Herbicides that persist in the forest litter and soil following their use for managing invasive plant species may negatively affect restoration efforts as well as minimise re-invasion via their residual phytotoxic activity. This study determined the impact of an herbicide mixture comprising triclopyr, dicamba, picloram and aminopyralid (‘TDPA’) for the control of dense infestations of a woody invader, Pinus contorta, on the germination of re-invading P. contorta and three New Zealand native species (Chionochloa rubra, Fucospora cliffortioides, and Leptospermum scoparium) used in restoration. Given the essential role of ectomycorrhizal fungi in facilitating pine re-invasion, the impact of residual herbicides present in mineral soil on the ectomycorrhizal infection of P. contorta seedling roots was also examined. Germination trials were conducted using intact forest litter-soil cores collected at 27, 112 and 480 days (after herbicide spraying) from sprayed and adjacent unsprayed dense P. contorta infestations. At the same time, mineral soil was also collected for the ectomycorrhizal infection study. Post-spray herbicide residue bound in the litter significantly decreased germination rate, root and shoot growth and survival, and also caused malformation of P. contorta seedlings. Similar results were recorded for native species germination, however, overall viability of native seed was poor resulting in low germination rates. There was no difference in levels of ectomycorrhizal infection rates of P. contorta between treatments. Results indicate residual levels of TDPA herbicide in forest floor litter negatively affect P. contorta re-invasion, native recruitment and active restoration management. Ectomycorrhizal fungi, however, are unaffected by this herbicide mixture and therefore remain a risk to facilitating re-invasion as residual herbicide declines.

Amaranthus species are problematic weeds in snap bean production systems. They reduce crop yields, and their stem fragments contaminate harvested pods. Knowledge of snap bean tolerance to different preemergence herbicides is limited; however, knowing this tolerance is essential for planning a reliable weed management system, breeding herbicide-tolerant cultivars, and registering herbicides for use on minor crops such as snap bean. Field trials were conducted in 2021 and 2022 to determine the tolerance of eight snap bean cultivars to preemergence herbicides with activity on Amaranthus species, including dimethenamid-P, flumioxazin, lactofen, metribuzin, saflufenacil, and sulfentrazone. Snap bean plant density (number of plants per square meter), plant biomass (grams per plant), and canopy biomass (grams per square meter) 21 d after treatment were used to assess crop tolerance to a range of herbicide rates. Linear mixed-effects regression models were fitted to quantify the relationships between preemergence herbicide rate and snap bean cultivar tolerance. Results indicated a high margin of crop safety with dimethenamid-P and lactofen for weed control in snap bean, and a low margin of crop safety with metribuzin and saflufenacil. Results indicated differential cultivar tolerance to flumioxazin and sulfentrazone, which could be driven by genetic variability among cultivars.

Soybean [Glycine max (L.) Merr.] that lack resistance to auxin herbicides [i.e., not genetically modified for resistance] have well-documented responses to those particular herbicides, with yield loss being probable. When a soybean field is injured by auxin herbicides, regulatory authorities often collect a plant sample from that field. This research attempted to simulate soybean exposures due to accidental mixing of incorrect herbicides, tank contamination, or particle drift. This research examined whether analytical testing of herbicide residues on soybean to aminocyclopyrachlor (ACP), aminopyralid, 2,4-D, or dicamba would be related to the visual observations and yield responses from these herbicides. ACP and aminopyralid were applied to R1 soybean at 0.1, 1, and 10 g ae ha−1; 2,4-D and dicamba were applied at 1, 10, and 100 g ae ha−1. Visual evaluations and plant sample collections were undertaken at 1, 3, 7, 14, and 21 d after treatment (DAT), and yield was measured. The conservative limits of detection for the four herbicides in this project were 5, 10, 5, and 5 ng g−1 fresh weight of soybean for ACP, aminopyralid, 2,4-D, and dicamba, respectively. Many of the plant samples were non-detects, especially at lower application dosages. All herbicide concentrations rapidly declined soon after application, and many reached nondetectable limits by 14 DAT. All herbicide treatments caused soybean injury, although the response to 2,4-D was markedly lower than the responses to the other three herbicides. There was no apparent correlation between herbicide concentrations (which were declining over time) and the observed soybean injury (which was increasing over time or staying the same). This research indicated that plant samples should be collected as soon as possible after soybean exposure to auxin herbicides.

