Following the discovery of potent insecticides and herbicides in the 19405 a euphoric attitude prevailed in which chemicals were seen as the panacea… [608358]

5 WEED CONTROL WITHOUT
HERBICIDES
5.1 INTEGRATED CONTROL
Following the discovery of potent insecticides and herbicides in the
19405 a euphoric attitude prevailed in which chemicals were seen as
the panacea of all ills, able to provide easy and effective control of pest
problems. Soon, however, insect resistance required the application of
higher rates, destruction of predators increased the severity of pest
infestations and sometimes actually created new pests. A reappraisal of
chemotherapy was begun as the ecological implications for wild life
and man himself of the highly persistent pesticides then in use became
known. Gradually the notion of integrated control has evolved, in
which pest and disease infestations are reduced below the economic
threshold by a judicious combination of all feasible means, chemical,
biological or cultural. The life cycles of pests and their parasites and
predators were re-examined and improved pesticide application
methods and pest and disease forecasting techniques were devised to
provide an adequate control cheaply and with the least possible con­
tamination of non-target organisms.
Weed control, however it is achieved, should be treated as an
integral part of good husbandry, but only recently has the concept of
integrated control been applied with conviction to the control of
weeds, although the problems are essentially similar to those met in
dealing with pests and pathogens (Fryer and Matsunaka, 1977); these
include herbicide resistant floras of grasses, sedges and other problem
weeds which have arisen by interspecific selection and an increasing
dependence on herbicides for weed control in most developed
countries. Furthermore, the occurrence of interaction between pres­
ence of insecticide residues and herbicide persistence times
(Chapter 6) and between weeds, pests and disease organisms
(Chapter 1), and of more direct effects of herbicides on the incidence
of disease (Altman and Campbell, 1977) make an integrated approach
to these problems essential.
In spite of intensive hand-weeding, control of weeds in the past was
probably much less effective than it is today. for example, there are
many historical references to the toxic effects of Darnel and
64
R. J. Stephens, Theory and Practice of Weed Control
© R. J. Stephens 1982

Weed Contral Without Herbicides 65
Corncockle contamination of wheat in Northern Europe, yet both
weeds have now almost disappeared. This improvement began with
innovations such as efficient mechanical separation of crop from weed
seeds, more uniform germination resulting from better drills, protec­
tion from pests and diseases, seed of guaranteed high germ in ability
and use of fertilisers to increase early crop growth; thus herbicides are
just one more of the factors in this general improvement. Plant breed­
ing has produced more vigorous crops, but some of the recent
cultivars, such as the short strawed cereals, are dependent on chemical
weed control for their success. However, the alternative methods of
controlling weeds, such as cover-cropping and mulching, crop rota­
tion, flooding, burning and intensive use of pigs and other animals can
often be combined with herbicides to obtain a better result more
economically. In addition, research in many countries is attempting
the control of certain problem weeds biologically, using insects, fish,
viruses and parasitic fungi.
5.2 CULTIVATION
Cultivation for weed control, most of it by hand, has been claimed to
be the most time consuming and exhausting human occupation;
certainly, it remains the most important method of weed control
everywhere except in the developed nations. According to Fryer and
Makepeace (1977), cultivations control weeds by burying them, by
cutting them off near soil level, by stimulating weed seeds to ger­
minate so that they can be destroyed, by desiccation of roots and
rhizomes brought to the surface and finally by continually cutting back
the growth of deep rooted perennials, thereby exhausting them. It is
now believed that soil disturbance beyond the minimum necessary for
weed control often reduces yields and that provided weed infestation
can be prevented in same other way much of the tradition al soil
disturbance can be dispensed with (Chapter 8). However, well-timed
overall cultivations can be a cheap and effective method of killing
weed seedlings that can be combined with residual herbicide treat­
ment for prolonged weed control. In the stale seedbed technique a
tilth is prepared in the usual way, but by delaying drilling of a crop for
two or three weeks the initial flush of weed seedlings is easily
destroyed with a contact herbicide before the crop germinates. The
roots and rhizomes of many perennial weeds cannot withstand
prolonged desiccation, and infestations of both Common Couch and
the tropical Purpie and Yellow Nutsedges (Cyperus ratundus and C.
escu/entus) can be reduced by exposure to sun and wind following
cultivation.
Ploughing has been the primary process in the creation of a seedbed