At the Third British Weed Control Conference in 1956 I had the pleasure to discuss weed control research and development in Sweden (3). Some of the statements I made at that time will be used as a background to this paper. It was emphasized that there has been continuous change in weed control methods since World War I depending on the introduction of chemical weed killers but also on improved methods for cultivating and using the lands. Use of chemicals have brought with it changes in the weed flora meaning that certain weeds which were not serious in earlier days have now become a menace to the farmers. This brings with it a need for new chemicals with other weed killing properties. But it also brings with it a need for new cultivating methods and a better crop sequence. Cultivated plants are influenced by the chemicals. The effect on the cultivated plants depends on the stage of development of these plants. In uneven fields the harmful effect can be serious as there are always some plants that are sensitive at the time of spraying. The effect of the chemicals may depend on the purity of the acids. In MCPA, for example, the percentage of the 4–isomer, i.e. the 2–methyl–4–chlorophenoxyacetic acid, is important. It should be 95 per cent or slightly above as shown by åberg (1). The after effect of chemicals in the soils may be longer and more severe under northern conditions than under southern. For the choice of crops this must be considered. Thus there is no doubt that today, more than at any earlier period, weed control is a crop production problem. It can not be treated without considering crop production problems. Sometimes there is a feeling over in our part of the world that since the end of World War II chemicals and their properties have been emphasized too much in weed control discussions. Chemicals are, no doubt, important, but they can never get the place they are entitled to, unless our research work in weed control pays attention also to biology and ecology of those plants that we treat; cultivated plants as well as weeds. I believe we are in Europe thinking more about this particular end of the weed control work than you need to do over here. We usually have small farms with a number of different crops on each one of them. Thus we must learn to apply our chemical weed killers or use our cultivated methods in different ways for each one of these crops. For economical reasons and with regard to the supply of food and feed we need high yields per hectare. Every measure on a farm must be chosen with this in mind. Our problems, progress and organization of weed control must then be seen against the background I have now tried to give you.

Canada thistle (Cirsium arvense Scop.), a widely distributed weed in northern United States, according to Dewey (3), was introduced from Europe in 1777. In 1901 Blankenship (1) reported it to be rare in Montana having been identified in only five locations. However, a report issued in 1956 by Heikes (6), showed an estimated 625,000 infested acres in Montana. Another report of a survey of the four northwestern states including Montana, Idaho, Oregon, and Washington estimated their total of Canada thistle at more than 1.5 million acres infested (4). This was considerably more than any other weed of this region. Control measures for this weed are, therefore, of primary importance.

Weeds have plagued man in his agricultural pursuits since biblical times in spite of backbreaking efforts to control them by hand and mechanical methods. At this second meeting of the Weeds Journal of Weed Society of America it appears appropriate to review briefly some of the modern accomplishments in weed control and to take stock of future needs.

The control of annual weeds in row-planted crops such as soybeans and corn may become a major production problem whenever weather conditions limit the scope and effectiveness of tillage operations. With corn, the use of 2,4–D has greatly minimized this problem, but with soybeans the utilization of herbicides has been limited. Adaptation of herbicides to soybean production depends not only on herbicide performance but also on economic aspects of the problem. Recent studies (6, 7) reported average bean yield reductions of approximately 10 percent from the competition of average annual weed infestations. This emphasizes the importance of effective weed control and indicates some economic limits to the choice of methods to be utilized in practical weed control for soybeans.

A review of literature indicates that there are considerable variations in the results obtained in using herbicides for selective pre-emergence weed control. These variable results have been attributed in part at least to a large number of possible factors, a few of which are: soil type, soil moisture, soil surface condition, rainfall, plant varietal difference, and application variations and errors. In 1952 a study of the literature reporting research studies and recommendations indicated that some method of rolling or smoothing prior to pre-emergence applications might prove to eliminate those variations in results from such applications as could be attributed to the rough, cloddy condition of the soil surface. This was particularly noticeable in early bulletins of the cotton producing states, a number of which (1, 3, 4, 6, and 7) recommended that the soil surface be rolled smooth prior to the application of pre-emergence herbicides for weed control in cotton.

I feel greatly honoured to have been invited to cross the Atlantic to address so distinguished a gathering in the weed research and control fields.

In studying herbicides in relation to a complex weed like nut grass (Cyperus rotundus L.), it soon becomes clear that high efficiency is attained only if the herbicide is able to penetrate and translocate readily to all parts of the plant, still retaining sufficient toxicity to kill every one of the vital organs.

Grains of wild oats (Avena fatua L.) may remain dormant in the soil for several years or even for decades. The reasons for this dormancy are not known, but various hypotheses have been advanced to explain it, such as that the hull acts as a barrier to oxygen (1,7), and that a toxic substance is present in the hull as in cultivated oats (5). The tests here reported were undertaken to further explore some aspects of the problem of dormancy in wild oats.

Not more than 5 years ago the subject of herbicide residues on food probably would not have aroused too much interest. Such is not the case today. Aware as you are of the improvement in agriculture practice that can stem from herbicide usage, you doubtless are also familiar with the associated residue problem. My purpose this morning is to comment on some aspects of that problem from the standpoint of recent legislation.