Recently, there has been emphasis on the need to shift away from the use of synthetic chemical herbicides to low-risk alternatives derived from natural sources. This is aimed at lowering or averting the negative impact synthetic herbicides have on the environment and dealing with the emergence of weed species resistant to these chemicals. As a result, more stringent measures or outright bans on the use of most synthetic herbicides have been put in place by regulatory bodies. As seaweeds are abundant resources in the marine environment that have the capacity to produce diverse bioactive compounds, they could serve as sustainably viable, natural, and low-risk alternatives/sources to explore for potential phytotoxic capabilities. This could in turn help to enhance or boost the availability of effective solutions in the global bioherbicide market. This review highlights the prospects of using seaweeds as novel biopesticides for the control and management of various plant pests, including weed species, and for the development of sustainable agriculture/forestry practices. More specifically, it focuses on their use as a rich natural source for novel bioherbicide development, a potential that has remained underexplored for many years. However, to unlock the full potential of seaweed-derived bioherbicides and to create a potential path toward their development, increased research and development efforts are urgently needed to tackle and overcome possible constraints posed in this novel area, such as variability in seaweed chemical composition, formulation technologies, stability and efficacy of seaweed bioactive compounds, cost and scalability, and environmental considerations.

Australian grain growers are showing interest in winter-planted sorghum to avoid heat and water stress during the grain-filling stage. Winter-planted sorghum may face competition from winter weeds, including sterile oats, and no herbicides are registered for controlling winter weeds in winter-planted sorghum. The objectives of this study were to (1) identify alternate herbicide options for sterile oats control in winter-sown imidazolinone (IMI)-resistant sorghum and (2) assess the crop injury levels due to herbicides. Sterile oats control with pendimethalin at 0.59 kg ai ha−1, trifluralin at 0.38 kg ai ha−1, and prosulfocarb + S-metolachlor at 2.3 kg ai ha−1 was poor (<30%). Atrazine at 2.7 kg ai ha−1, imazamox + imazapyr at 0.048 kg ai ha−1, and atrazine at 2.7 kg ai ha−1 followed by imazamox + imazapyr at 0.048 kg ai ha−1 reduced the sterile oats biomass by 93%, 96%, and 100% and increased yields by 116%, 136%, and 140%, respectively, compared with nontreated control. Pendimethalin at 0.59 kg ai ha−1 and trifluralin at 0.38 kg ai ha−1 caused phytotoxicity to the crop and gave similar yields to nontreated control. Triallate at 0.8 kg ai ha−1, pyroxasulfone at 0.1 kg ai ha−1, and terbuthylazine at 1.0 kg ai ha−1 provided moderate weed control (44% to 65%) and increased yields by 68%, 108%, and 80%, respectively, compared with nontreated control. This research identified herbicide treatments for the effective control of sterile oats in winter-sown IMI-resistant sorghum that could be used in rotations to reduce the reliance on single herbicide treatments.

Acacia dealbata Link is one of the main invasive species in southwestern Europe and a resource with potential value for agriculture. Our objective was to assess the value of A. dealbata vegetative aerial biomass used as green manure and as a tool for weed control in maize crops through three sequential experiments. In 2017, an experiment was carried out with acacia green manure vs inorganic fertilization of pots sown with a field corn and a sweet corn hybrid with strong and weak nutrient demand, respectively. Nutrients were not released from acacia green manure at an appropriate timing, and maize suffered nutrient deficit. In 2018, a pot experiment was made outdoors incorporating acacia green manure at different times before maize sowing, and we found that a 4-month period was required for maximum nutrient release from acacia green manure. In 2019, an early and a late-field experiments were performed by incorporating acacia green manure 4 months before maize sowing. Physiological and agronomic data were recorded in maize, along with soil data, for all years, and weed data the last year. Altogether, most effects and interactions between genotype or environment and fertilization treatment were not significant, and some deficiencies caused by acacia green manure fertilization depend on genotype and environment. Incorporation of acacia green manure 4 months before maize sowing partially controlled weeds and replaced inorganic fertilization. However, deficiencies should be corrected with additional weed control practices and fertilization treatments, according to the nutrient demand of the crop and the soil environment.

