British Agriculture has existed for at least a thousand years, and it would require a book to record the major developments in weed control. My concern today is with the events of the past 25 years during which weed control has become established as a science, and herbicides have been given a widespread introduction. I hope to set these events against a historical background as they relate to Great Britain, and thereafter, to draw out some thoughts on the impact that modern weed science is making and will make in the future on crop production and land use.

Concentrations of 2,2-dichloropropionic acid (dalapon), trichloroacetic acid (TCA), and (2,4-dichlorophenoxy) acetic acid (2,4-D) were determined in irrigation water following bank applications for weed control. Maximum concentrations of dalapon in the water varied from 23 to 365 ppb from application rates of 6.7 to 20 lb/A. The highest levels of TCA ranged from 31 to 128 ppb following applications of 3.8 to 5.9 lb/A. Applications of 1.9 to 3 lb/A of 2,4-D produced maximum concentrations of 25 to 61 ppb. Reduction of herbicide levels appeared to be due to dilution as the water flowed downstream. Rates of reduction in herbicide levels showed that negligible concentrations would remain after the water traveled a distance of 20 to 25 miles. The low concentrations of herbicides observed in the irrigation water likely would not be hazardous to crops or animals.

Root application of N,N-dimethyl-2,2-diphenylacetamide (diphenamid) caused reduction of root and shoot growth of oats (Avena sativa L., var. Jaycee) seedlings. Shoot application did not affect plant growth, but studies with labeled diphenamid showed that diphenamid will enter also through the shoot. In ivyleaf morningglory (Ipomoea hederacea L.), a considerable amount of label was translocated from the roots to the shoots, but not in oats seedlings. The difference in tolerance between these two species (oats susceptible, morningglory resistant) may lie in the ability of morningglory to translocate diphenamid out of the roots into the shoots faster than oats. The inhibitory effect of diphenamid was restricted to the site of uptake. Reduction in shoot growth of treated plants was the result of the limited root system and it was not a direct effect of diphenamid. Diphenamid was 10 times as toxic to oats as its metabolites. Oats seedlings inhibited by diphenamid for up to 5 days, and then placed in water recovered from the diphenamid caused inhibition. The resumed root growth appeared to be normal. The amount of uptake of 14C-labeled sucrose by excised roots treated with 10−5 M diphenamid was equal to that in untreated roots; however, more sugar was incorporated into the untreated roots than the treated roots. It appears that diphenamid is a reversible metabolic inhibitor; it inhibits cell division in the root tip perhaps by limiting utilization of substrates in the cells.

Plant litter that covers the soil surface acts as a layer of insulation moderating temperature and moisture, and creating favorable microsites for germination and the establishment of annual weed species in rangeland communities. Litter cover is an important factor in succession among annual species which culminates in dominance by downy brome (Bromus tectorum L.) and medusahead (Taeniatherum asperum (Sim.) Nevski) in these communities.

Combinations of copper sulfate pentahydrate (hereinafter referred to as CSP) at 1.0 ppmw copper plus 0.1 to 2.0 ppmw 6,7-dihydrodipyrido-(1,2-a:2′,1′-c)pyrazinediium ion (diquat) resulted in higher accumulations of copper in hydrilla (Hydrilla verticillata Casp.), egeria (Egeria densa Planch.), and southern naiad (Najas guadalupensis Spreng. Magnus) when compared to plants which received only CSP. A contact period greater than 24 hr was necessary before the higher amounts of copper were detected in those plants treated with the combinations. Water samples from outdoor, plastic pools 7 days after treatment with CSP at 1.0 ppmw of copper plus 1.0 ppmw of diquat contained 25% less copper than pools treated with CSP. Samples of hydrilla and southern naiad removed 7 days after treatment of the pools with the combination contained 77 and 38% more copper, respectively, than samples from those pools treated with only CSP.

