nevertheless, emphasis in all approaches will 
be on the development of herbicides that con- 
trol weeds through mechanisms found in plants 
but not in animals and man and that are harm- 
less to man and all other nontarget organisms 
in the total environment. 
Not only will fundamental chemical and bio- 
logical information be required, but it will be 
essential in shaping engineering objectives in 
weed control in the future. The types of equip- 
ment needed will undergo significant changes 
and the need for engineering research will 