Pigeonpea has great potential as a profitable summer legume rotational crop in cereal farming systems of subtropical Australia. Pigeonpea requires season-long weed control, but options for controlling broadleaf weeds in pigeonpea with POST herbicides are limited. The objective of this study was to evaluate the performance of different herbicides (PRE: pendimethalin; POST: acifluorfen, bentazon, and imazapic) applied singly or in sequence for horse purslane control in pigeonpea and their impact on pigeonpea yield. Field experiments were conducted in 2017 and 2018 at Gatton, Australia. Pendimethalin applied PRE at 1.14 kg ai ha−1 reduced horse purslane biomass by 87% and 92% and produced 32% and 105% higher grain yield compared with the nontreated control in 2017 and 2018, respectively. Imazapic applied POST at 0.10 kg ai ha−1 reduced horse purslane biomass by 79% and 82% and increased grain yield by 60% and 88% compared with the nontreated control in 2017 and 2018, respectively. Acifluorfen applied POST (0.34 and 0.42 kg ai ha−1) caused 16% to 48% injury to pigeonpea at 45 d after treatment. Control of horse purslane ranged from 87% to 92% (biomass reduction) with pendimethalin applied PRE at 1.14 kg ai ha−1 and was comparable with pendimethalin applied PRE at 0.91 kg ai ha−1 in the sequential application, and imazapic at 0.08 kg ai ha−1 or bentazon at 0.96 kg ai ha−1. The study findings suggest if farmers miss the PRE application of pendimethalin or are unable to achieve season-long weed control, POST application of imazapic is an alternate. This research provided herbicide options for control of horse purslane in pigeonpea that could be used in rotations for reducing the selection pressure of weeds.

Plants that release molecules affecting other plants are a source of potential bioherbicides. Silver wattle (Acacia dealbata Link), considered invasive worldwide, was found to be phytotoxic to various other plant species. Combining the search for alternative bioherbicides while reducing the spread of this invader by preventing seed formation is a good potential strategy to solve both agricultural and environmental problems. This study aimed to identify nonvolatile compounds from A. dealbata flowers and explore their phytotoxicity on the germination process and seedling and plant growth of lettuce (Lactuca sativa L.), wheat (Triticum aestivum L.), and rigid ryegrass (Lolium rigidum Gaudin). We identified methyl cinnamate and methyl anisate as potential phytotoxins in the extracts, but we used pure commercial molecules to conduct bioassays. Methyl cinnamate showed higher phytotoxicity than methyl anisate and was selected for further bioassays. Methyl cinnamate reduced guaiacol peroxidase activity by 57% and 85% in L. rigidum and lettuce, respectively, and α-amylase by 6% in L. rigidum. This compound also inhibited early stem and radicle growth of dicotyledonous lettuce (60% and 89%, respectively) and monocotyledonous L. rigidum (76% and 87%, respectively), both species having small seeds. However, wheat with a larger seed size was not affected by the phytotoxin. The results obtained indicate a potential bioherbicidal effect for methyl cinnamate, and its application might be useful in wheat crops infested by L. rigidum. We suggest that collecting A. dealbata flowers would prevent Acacia seed formation and thus play a role in invasive pest management, as well as serving as a source of potential herbicides to other species.

Synthetic-auxin herbicides are often applied for horseweed control before soybean planting. However, certain days of planting interval must be maintained before soybean planting, depending on the product and rate used, because of potential crop phytotoxicity. Halauxifen-methyl is a new synthetic-auxin herbicide for horseweed control in preplant applications in soybean. Field experiments were conducted in 2015 and 2016 in Indiana to evaluate soybean phytotoxicity in response to applications of halauxifen-methyl (5 g ae ha−1) at five preplant intervals (0, 1, 2, 3, and 4 weeks before planting [WBP]). In 2015, soybean phytotoxicity was not observed for any of the preplant intervals at any of the sites. In 2016, 0% to 15% phytotoxicity was observed at 14 d after planting (DAP) when halauxifen-methyl was applied at planting, 1 WBP, and 2 WBP at different sites. Soybean phytotoxicity was expressed in the unifoliate leaves only at 14 DAP. However, the first trifoliate did not show any injury symptoms at 21 DAP from any preplant application timing. Preplant application intervals for halauxifen-methyl did not affect soybean stand counts or grain yield in any site-year. Therefore, field results indicated that halauxifen-methyl applied alone can cause slight soybean phytotoxicity in preplant applications. In growth-chamber bioassays, reductions in soybean biomass, plant length, and emergence were accentuated at 30 C, compared with 20 or 15 C, when halauxifen-methyl was applied at 20 or 40 g ae ha−1. These results contradict the currently held paradigm in which lower temperatures generally increase crop phytotoxicity levels to herbicide soil residual.

