Antagonistic interactions between herbicides or between herbicides and nonherbicides are common and prevent the use of certain mixtures. Although antagonism is detrimental when it reduces weed control, the degree of antagonism is critical to determine whether a mixture is agronomically useful. If antagonsim is not severe, herbicides with different weed spectrums can be used together to complete the weed spectrum. Antagonism can be beneficial when it increases crop safety. In cases where antagonism significantly reduces control, key factors must be identified and managed to decrease antagonism and increase efficacy. These factors, which include the herbicide rate(s), mode of action, plant species, formulation, adjuvants, timing, stage of growth, and environment, often are complex and poorly understood at the whole plant level. The growing dependence on herbicide mixtures has increased the practical importance of understanding herbicide antagonism.

Adjuvants are used to improve spray delivery, to increase spray retention on weed foliage, and to enhance foliar penetration by postemergence herbicides for increased herbicide efficacy. Numerous factors govern adjuvant efficacy, including the specific herbicide, plant species, and environmental conditions. Adding superior surfactants as adjuvants to a sethoxydim tank mix with Na-bentazon may overcome the Na-bentazon antagonsim of sethoxydim activity by facilitating increased sethoxydim absorption and by masking the antagonism similar to increasing the sethoxydim rate. Herbicides such as Na-bentazon contribute alkaline or alkaline earth cations that can form salts of a weakly acidic herbicide such as sethoxydim that are not absorbed readily by the plant. Adding abundant ammonium from ammonium sulfate or other sources may prevent and/or may overcome this antagonism.

Mortality of potted huisache seedlings averaged 87 to 100% 18 weeks after picloram or clopyralid was applied with a model carpeted roller at concentrations as low as 7.5 g ae/L. A 1:1 mixture of picloram and clopyralid was equally effective. Triclopyr was effective only when applied to small seedlings in the spring, and an ester was more effective than an amine formulation. A 1:1 mixture of clopyralid and triclopyr ester was intermediate in effectiveness between the two herbicides applied alone. Adding surfactant increased canopy reduction and plant mortality (P = 0.1 to 0.01), especially at lower herbicide concentrations.

Experiments were conducted in 1985 and 1986 at three locations in eastern Arkansas to evaluate red rice control in soybeans with postemergence grass herbicides and plant growth regulators applied singly or sequentially at early to late-tillering growth stages of red rice. Haloxyfop at 0.21 kg ai/ha and quizalofop at 0.14 kg ai/ha applied singly or sequentially and fluazifop at 0.21 kg ai/ha applied sequentially consistently controlled red rice and suppressed seedhead production in soybeans. Mid-season treatments were not beneficial when high soil moisture stress conditions existed. Mefluidide or sethoxydim applied singly or sequentially or amidochlor applied singly provided erratic control and seedhead suppression of red rice in soybeans.

A survey of 146 fields was conducted to investigate herbicide performance in winter wheat-producing areas of southwestern Nebraska during August and September of 1986. Only 55% of the fields received an excellent rating for weed control and stubble quality; one third rated as unacceptable. Weed control after wheat harvest was improved by planting ‘Bounty 310’, ‘Siouxland’, ‘Vona’, and ‘Centura’ winter wheat cultivars rather than ‘Mustang’, ‘Hawk’, ‘Pioneer 2656’, and ‘Wings'. Fertilizing winter wheat in the fall, planting wheat at the optimum date, high wheat stem density, using a winter wheat-corn-fallow rotation, not spraying herbicides after wheat harvest on days that it rained or air temperature exceeded 35 C, and spraying weeds when they were small also improved weed control in wheat stubble. Nine months after wheat harvest, fields treated with atrazine before July 16 had more volunteer wheat than fields treated later.

Weed control with thiameturon applied postemergence at 8 to 32 g ai/ha varied with species, growth stage, addition of surfactant, and environmental conditions. Pennsylvania smartweed control was excellent, velvetleaf control ranged from poor to excellent, and common lambsquarters control was poor with thiameturon applied without surfactant. When surfactant was added to the thiameturon spray mixture, velvetleaf and common lambsquarters control was >85% at 8 g/ha when weeds were growing actively and at 16 g/ha when weeds were under moisture stress. However, in weed-free studies, corn yields were reduced by 35 kg/ha for each 1 g/ha thiameturon applied in 1986 and by 10 kg/ha for each 1 g/ha thiameturon applied in 1987 when surfactant was added to the spray mixture. When no surfactant was added, yields were reduced 12 kg/ha for each 1 g/ha thiameturon applied in 1986 but were not reduced in 1987.

