Tải bản đầy đủ - 0trang
II. Tillage, Cropping, and Competition in the Control of Weeds
to erosion control and weeds must be handled by other means. A complete consideration of the methods of cultural, tillage, and competition
methods of weed control is given in Robbins et al. (1942). Further
reference material will be found in Korsmo (1930), Muenscher (1935),
Herbage Publication Series Bulletin 27 (1940), Reports of the Cooperative Weed Investigations of the U.S.Department of Agriculture, Timmons (1941), etc.
It should also be obvious that the simplest method of controlling
weeds in row and checkrowed crops is the frequent shallow tillage so
commonly practiced. A point that has not been so well recognized is
that, a t least in some soils, frequent tillage destroys the structure of soil
colloids, rendering the soil less pervious to air and water, and hence less
favorable as a medium for plant growth. Harvey (1944) observed that
2 seasons of tillage operations resulted in alteration of soil structure to
the extent that seedbed preparation was difficult and irrigation furrows
tended to wash out. Many citrus growers in California have adopted
nontillage as a preferred program in their groves and all weed control
is carried on with chemical sprays. Parker and Jenny (1945) have made
preliminary investigations on nontilled and compacted soils, finding that
tillage restricts percolation. Whereas nontillage may be readily pract.iced
in an orchard, the necessity for seedbed preparation requires a certain
amount of manipulation of the soil for many field crops and frequent
tillage for weed control has been considered an essential feature in the
production of row crops. If tillage is as harmful to soil structure as some
of the experimental work indicates, the substitution of chemical weed
control for cultivation can be readily made, a t least in row crops. I n
view of the benefits claimed for nontillage, the advantages of growing
perennial forage crops such as alfalfa and clover may come, partly a t
least, from the absence of tillage during relatively long periods.
The role of competition in cropping methods of weed control has
been studied in Canada by Godel (1935, 1938), Pavlychenko and Harrington (1934, 1935), Pavlychenko (1940), in Germany by Rademacher
(1940), and in the United States by Timmons (1941), and others. The
work up to 1942 is reviewed in Robbins et al. (1942).
Although the advent of the dinitro compounds, 2,4-D and related
compounds, and other selective herbicides has lowered costs of weed control in cereal crops to a level that rules out many cultural and cropping
procedures, the sowing of clean seed, the prompt control of new infestations, the elimination of weeds on fence lines, roadsides, ditchbanks, and
obher noncropped areas, and other preventive measures are still economiaally sound. Research on competition and cropping methods is necessarily
time-consuming and costly; publications, therefore, are not so numerous
A. 8. CRAFTS AND W. A. HARVEY
as in the field of chemicals. This in no way reflects upon the relative
value of the former methods. I n any educational work on weed control
the merits of t.hese “good farming” methods should be stressed.
One real point of departure from the older methods included under
“good farming” is t.he changed view regarding frequent tillage that has
been expressed above. It has been proved that the periodic stirring of
the soil that constitutes the essential feature of “summer fallow,” “black
fallow,” and mechanical weed control in trees, vines, and row crops does,
a t least in semi-arid regions, have a deleterious effect on the colloidal
structure of the soil. Furthermore, such tilled soils are often subject
to severe wind or water erosion. In tree and vine crops sprays are proving a satisfactory and effective substitute for tillage (Aldrich, 1948;
Johnston and Sullivan, 1949; Moore, 1945a, b; Puffer, 1947; Puffer
and Yarick, 1948; Sullivan and LaRue, 1947; Yarick, 1947). I n some
row crops such as sugar cane, corn, milo, soy beans, and cotton, experimental work in progress promises some relief from the deleterious effects
of cultivation (Anderson and Ahlgren, 1947; Anderson and Wolf, 1947;
Crafts, 1948b; Harvey and Robbins, 1947; Lee, 1948a, b; Leonard et al.,
1947, 1948; Van Overbeek and Velez, 1946a, b; Van Overbeek, 1947; and
Willard, 1948). Work on the substitution of chemical control for fallow
has just started (Pavlychenko, 1947). Much more will have to be done
before satisfactory substitutes for all fallow procedures will be found.
Furthermore, there are undoubtedly soils that are definitely benefited by
tillage. A case in point is a heavy tideland soil in Puerto Rico which
was reclaimed by drainage and irrigation with a good quality irrigation
water. By plowing in bagasse and sugar cane trash this land was gradually reclaimed and brought into a very productive condit.ion. Hester
and Isaacs (1948) report a case in Pennsylvania where carrot production
was much lower on nontilled than on tilled land. I n this situation it
should be emphasized, however, that one season of nontillage will not
compensate for harm done by years of frequent cultivation.
Biological methods of weed control are reviewed in Robbins, et al.
(1942). Reports in recent years indicate the successful control of St.