Flax yield can be severely reduced by weeds. The combination of limited herbicide options and the spread of herbicide-resistant weeds across the prairies has resulted in a need for more weed control options for flax producers. The objective of this research was to evaluate the tolerance of flax to topramezone, pyroxasulfone, flumioxazin, and fluthiacet-methyl applied alone as well as in a mix with currently registered herbicides. These herbicides were applied alone and in mixtures at the 1X and 2X rates and compared with three industry standards and one nontreated control. This experiment was conducted at Carman, MB, and Saskatoon, SK, as a randomized complete block with four replications. Data were collected for crop population, crop height, yield, and thousand-seed weight. Ratings for crop damage (phytotoxicity) were also taken at three separate time intervals: 7 to 14, 21 to 28, and 56+ d after treatment. Crop tolerance to these herbicides varied between site-years. This was largely attributed to differences in spring moisture conditions and the differences in soil characteristics between sites. Herbicide injury was transient. Hence, no herbicide or combination of herbicides significantly impacted crop yield consistently. Flumioxazin was the least promising herbicide evaluated, as it caused severe crop damage (>90%) when conditions were conducive. Overall, flax had excellent tolerance to fluthiacet-methyl, pyroxasulfone, and topramezone. Flax had excellent crop safety to the combination of pyroxasulfone + sulfentrazone. However, mixing fluthiacet-methyl and topramezone with MCPA and bromoxynil, respectively, increased crop damage and would not be recommended.

Field experiments were performed in 2016 and 2017 in Missouri to determine whether interactions exist between PRE herbicides and seed treatments in soybean. The experiments consisted of a randomized complete block design with factorial arrangements of varieties, seed treatments, and herbicides. We selected two genetically similar varieties of soybean, one with known tolerance to PPO-inhibiting herbicides and one with known sensitivity. Each variety of seed received three separate seed treatment mixtures (STMs): (1) STM1, imidacloprid plus prothioconazol+penflufen+metalaxyl plus metalaxyl plus Bacillus subtilis+B. pumilis, (2) STM2, Pasteuria nishizawae plus thiamethoxam plus prothioconazol+penflufen+metalaxyl plus metalaxyl plus B. subtilis+B. pumilis, and (3) STM3, fluopyram plus imidacloprid plus prothioconazol+penflufen+metalaxyl plus metalaxyl plus B. subtilis+B. pumilis. Chlorimuron-ethyl+flumioxazin+pyroxasulfone, chlorimuron-ethyl+flumioxazin+metribuzin, and chlorimuron-ethyl+sulfentrazone were applied PRE to each variety and seed treatment combination at 1× and 2× the labeled use rate. Chlorimuron-ethyl+sulfentrazone treatment at the 2× rate resulted in greater injury of 8% and 14% to the sensitive variety than the tolerant in 2016 and 2017, respectively; this was the highest injury observed from any herbicide treatment in either year. In 2017, chlorimuron-ethyl+sulfentrazone resulted in the greatest height reductions in both varieties, but this reduction was more evident in the sensitive (19%) than in the tolerant (6%) variety. Overall, yield differences between the two varieties were not consistent between years, and for both varieties, the sulfentrazone-containing treatments resulted in the highest yield losses. The results of this research indicate that there is a larger interaction between herbicides and varieties than there is between herbicides and seed treatments, or seed treatments and varieties.

A sustainable practice for weed control and crop protection is the incorporation of green manures with phytotoxic potential. It is gaining attention as a way to reduce the use of synthetic herbicides in agriculture and so pot experiments and field trials were conducted to explore the possible use of residues of Acacia species to alleviate weed emergence. We assessed, under greenhouse conditions, the herbicidal effect of phytotoxic manures from Acacia dealbata and Acacia longifolia applied to soil at different doses (1.5 and 3% w/w) on maize growth, some accompanying weeds, and the physiological profile of soil microbes. Applied at a higher dose, A. dealbata residues reduced the emergence of dicotyledons in the short-term (P < 0.05) and, after 30 days, there was a decrease in total weed emergence (P < 0.005) and a mild effect on weed composition, while total weed biomass remained unaffected. Regardless of the inclusion of Acacia residues, the physiological profile of the soil bacterial community did not show significant alterations. Additionally, we tested A. dealbata residues as a mulch or a green manure at the field scale. Although the effects of manures were site-dependent and affected monocot and dicot weeds differentially, dicots were more sensitive. The herbicide potential of acacia residues was only evident for dicots at sites with low-weed density in the seed bank. Nevertheless, due to the absence of phytotoxic effects on maize and minor modifications in the functional profile of bacterial communities, residues of acacia could be used as a complementary tool used together with other practices to reduce the reliance on synthetic herbicides in maize-based cropping systems.