Combinations of grass and legume mulches were planted in growing corn during the fall in 1985 and 1986, and the following spring no-till corn was planted into these living mulches. Mulch treatments consisted of a single species or grass plus legume mixtures. Fluazifop-P, 2,4-D, and atrazine were broadcast applied in late April to suppress the mulches and reduce their competition with corn. Chewings fescue and ladino clover competed least with dryland corn. Weed growth associated with chewings fescue and the ladino clover mulches was similar to that in the conventional disk-plant treatment, but corn yields were lower. Hairy vetch mulch was killed by 2,4-D. The winter rye mulch competed with corn and reduced yield.

Field experiments were conducted from 1983 through 1987 to evaluate various postemergence grass herbicides on bermudagrass in peanuts. Fluazifop, haloxyfop, and SC-1084 applied once effectively controlled common bermudagrass when less than 8 cm tall, but resulted in variable control when applied to bermudagrass 15 cm or taller. Sethoxydim effectively controlled common bermudagrass one of 2 yr. ‘Coastal’ bermudagrass required two applications of BAS 517, fluazifop-P at 0.21 or 0.28 kg ai/ha, and haloxyfop for control of better than 88%. Clethodim and sethoxydim provided poor to fair control, respectively.

Experiments were conducted at Bridgeport, NE, during 1983 through 1987 to select alternatives for silvex and 2,4,5-T for sand sagebrush and brittle pricklypear control. Of the six herbicides examined, the butoxyethyl ester of 2,4-D at 2.2 kg ae/ha was equivalent to the propylene glycol butyl ether ester of silvex or 2,4,5-T for sand sagebrush control. The potassium salt of picloram at 0.3 kg ae/ha was equal to silvex for brittle pricklypear control.

Variation in size and distribution patterns of granules of a blank hexazinone formulation were studied in both ground and aerial applications. Granule size ranged from 2.5 to 93 mg with 90% of the granules between 5 and 35 mg. The granule size frequency among weight classes did not change within a 5-m wide test swath from a truck-mounted applicator, indicating no impact on the distribution of active ingredient. The ground applicator did not produce a uniform deposition of granules. Average granule depositions were 11 ± 5 kg/ha and 16 ± 8 kg/ha for the target rates of 20 and 40 kg/ha, respectively. Aerial application with a helicopter-mounted bucket spreader produced a 27-m wide effective swath. Average deposition rates were 19 ± 5 kg/ha and 39 ± 9 kg/ha for the target rates 20 and 40 kg/ha, respectively. The optimal sample sizes for aerial application were estimated to be 27 and 50 samples for 40 (±10%) and 20 (±10%) kg/ha rates, respectively, with 90% certainty.

An area of a shrink-swell clay soil (Tunica clay, Vertic Haplaquept) with an established population of redvine, trumpetcreeper, honeyvine milkweed, redberry moonseed, and maypop passionflower was treated with dicamba once in the fall of 1983. The effect on perennial vines was determined for the following 4 yr in three rotational cropping systems involving winter wheat, soybean, corn, and sorghum, all with and without irrigation. Dicamba reduced the population of perennial vines 80% over 4 yr. Redvine and trumpetcreeper, the first and second most abundant species, were reduced by over 83 and 76%, respectively. Yield of soybean increased 17% in 1985 and 1987 while corn yield increased 9% in 1986 with dicamba use. In 1984 no effects on crop yield were measured. This inconsistent crop yield reponse after dicamba treatment, even though perennial vines were suppressed, must be considered in evaluating the economics of using dicamba for perennial vine control.

Tolerance and subsequent yield response of established alfalfa, red clover, alsike clover, sainfoin, birdsfoot trefoil, and cicer milkvetch to sethoxydim and fluazifop spring applied and to hexazinone, metribuzin, and terbacil fall applied were determined in a field study. All legumes tolerated sethoxydim. Fluazifop was safe on all legumes except sainfoin. Alfalfa and cicer milkvetch tolerated hexazinone, metribuzin, and terbacil. Alfalfa dry matter yield was not affected by any of the herbicide treatments, but cumulative cicer milkvetch yield increased 9% over a 3-yr period with hexazinone applications. Sainfoin yield increased 20% with hexazinone and terbacil treatment. Hexazinone injured red clover and reduced yield. Alsike clover was the most susceptible legume to the residual herbicides. Weed dry matter yield associated with the legumes indicated that alfalfa and sainfoin were the most and the least competitive species, respectively. Plots treated with hexazinone contained the least amount of weeds, regardless of the legume species. Field peas seeded in rotation was not affected by herbicide residues; however, residues from terbacil applied at 1.0 kg/ha reduced seed yield of lentils.