Three tractor cultivations that severely pruned the root systems of hand-weeded plots of sweet corn [Zea mays L. var. rugosa Bonaf., Iochief] grown on a Codorus-Elkton silt loam under extreme drought conditions did not reduce the yield of marketable ears when irrigation totalling 2 acre-inches was applied during pollination and ear-filling. Pre-emergence application of 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine [atrazine] to the soil surface without cultivation reduced brace root growth and yields significantly in the year of most severe drought. These effects of atrazine were avoided when the herbicide treatments were supplemented with tractor cultivations that covered the brace root zone of the corn plants with soil.

The effect of dimethyl tetrachloroterephthalate (DCPA) applied during the establishment of Tifgreen bermudagrass (Cynodon dactylon (L.) Pers. X C. transvaalensis Burtt-Davy) on subsequent winter injury was studied. DCPA treatments to plots sprigged in July or later increased winter injury. DCPA applied in July, August, or September to plots planted in June resulted in approximately 15% of the turf being killed during the winter; however, DCPA applied before the middle of June did not significantly influence winter injury. Significant increases were found in total nitrogen, nitrate, amino acids, and protein of stolons harvested from DCPA-treated plots. These data suggest that DCPA treatments applied during establishment resulted in a less winter hardy condition by maintaining vegetative growth into the normal period of winter hardening.

A split-stem technique was devised for the rapid evaluation of herbicide effect on root growth and herbicide translocation. Four rows of adventitious root initials were prominent on the stem of red kidney bean [Phaseolus vulgaris L.] seedlings. They were excised at the soil level above the vascular plate. Excised stems were longitudinally split for a distance of 75 to 90 mm leaving two rows of root initials on each stem portion. Split-stem halves were immersed in herbicide-nutrient and nutrient solutions, respectively, contained in paired 25 by 150-mm test tubes. Subsequent plant growth and development of treated plants reflected effects of the herbicide on root growth. Herbicide translocation often was indicated by abnormalities of growth of plant tissues remote from the herbicide solution.

A variable-dosage sprayer system with a toroidal mixing chamber was constructed and used successfully in evaluation of equipment and herbicides for 2 years at the U. S. Cotton Research Station, Shafter, California. The primary advantage of this system compared to other systems is that the dilution rate is the same as predicted by theory over a wide range of conditions. Therefore, the rate of dilution can be calculated for different combinations of nozzles, or metering orifices, pressures, and spray rates without the need for direct calibration. Since the inside of the chamber is visible, some application errors are avoided and others detected that are not known with other systems. The system should be adaptable to a wide range of plot sizes and half-dosage distances by changing the volume of the toroid, the limiting factor being the availability of proper size propellers and motor.

In eastern Washington Dalmatian toadflax (Linaria dalmatica (L.) Mill) produces seeds for about 3 months, beginning in early July, with over 95% produced during the first 5 weeks. A large plant will produce a half million seeds under good conditions. Seeds are sharply angular, slightly winged, and 1 to 2 mm long. Approximately 7,000 weigh 1 g. Seeds are spread by wind and browsing animals. Some seeds will germinate under laboratory conditions the same season in which produced, but germination from dry storage increases for 2 to 3 years, with a periodic low in November and a high in April. Over 90% germination is obtained with 2 to 3-year-old seeds in the laboratory. Germination in the field occurs in both spring and fall, and seedlings can emerge from a soil depth of 2 to 2.5 cm.

The selectivity of 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) in winter wheat (Triticum aestivum L., var. Nugaines), a resistant species, and coast fiddleneck (Amsinckia intermedia Fisch. & Mey.), a susceptible species, was concluded to be the result of a complex of interactions. Wheat was 109 times more tolerant of bromoxynil than was coast fiddleneck. More spray solution was retained by coast fiddleneck plants than by wheat. Bromoxynil penetrated more rapidly into leaves of coast fiddleneck than of wheat. However, these factors did not fully explain bromoxynil selectivity. When differential spray retention and bromoxynil penetration was accounted for, it was estimated that coast fiddleneck was 30 times more susceptible than wheat.