Kochia (Kochia scoparia L.), also commonly called fire weed, is one of the most widespread annual weeds in Kansas and the Great Plains region. The origin of the weed in Kansas is obscure but it was introduced into the United States from Europe (1). The plant had been cultivated as an ornamental and apparently much of its present wide distribution is a result of escape from ornamental plantings.

The mesquite problem on southern Arizona ranges has been comprehensively described by Parker and Martin (9), and certain ecological relationships of this noxious tree, insofar as are known, have recently been considered by Glendening and Paulsen (5). Although a serious problem only in the states of Arizona, New Mexico, and Texas, at least one of the three common varieties of Prosopis juliflora now may be found in each of eight southwestern states (2). Its progressive invasion is unquestioned.

The biological effectiveness of any systemic herbicide is vitally dependent upon the absorption of the chemical by the plant. The process of absorption is of particular interest in the use of herbicides and it is only as this process is understood and turned to advantage that maximum effectiveness of applied chemical can be achieved. In foliar absorption, with which this paper is concerned, the cuticle plays a very important role (2). The thickness and general state of the cuticle and the attendant stomata have been shown to vary markedly with growth conditions (10). The submicroscopic structure of cuticles and the accretion of wax deposit on leaf surfaces have been studied (7, 8). These factors are of significance in the consideration of the intimacy of the contact of a particle impinging on the leaf. The chemical nature of the cuticle (6) is undoubtedly of great importance in the partitioning of certain herbicides between a particle of the chemical and the interior of this leaf.

In Part I of the present study, the effects of amount, intensity, and frequency of simulated rainfall on the distribution of monuron [3–(p–chlorophenyl)–1,1–dimethylurea] in Lakeland sand soil were studied (9). Part I also indicated the general importance of leaching in herbicide usage. The present study includes consideration of soil temperature, soil organic matter, soil moisture content, and amount of herbicidal application as they influence monuron distribution.

The plant growth regulating properties of 2,4–D (2,4–dichlorophenoxyacetic acid) (26) and the resistance of most monocotyledonous plants to the toxic effect of this chemical (17) have been known for many years. This difference in sensitivity of various species to the toxic effects of 2,4–D has had wide practical use in the field of weed control. However, there is no generally accepted mechanism for the selectivity between sensitive and resistant plants or for the actual mechanism of 2,4–D action. The four areas of research which have been investigated to elucidate the mechanism of action are absorption, translocation, and two distinct metabolic approaches. Absorption and translocation are often studied simultaneously. Metabolism is studied from two approaches, either the fate of 2,4–D in the plant or the effect of 2,4–D on plant metabolism. All of these processes are operating simultaneously in the plant and the ultimate explanation as to the mechanism of 2,4–D action must incorporate all of the contributions from these areas of research.

A relatively simple method for screening and evaluating chemicals for algicidal properties has been developed which embodies spray application of chemicals and greenhouse culture conditions. The method utilizes equipment and facilities common to the majority of industrial herbicide screening programs.

In many areas of the southwestern states where cotton is a major crop, the use of 2,4–dichlorophenoxyacetic acid (2,4–D) is hazardous. Its use is restricted by law in many localities during the growing season. In these areas many perennial broad-leaved weeds which are susceptible to 2,4–D, at least to some degree, grow in abundance. Because of extreme arid conditions and other factors these weeds have not responded favorably to soil sterilants. There is urgent need for more effective herbicides for these weeds.

The use of herbicides among nursery crops has been practiced only to a limited extent. Possibly this is because: slight herbicidal damage is serious in an expensive crop of nursery plants; the diversity of plants in a nursery makes it difficult to find suitable selective herbicides; the relatively small area of many nurseries makes cultivation and hand hoeing more practical than for large acreages; and damage to foliage of nursery crops makes the plants unsaleable whereas in farm crops foliage damage is less significant.

Establishment of new seedings of alfalfa is often difficult because of the many hazards encountered during the first season of growth. In the Central Great Plains moisture is often the major factor determining the degree of success of the operation. This condition can be accentuated by the presence of a population of annual grasses such as the foxtails (Setaria spp.) and barnyard grass (Echinochloa crusgalli L. Beauv.). These grasses have a more rapid seedling growth rate than alfalfa and tend to produce a drouthy condition in the soil by rapid utilization of the available moisture. The same effect often results from the use of oats or other small grains as companion crops.

Flax in Uruguay is normally sown in July and August. The common weeds in flax, radish (Raphanus sativus L.) and Rapistrum rugosum L., in the Cruciferae, and Ammi majus L., in the Umbelliferae, are readily controlled by hormone-type weed killers. Experimental plots of flax may be sown from May to September or even later. The weed problem in plots is different from that in farm fields. Plots sown in May or June are usually relatively free of weeds; plots sown in July or later may be heavily infested by various weed species, both broad-leaved and grasses.

In 1954 the authors reported studies involving the application of sodium 2,2–dichloropropionate (sodium salt of dalapon) to wild oats (Avena fatua) and concluded that treatment in the boot stage showed no promise in controlling viable seed production in wild oats. However, a lag in germination in seed from treated plants was noted in some instances (1). When seeds from these plants were planted in the greenhouse, many seedlings reached the coleoptile stage only, and failed to develop further (2).