Laboratory and greenhouse experiments were conducted to determine if activated charcoal can remove norflurazon from water and from a sand matrix, thereby reducing its availability for plant uptake. A ratio of approximately 10:1 activated charcoal:norflurazon completely inactivated norflurazon in water. In a sand matrix, activated charcoal: norflurazon ratios of approximately 100:1 and 200 to 300:1 completely reduced norflurazon injury to cotton and to soybean and corn, respectively. The amount of activated charcoal required to remove norflurazon from soil was estimated from bioassay and regression analyses. The accuracy of this method to eliminate injury to cotton without reducing control of prickly sida and seedling johnsongrass was confirmed by bioassay using a sandy loam soil.

Growth chamber experiments were conducted to determine if certain genetic, environmental, and chemical factors or their interactions promote chloroacetamide herbicide injury to corn seedlings. Greater chloroacetamide injury occurred with ‘Pioneer 3320’ than with ‘Pioneer 3780’ hybrid corn, with 15 C than with 30 C soil temperature, with soil moisture at 105% field capacity (FC) than at 75% FC, with alachlor than with metolachlor, and with a herbicide rate of 2.2 kg/ha than with 1.1 kg/ha. Covering the plant containers with clear plastic until seedling emergence caused alachlor to be more phytotoxic to corn than metolachlor. Most factors evaluated increased the injury to corn in an additive manner.

Preliminary evaluation of 11 pepper genotypes indicated a high degree of variability in bentazon tolerance. ‘Bohemian Chile’ and ‘Santanka’ hot pepper tolerated bentazon similarly in subsequent greenhouse and field experiments. Yields or shoot fresh weights of these two cultivars were not reduced by up to 9.0 kg ai/ha bentazon in the field. ‘Keystone Resistant Giant’ sweet pepper was more susceptible to bentazon compared to the tolerant cultivars, but it was more tolerant than the highly susceptible cultivar, ‘Sweet Banana’. An F1 hybrid of Keystone Resistant Giant and Santanka tolerated bentazon better than Keystone Resistant Giant but slightly less than Santanka. Thus, bentazon tolerance in this genotype is genetically transferrable, and increasing bentazon tolerance through conventional plant breeding techniques may be possible.

The duration and intensity of unicorn-plant interference on lint yield of cotton were evaluated in the field. Random densities of 5.5 ± 1.1 unicorn-plant/m2 reduced lint yield by 41 kg/ha or about 5% for each week that unicorn-plant was present. Interference by 4, 8, and 12 weeds/10 m row decreased yield by 22, 49, and 56 kg/ha, respectively, for each week of weed interference. Each 1 kg/ha of unicorn-plant dry weight reduced lint yield by 0.26 kg/ha. Linear regression of weed dry weight could be used to predict cotton lint yield changes regardless of duration or intensity of weed interference.

Several plant growth regulators applied to established sod driveways in an apple orchard suppressed growth of the ground cover sufficiently to eliminate one to three mowings. MH at 4.5 or 6.7 kg ai/ha applied spring and fall reduced the growth of a single species sod cover crop, ‘Kentucky 31’ tall fescue, the year after treatment. MH at both rates also reduced the dandelion population growing in the mixed species orchard sod. Paclobutrazol or EPTC applied in the spring before or during initial grass growth reduced dry matter production in the fescue sod cover crop and the number of mowings compared to the mowed and non-mowed control plots.

A scentless plant bug feeds on velvetleaf seeds. Fungi, dominated by the genera Fusarium and Alternaria, were isolated from insect-attacked seeds at levels related to insect density on the plants. The combined effects of insect feeding and fungal infection decreased seed germination. Burial of insect-attacked seeds in soil for 24 months reduced seed survival and increased Fusarium infection. Decreases in velvetleaf seed viability and survival in soil caused by a seed-feeding insect and associated seed fungi suggests that subsequent infestations by velvetleaf can be decreased through integrated use of the two biological control agents.