Johnswort in Australia (Currie and Garthside, 1932; Wilson, 1943).
Of interest to Westerners is the fact that, Chrysolinu hyperici and
C . gernellata, introduced in California in 1945 by Smith and Holloway
(Holloway, 1948), are increasing, and give all indications of producing
satisfactory results on St. Johnswort under Pacific Coast conditions.
This is encouraging to the stockmen because in the four states of Idaho,
Washington, Oregon, and California upward of a million acres have become infested with St. Johnswort. This weed takes over the open productive areas of the range and become so dense that little or no forage is
produced. Because of the wide dissemination of this species, and the fact
that it infests low- to intermediate-value lands, biological control is the
only hope for holding it in check.
Early reviews on chemical weed control include Bolley (1908)) Cook
and Halferdahl (1937), Long (1934) , Long and MacDowall (1935),
Crafts and Raynor (1940), and Robbins et al. (1942).
More recent reviews have been written by Mitchell and Marth (1947),
Avery et al. (1947), Akamine (1948), Avery and Thompson (1947), and
Van Overbeek (1947).
Current papers on weed control, particularly concerned with the
hormone weed killers are appearing in Botanical Gazette. Of special
interest is the group of contributions from the Special Projects Division,
Chemical Warfare Service, Camp Detrick, Maryland, published in Bot.
Gaz. 107,476-634, 1946, and the paper on “Growth Regulating Substances
as Herbicides’’ by Kraus and Mitchell (1947). T h e JournaZ of the
American Society of Agronomy, Science, American Journal of Botany,
and other scientific periodicals are including papers on weed control.
The trade journal, Agricultural Chemicals, carries articles on weed control and has many advertisements describing the current products being
offered for sale. The organ “Down to Earth” of Dow Chemical Company also carries timely articles on pest control, including weeds. Many
research and extension publications are coming from Agricultural Colleges both in this country (e.g., Weeders Readers, Oregon State College
of Agriculture, Extension Service) and abroad. Mimeographed reports
of regional weed control conferences contain many valuable data (Anonymous, 1948b; North Central Weed Control Conf., 1947; North Eastern
Weed Control Conf. Minutes, 1947; Western Weed Control Conf.
Minutes, 1946, 1947, Proc. 1948).
There are many mechanisms by which the cells of plants may be
killed (Aberg, 1947; Crafts, 1939b, 1948c; Crafts and Reiber, 1945;
Harvey, 1931; Offord and d’Urbal, 1931; Van Overbeek, 1947) ; salts such
as sodium chloride will take water osmotically causing plasmolysis and
death; strong acids or bases cause violent shifts in cell reaction from
which cells die; some herbicides such as sodium arsenite are protoplasmic
poisons that denature the proteins of cells; some partake of the nature
of enzymes or stimulators (e.g., the dinitro compounds seem to accelerate
respiration) ; many new chemicals such as the naphthoxy-, phenoxy-, and
benzoic acids and phenyl carbarnates have hormone properties causing
A. S . CRAFTS AND W. A. HARVEY
disturbances in the basic metabolism of plants; the unsaturated hydrocarbons occurring in petroleum have a compatibility and a reactivity
with protoplasm that enables them to enter cells readily and cause a
breakdown of their semi-permeable properties (Crafts and Reiber, 1948) ;
substituted benzenes seem particularly effective (Crafts, 1945b).
Because of the number of reagents used and the variety of actions
noted, it seems obvious that different herbicidal properties should result.
Not only do the large general groups named above have different mechanisms of herbicidal action, but individual compounds within these groups
have specific toxic properties and different plant species, or even different
individuals wit,hin a species may react differently. And, finally, variations in environmental conditions cause wide differences in response of
plants to herbicides, often making for success or failure of a given treatment.
Because of the variations in response of plants to herbicides chemical
weed control is complicated and often confusing; on the other hand these
same variations result in the high degree of specificity that permits
selective use of herbicides. An example is the selective use of dinitro
compounds and chlorophenoxy compounds against broad-leaved weeds
in cereal crops. An even finer degree of selectivity is that of 2,4-D in
killing mustards, Amsinckia, and knotweed in flax, or the use of Stoddard
Solvent to control mixed weeds including both grasses and broad-leaved
annuals in crops of the carrot family. As research continues, even finer
degrees of selectivity are found; 2,4,5-trichlorophenoxyaceticacid is more
toxic than 2,4-D on potatoes and a number of woody species; mixtures
of Stoddard Solvent and kerosene will kill wild oats in flax; and combinations of chemicals such as 2,4-D and I P C (isopropyl phenylcarbamate) have proved successful in controlling mixed weeds including
grasses in Ladino clover.
I n discussing herbicides, it has been found convenient to group them
into the following categories :
Pre- and post-emergence