Dormant (fresh) and nondormant (afterripened) prickly sida (Sida spinosa L.) seeds were extracted and bioassayed for both inhibitory and promotory growth regulators. Both dormant and nondormant prickly sida seeds contained water-soluble inhibitors, but these inhibitor levels in nondormant seeds did not change after 8 hr of incubation. A basic inhibitor was present in dormant seeds, but not in nondormant seeds. Exogenous growth regulators stimulated germination of dormant seeds only when a portion of the seed coat was removed. Promoter levels in nonincubated, dormant and nondormant seeds were similar, but there were increases in promoter levels in nondormant seeds after 8 hr of incubation. However, it was not determined whether the promoter increases were a cause or a result of germination.

Polymerized and conventional (2,4-dichlorophenoxy)-acetic acid (2,4-D), (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), 3,6-dichloro-o-anisic acid (dicamba), and 4-amino-3,5,6-trichloro-picolinic acid (picloram) were applied alone and in certain combinations to several brush species. Generally there were no significant differences in effectiveness between conventional and polymerized herbicides on honey mesquite (Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockrell), huisache (Acacia farnesiana L. Willd.), Macartney rose (Rosa bracteata Wendl.), live oak (Quercus virginiana Mill.), whitebrush (Aloysia lycioides Cham.), and winged elm (Ulmus alata Michx.). Conventional granular picloram was more effective on yaupon (Ilex vomitoria Ait.) than polymerized formulations.

Field evaluations were made on the effects of imazaquin {2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid} on Florida beggarweed [Desmodium tortuosum (SW) DC. # DEDTO] and sicklepod (Cassia obtusifolia L. # CASOB) control in peanuts (Arachis hypogaea L.). Crop yield was reduced at 392 and 504 g ae/ha and with preemergence applications. Florida beggarweed control was not achieved with this herbicide. Maximum sicklepod control (approximately 80%) was achieved with 392 and 504 g/ha applied postemergence. In a separate study, several herbicide systems utilizing imazaquin were compared to a standard weed control program. All these systems provided weed control and yields that were comparable to the standard weed control system; thus the inclusion of imazaquin offered no advantage over the standard system.

Seedlings of switchgrass (Panicum virgatum L. ‘Blackwell’), sideoats grama (Bouteloua curtipendula [Michx.] Torr. ‘Premier’), and native vine mesquite (Panicum obtusum H. B. K.) tolerated 0.28 kg/ha of 3,6-dichloro-o-anisic acid (dicamba) applied preemergence. After emergence, sideoats grama tolerated 0.56 kg/ha dicamba. Preemergence or postemergence applications of 1.12 and 2.24 kg/ha severely retarded shoot production of all three species. Soil residues in the surface 2.5 cm reduced root production of grass seedlings, whereas placement of dicamba 7.6 or 15.2 cm deep was not detrimental. All grass species germinated and grew for 21 days in soil containing up to 63 ppb of dicamba without reduction in shoot production. Overall order of susceptibility to dicamba was ranked as vine mesquite > switchgrass > sideoats grama.

In three field experiments with weed-free plots, delaying the application of bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] reduced toxicity to cucumbers (Cucumis sativus L. ‘Poinsett 76′). Maximum injury and yield reduction were observed when bentazon was applied at the first leaf stage, whereas injury and yield reduction were slight when bentazon was applied at the fourth leaf stage. Increasing illuminance after application increased injury and tended to override the effect of leaf stage. In greenhouse studies, the slicing variety ‘Poinsett 76’ was more tolerant of bentazon than the pickling variety ‘Calypso’. The insecticide diazinon [O,O-diethyl-O-(2-isopropyl-6-methyl-4-pyrimidinyl)phosphorothioate] increased bentazon toxicity to cucumbers compared to toxicity caused by bentazon alone. Four other insecticides and two fungicides had no effect on phytotoxicity.

The absorption and distribution of 14C terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine] and 14C-fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] in cotton (Gossypium hirsutum L. ‘SJ-1’) and snapbean (Phaseolus vulgaris L. ‘Tenderette’) seeds during germination was studied. Seeds of the susceptible snapbean absorbed larger amounts of both herbicides from soil than the resistant cotton seeds. The herbicides accumulated mainly in the seed coat of cotton which shed following emergence, with no translocation to the seedling leaves. In snapbean seeds the herbicides accumulated in the cotyledons, which served as a source for acropetal flow to the developing foliage of the young seedling.