Laboratory and field studies were undertaken to establish the efficacy of some selected botanicals in the control of the tea red spider mite, Oligonychus coffeae Nietner, a major pest of tea. Bioassays were performed to evaluate the dose-mortality response of adults and eggs of O. coffeae to aqueous plant extracts. The order of adulticidal toxicity based on LC50 and slope values was Sapindus mukorossi L. > Nyctanthes arbor-tristis L. > Phlogacanthus thyrsiformis Nees. The order of ovicidal activity was reversed and was P. thyrsiformis > N. arbor-tristis > S. mukorossi. In addition, different concentrations of the aqueous extract of these plants exhibited repellent properties against adult mites and also significantly decreased the deposition of eggs by the mites on treated tea leaf surfaces. In field conditions, the extracts significantly reduced O. coffeae populations compared to propargite, a synthetic acaricide. There was no phytotoxic effect and the quality of tea was not adversely affected by the treatments. Further, the plant extracts caused no mortality or decrease in the predation efficacy of the adults and fourth instar larvae of Stethorus aptus Kapur, a natural predator of O. coffeae. Using the procedure adopted in this study, tea growers can easily formulate these low-cost eco-friendly botanicals for O. coffeae management in tea plantations.

‘Wilmington’ and ‘Pensacola’ cultivars of bahiagrass (Paspalum notatum Flügge) were found to exhibit a differential level of tolerance to certain triazine herbicides. Pensacola was more susceptible to postemergence treatments of atrazine [2 - chloro - 4 - (ethylamino) - 6 - (isopropylamino) -s - triazine], simazine [2 - chloro - 4,6 - bis (ethylamino) - s-triazine], and cyanazine {2 - [ [4 - chloro - 6 - (ethylamino) -s - triazin - 2 - yl] amino] - 2 - methylpropionitrile} than Wilmington. Injury to Pensacola was noted as early as 2 weeks after the application of 2.4 kg/ha atrazine and cyanazine, whereas similar injury to Wilmington was not observed until later than 4 weeks after the treatment date. Results of 14C-atrazine treatments indicated that Wilmington bahiagrass accumulated approximately twice as much 14C-atrazine: peptide conjugate as the Pensacola cultivar.

Yellow nutsedge (Cyperus esculentus L. # CYPES) was treated with unlabeled and 14C-labeled glyphosate [N-(phosphonomethyl)glycine] to study the toxicity and translocation of glyphosate into parent tubers of this weed. When plants of 2 and 4 weeks of age were treated with 0.6 kg ai/ha of glyphosate, fewer parent tubers from treated plants resprouted (14 and 32%) than tubers from control plants (73 and 59%, respectively) of equal age. Also, fewer tubers resprouted from 2-week than from 4-week-old treated plants. The radioactivity of parent tubers from plants treated at 2 weeks of age, which exceeded that of tubers from plants of 4 and 6 weeks of age, indicated that translocation of 14C into parent tubers decreased as the treated plants increased in age. The differential amounts of 14C translocated appear to explain why applications of glyphosate were more effective in suppressing resprouting of parent tubers from 2-week than 4-week-old plants.

The relative effectiveness of eight acetanilide herbicides for controlling annual weeds in processing peas (Pisum sativum L.) was evaluated in field studies and the susceptibility of peas to injury from these herbicides was studied in both the field and greenhouse. Alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] was the most phytotoxic to peas grown in washed silica sand in the greenhouse when the herbicides were applied at concentrations of 2 and 8 mg/L. Field trials conducted over a 2-yr period and simulated rainfall experiments in the greenhouse demonstrated that the phytotoxicity to peas by acetanilide herbicides applied preemergence at 2.2 and 4.5 kg/ha was greatly influenced by subsequent rainfall. When 2.5 cm of simulated rainfall were applied immediately after herbicide application, pea injury increased. Injury to peas was avoided in the field by delaying alachlor application at 2.2 kg/ha until peas began to emerge. This treatment also gave excellent control of annual grass weeds. Greenhouse studies demonstrated that injury to peas by all eight acetanilide herbicides at 2.2 and 4.5 kg/ha could be avoided by delaying application until pea emergence. Alachlor, propachlor (2-chloro-N-isopropylacetanilide), and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] gave slightly superior weed control than butachlor [N-(butoxymethyl)diethylacetanilide] and H 22234 [N-chloroacetyl-N-(2,6-diethylphenyl)-glycine ethyl ester].