The relation of translocation and degradation to selectivity of 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) in winter wheat (Triticum aestivum L., var. Nugaines), a resistant species, and coast fiddleneck (Amsinckia intermedia Fisch. & Mey.), a susceptible species, was examined. Radioautographic and extraction analysis revealed that the label from 14C-bromoxynil was more mobile in coast fiddleneck than in wheat. Higher levels of soluble radioactivity were found in extracts of coast fiddleneck than wheat. Higher levels of insoluble label were found in tissue residues of wheat than coast fiddleneck. In both species, most of the activity remained in treated leaves. A high percentage of the soluble activity was attributed to 14C-bromoxynil in both wheat and coast fiddleneck. The evolution of 14CO2 by wheat treated with 14C-bromoxynil significantly exceeded that of coast fiddleneck. The difference in susceptibility between winter wheat and coast fiddleneck appears to be explained partly by differential translocation and degradation of bromoxynil.

We sprayed yellow nutsedge (Cyperus esculentus L.) with 0.56, 1.12, and 2.24 kg/ha of (2,4-dichlorophenoxy)acetic acid (2,4-D) at the two and four-leaf stages in both field and growth chamber studies. Compared to non-treated plants, these rates significantly reduced shoot weight, rhizome weight, and rhizome number 12 days after application. There were no significant differences in these growth responses among the rates tested, when applied one, two, and three times at consecutive 12-day intervals. We pretreated two and four-leaf plants with 0.56, 1.12, and 2.24 kg/ha of 2,4-D immediately before foliar application of 14C-2,4-D. The increase in pretreatment rates decreased the percentage of the applied radioactivity which was absorbed and translocated. Of the radioactivity absorbed, a rather constant percentage translocated from the treated area in the different treatments. The radioactivity translocated from the application site for at least 288 hr after treatment. Less than 19% of the 14C-2,4-D applied was absorbed into the leaves 24 hr after application. Ninety-eight to 100% of the methanol-soluble radioactivity that translocated to developing leaves, rhizomes, and roots chromatographed identically to 14C-2,4-D.

There is little evidence that herbicides from agronomic or industrial usage are reaching or accumulating in our water supplies in amounts to cause a pollution problem. Fish tolerance to weed killers varies with their size and species as well as with differences in the aquatic site. There may be variation to different forms of the active ingredient. Other components in a formulation may be more toxic than the herbicide itself. Therefore, the LD50 of different formulations should be determined. In addition, the manner and other circumstances involving application of a herbicide may change its toxicity to water inhabitants. Our current knowledge of the effects of herbicides on fish, plankton, and other water inhabitants indicate that harmful effects with our present herbicides, when such exist, are only temporary. Available evidence suggests there is no biological magnification problem with herbicides.

We evaluated the effectiveness of aqueous sprays and granules of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), 1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea (fluometuron), isopropyl m-chlorocarbanilate (chlorpropham), α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin), and 2-chloro-2′, 6′-diethyl-N-(methoxymethyl) acetanilide (alachlor) incorporated shallow for weed control in cotton (Gossypium hirsutum L., var. Stoneville 7A). In general, liquid formulation provided better weed control than did granular formulation. Applications of alachlor resulted in good weed control, but in a dry season the liquid spray form of the herbicide at high rates reduced the yield of cotton.

The selective action of α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) for the control of green foxtail (Setaria viridis (L.) Beauv.) in wheat (Triticum aestivum L.) was studied. Seedlings of both species, especially in the early stage of growth, were particularly susceptible to herbicide applied in the region of the coleoptile node. Selective control with trifluralin was possible, however, because of the morphological differences between the seedlings of the two species. Thus, the sensitive coleoptilar node area of green foxtail was located at a depth of less than 1 cm from the soil surface regardless of planting depth. The coleoptile node of wheat remained closely associated with the caryopsis. When a protective band of untreated soil was used to separate the caryopses from a 5-cm band of surface soil containing 1.12 ppmw of trifluralin, the shoot growth of wheat was normal. Much lower rates of trifluralin effectively controlled green foxtail, as surface or subsurface placement of a 1.3-cm layer of soil containing 1.12 ppmw of chemical above the foxtail caryopses completely inhibited shoot development. In addition to the differences in morphological development between the two species, the fact that green foxtail was considerably more susceptible to trifluralin than wheat was considered as an important factor contributing to the selective control.