Sainfoin (Onobrychis viciaefolia Scop. ‘Melrose’) seedlings tolerated at 15:1 commercial formulation of the sodium salts of MCPB {4-[(4-chloro-o-tolyl)oxy] butyric acid} and MCPA {[(4-chloro-o-tolyl)oxy] acetic acid} applied at rates up to 2.2 kg/ha in the greenhouse and 3.4 kg/ha in the field. Broadleaf weed control was only fair. Applications of 1.1 kg/ha, 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid] ester caused moderate damage to sainfoin initially, but no reduction in yield. Weed control was fair to good. Higher rates of 2,4-DB caused excessive injury unless a heavy growth of weeds protected the sainfoin from contact with the spray. The use of 2,4-D [(2,4-dichlorophenoxy) acetic acid] amine damaged sainfoin severely, although weed control was fair. Nitrofen (2,4-dichlorophenyl-p-nitrophenyl ether) at 3.4 kg/ha caused spotting on sainfoin leaves with no reduction in growth. Initial weed control was good, but sainfoin seedlings did not suppress new weed growth. Herbicide applications made when sainfoin had 1 to 3 leaves provided the best combination of crop tolerance and weed control.

Studies determined the effect of sodium bisulfate (NaHSO4) on the phytotoxicity, retention, uptake, and translocation of the suspension concentrate formulation of imazamethabenz in wild oats. NaHSO4 completely solubilized this herbicide formulation when added in an equimolar concentration and did not affect herbicidal activity at NaHSO4 concentrations below 28 mM when used in a carrier volume of 100 L ha−1. NaHSO4 improved phytotoxicity at a carrier volume of 33 L ha−1. NaHSO4 at 28 mM increased the efficacy of imazamethabenz applied as individual drops on growth chamber-grown wild oats. The herbicide retention was not changed but foliar absorption and acropetal translocation were increased, while basipetal translocation was decreased. The increase in phytotoxicity of imazamethabenz with NaHSO4 was related to absorption of imazamethabenz by the target plants.

Field trials were conducted from 1991 to 1993 in the northern cotton-producing area of Alabama to evaluate the interaction of various production inputs for pest management and cotton development, maturity, and yield. Two levels of tillage (conventional and no-till), herbicide (1.12 and 2.24 kg ai ha−1 fluometuron, preemergence, with post-directed herbicides), insecticide (0.5 and 0.85 kg ai ha−1 aldicarb, in-furrow), and fungicide (0.9 kg ai ha−1 quintozene plus ethridazole, in-furrow or 0.14 kg ai ha−1 metalaxyl, hopper-box) programs were evaluated. The inputs investigated did not interact significantly to change the overall production strategy. Prickly sida required a higher level of herbicide input than did the entireleaf/ivyleaf morningglory complex. Both species were sufficiently controlled using reduced levels of fluometuron without sacrificing yield; however, cultivation was necessary in conventional tillage treatments to maintain control. A postemergence-directed herbicide treatment was also necessary for weed control, regardless of tillage. Decreasing the levels of each input simultaneously did not interact to affect cotton stand, height, early-season thrips counts, cotton maturity, or yield. Cotton was shorter in no-till than conventional plots.

The plant hormones gibberellic acid (GA) and benzyladenine (BA) stimulated the development of dipeptidase activity in distal-half cotyledons of squash (Cucurbita maxima Duch. ‘Chicago Warted Hubbard’). The herbicides 2,4-D [(2,4-dichlorophenoxy)acetic acid], dicamba (3,6-dichloro-o-anisic acid), and naptalam (N-1-napthylphalamic acid) inhibited the development of dipeptidase activity in this tissue. Herbicide concentrations which inhibited the development of this enzyme activity about 30% were used for the herbicidehormone interaction study. The addition of GA or BA counteracted the inhibition induced by 2,4-D and naptalam. Neither hormone grossly altered the inhibition induced by dicamba; however, BA did significantly reduce the inhibition caused by dicamba.