Field studies were conducted from 1985 to 1989 on a Sharkey clay to examine injury to a semi-dwarf rice cultivar, ‘Lemont’, from triclopyr or triclopyr plus propanil. Triclopyr applied in the booting stage reduced yield two of three years, with the observed yield reduction possibly caused by epinasty of the rice flag leaf. Triclopyr application to three- to four-leaf rice caused hyponasty. Triclopyr did not reduce plant height, seed weight, germination, or total milling yield. Triclopyr plus propanil caused more leaf burn that triclopyr alone, but yields were not reduced compared with the untreated control. This research indicated that triclopyr and triclopyr plus propanil can be used in rice production with the semi-dwarf cultivar, Lemont, with the potential to minimize drift to non-target crops due to the greater flexibility in application timing compared with 2,4-D application.

The primary goal of this research was to determine whether crop damage from herbicides measured soon after treatment could estimate relative crop yield loss. Two to 4 wk after spraying soybeans with an unregisrered mixture of rhifensulfuron plus sethoxydim at various rates plus crop oil concentrate, percent stunting was visually rated and percent projected ground cover of soybeans was determined from photographs. In each of 3 yr, relative percent soybean yield was a negative linear function of relative herbicide rate from 0.25× to 2× the registered (1×) rate. The 1× rate of thifensulfuron and sethoxydim was 17.5 and 420 g ai ha−1, respectively. Relative soybean yield was also negatively related linearly to stunting but positively related linearly to soybean ground cover over 3 yr. Linear regression equation models of relative soybean yield versus percent soybean ground cover explained more model variability, more consistently (R 2 ≥ 0.60 in two of 3 yr) than did either stunting (R 2 ≥ 0.60 in one of 3 yr) or relative herbicide rate (R 2 = 0.37 to 0.48 over 3 yr). However, linear regression models for each independent variable differed from year to year and were related to differences in rainfall following treatment. Ways are suggested to increase regression model precision and between-year reproducibility.

Quackgrass [Agropyron repens (L.) Beauv.] was controlled by spring application of pronamide [3,5-dichloro(N-1,1-dimethyl-2-propynyl)benzamide] in 1970 and 1971, but when spring rainfall was below nornal in 1973, only pronamide applied the previous fall gave control. Terbacil (3-tert-butyl-5-chloro-6-methyluracil) gave 90% control of quackgrass for 2 yr at 2.2 kg/ha. Some control continued for 3 yr at 4.5 kg/ha. Three applications of glyphosate [N-(phosphonomethyl)glycine] at 3 kg/ha applied during a 2-yr period controlled quackgrass 97 to 100%. Phytotoxicity was not observed on 14-yr-old highbush blueberries (Vaccinium corymbosum L.) from pronamide or terbacil, on 4-yr-old blueberries from pronamide, or on 2-yr-old rhododendrons (Rhododendron × ‘A. Bedford’) from dichlobenil (2,6-dichlorobenzonitrile), pronamide, or directed sprays of glyphosate or amitrole (3-amino-s-triazole).

The effects of hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4 (1H, 3H)-dione] and its metabolites, A [3-(4-hydroxycyclohexyl)-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4 (1H,3H)-dione], B [3-cyclohexyl-6-(methylamino)-1-methyl-1,3,5-triazine-2,4 (1H,3H)-dione], C [3-(4-hydroxycyclohexyl)-6-(methylamino)-1-methyl-1,3,5-triazine-2,4 (1H,3H)-dione], D [3-cyclohexyl-1-methyl-1,3,5-triazine-2,4,6 (1H, 3H,5H)-trione], and E [3-(4-hydroxycy clohexyl)-1-methyl-1,3,5-triazine-2,4,6 (1H,3H,5H)-trione], on photosynthesis of loblolly pine primary needle segments were studied using a bioassay technique. Hexazinone, at concentrations equal to or greater than 1.0 × 10-6 M, inhibited photosynthesis, while 1.0 × 10-8 M and 1.0 × 10-7 M concentrations increased photosynthesis. Metabolite B at 1.0 × 10-4 M also inhibited photosynthesis. Metabolites A, C, D, and E did not affect photosynthesis at 1.0 × 10-4 in pine needle segments. The bioassay proved simple and useful for determining effects of certain herbicides and metabolites on photosynthesis.