Dicamba (3,6-dichloro-o-anisic acid) and 2,3,6-trichlorobenzoic acid (2,3,6-TBA) applied in lanolin pastes to leaves or roots of host plants growing in soil were translocated acropetally and affected witchweed (Striga lutea Lour.) plants parasitizing roots of host plants. Emerged witchweed plants exhibited characteristic epinastic responses while emergence of other witchweed plants was delayed. Broadcast foliar treatments of dicamba and 2,3,6-TBA to corn (Zea mays L.) applied before witchweed emergence generally reduced the intensity of the infestation and delayed emergence. However, injury to corn, expressed as failure of kernel development and grain yield decreases, sometimes occurred. Injury was greater when herbicide application was delayed until corn was in the early tassel stage. Dicamba was more effective than 2,3,6-TBA for witchweed control. However, no treatment gave season-long control of witchweed. Yearly variations in corn injury precluded a selection of optimum treatment dates and rates for control of witchweed by this technique.

Three field experiments, each for a 2-yr duration, were initiated to evaluate the efficacy of TCA (trichloroacetic acid) and dalapon (2,2 - dichloropropionic acid), alone or in combination, for the selective control of alsike clover (Trifolium hybridum L.) and red clover (T. pratense L.) during the establishment of birdsfoot trefoil (Lotus corniculatus L.). TCA at 6.7 kg/ha gave 95% control of alsike control. TCA at 13.5 kg/ha gave 92% control of red clover and the combination treatment of 6.7 kg/ha of TCA plus 9.0 kg/ha of dalapon gave 95% control of red clover. Birdsfoot trefoil tolerated TCA and dalapon alone or in combination.

Weed-control evaluations in ratoon-cropped grain sorghum [Sorghum bicolor (L.) Moench.] indicated that acceptable broadleaf weed control (>80%) in the second crop could be achieved by the use of a contact herbicide plus a residual herbicide applied after first harvest. Annual grasses, especially volunteer grain sorghum, were controlled in the second crop with metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] plus propazine [2-chloro-4,6-bis(isopropylamino)-s-triazine] applied preemergence at planting and followed by metolachlor, cyanazine {2-[[4-chloro-6-(ethylamino)-s-triazin-2-yl] amino]-2-methylpropionitrile}, or pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] applied after first harvest. Second-crop grain yields were not adversely affected by any treatments, and it appeared that satisfactory weed control in the first crop lessened the need for a residual herbicide in the second crop.

Herbarium records indicate that Russian pigweed (Axyris amaranthoides L.) entered the flora of Manitoba in 1886, spreading from there, by its own devices and by secondary introductions, to other provinces of Canada and to the United States. It is best established (to the extent of being considered a weed) in North Dakota and the prairie provinces of Canada.

Field trials conducted in the three major peppermint-producing regions of Oregon showed that repeated applications of bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] at 2.2 kg/ha controlled Canada thistle [Cirsium arvense (L.) Scop.] in peppermint (Mentha piperita L.) selectively. Repeat applications of 2.2 kg/ha controlled Canada thistle better than single applications of 3.4, 4.5, or 6.7 kg/ha. Late May or June applications were more effective than early May applications. The number of Canada thistle shoots was reduced 82% 10 months after repeat applications but had almost returned to the original number 22 months after application. Basipetal translocation of bentazon was demonstrated by observing bentazon injury symptoms on untreated Canada thistle shoots which were attached by a root to bentazon-treated shoots.

Atrazine [2-chloro-4-(ethylamino)-6-(isopropyl-amino)-s-triazine] applied to the soil of potted tomatoes (Lycopersicon esculentum Mill. ‘John Baer’) reached its maximum concentration in the xylem exudate collected from the stumps of detopped plants in 6 to 8 hr but continued to accumulate in the petioles for 36 hr. An atrazine concentration of 3.8 μg/g fresh weight of petiole at 12 hr after treatment was approximately the same as the maximum concentration of atrazine in the xylem exudate indicating that atrazine had accumulated extensively in tissues outside of the xylem vessels.

Field experiments were conducted to compare the effects of 0.07, 0.14, 0.28, and 0.56 kg/ha of 2,4-D [(2,4-dichlorophenoxy)acetic acid] amine on plant growth, seed production, and seed quality when applied at four growth stages of summer rape (Brassica napus L. ‘Zephyr’) and turnip rape (Brassica campestris L. ‘Span’). At all growth stages, seed yield of turnip rape, was significantly reduced by each additional increment of 2,4-D. The difference in response of the two species may relate in part to differences in spray droplet retention of the leaf surfaces. The herbicide did not affect the germination of harvested seed although treatment with the higher rates at the bolting and flowering stages resulted in the production of some shrivelled seed that was lost on cleaning. Some herbicide treatments caused a marked increase in size of harvested seed. There was a trend towards decreased seed oil content with increasing rate of 2,4-D at the bolting and flowering stages, but oil production losses were mainly reflected through losses in seed yield. The time required for both species to reach maturity increased with increasing rate of herbicide when applied at the vegetative stages. In additional field studies, 2,4-D significantly reduced seed yield in turnip rape when applied at the 3- to 4-leaf stage at 0.035 kg/ha, and 0.018 kg/ha sometimes produced visible effects without noticeable loss in yield.

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.

Roots of 10- and 14-day-old seedlings and excised leaves of cotton (Gossypium hirsutum L. ‘Coker 310’), jimsonweed (Datura stramonium L.), peanuts (Arachis hypogaea L. ‘Florigiant’), and prickly sida (Sida spinosa L.) were treated with ring-labeled 14C-tetrafluron {N,N-dimethyl-N′-[3-(1,1,2,2-tetrafluoroethoxy)phenyl] urea} in liquid culture. Herbicide absorption and subsequent translocation were limited in peanuts, intermediate in cotton, and very high in jimsonweed and prickly sida. Absorption of 14C-tetrafluron increased with time in 10- and 14-day-old seedlings. After 6 h, 10-day-old jimsonweed had absorbed as much as 50% of the initial 10-μg dose, whereas peanuts absorbed only 10%. Almost 100% uptake was observed after 48 h with 14-day-old seedlings of jimsonweed and prickly sida. Methanol-extracted 14C increased with time, and was higher for the weeds than for the crops. Only a small fraction of methanol-insoluble radioactive material was obtained, with the largest value in 14-day-old cotton at 48 h (3.0%). Autoradiographs demonstrated more rapid translocation of radioactivity from roots to shoots in jimsonweed and prickly sida than in cotton and peanuts. Limited absorption and translocation of tetrafluron in seedlings of peanut and cotton, and the rapid absorption and subsequent accumulation of high concentrations in seedlings of jimsonweed and prickly sida, are proposed to explain the selective action observed.

Growth chamber experiments were conducted to determine the effects of soil temperature on the response of corn to imazaquin soil residues. In a silt loam soil, 24/30 C (night/day) or 18/24 C soil temperatures caused greater inhibition of shoot growth than 12/18 C soil temperature. However, in a sandy loam soil, inhibition of corn shoot growth was maximal at 18/24 C, and there was no difference in shoot-growth inhibition between the lowest and highest temperatures. Higher soil temperatures caused greater root-growth inhibition in the sandy loam soil but not in silt loam soil. Soil temperature did not affect 14C-imazaquin uptake from either soil. Higher soil temperatures increased the translocation of imazaquin from root to shoot tissue in both soils. In the sandy loam soil, imazaquin metabolism in root tissue decreased as soil temperature increased, with twice as much parent herbicide recovered from roots of plants grown under the highest compared with the lowest temperature treatments. Soil temperature had no effect on imazaquin metabolism in shoot tissue. Longer-term experiments (22 d) were conducted with the sandy loam soil to determine the effect of changes in air temperature on corn response to imazaquin soil residues. Plants exposed to 24/30 C for 7 or 14 d of the final 14-d growing period showed greater inhibition of shoot growth compared with plants maintained at 12/18 C. Uptake and translocation of 14C-imazaquin to shoots was greater in plants maintained at 24/30 C throughout the final 14-d period than in plants maintained at 12/18 C. Plants grown for 7 d at 24/30 C during the final 14-d period either preceding or following 7 d growth at 12/18 C showed increased translocation of imazaquin to shoots but no difference in imazaquin uptake compared with plants maintained at 12/18 C. Neither air nor soil temperature treatments had any effect on imazaquin concentration in soil water.