Field studies were established to evaluate several herbicides for buckbrush control in tall fescue pastures. Buckbrush was controlled most consistently by 2,4-D amine or low volatile ester, or package-mixes of 2,4-D plus dicamba or triclopyr. Two consecutive years of herbicide application was more effective for reducing buckbrush stem populations than one application.

Honey mesquite (Prosopis glandulosa Torr. # PRCJG) was treated with four herbicides at 1.1 kg ae/ha at 14 dates from April 30 through September 23 over a 2-yr period. Overall, clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) was the most effective herbicide, killing 80% or more of the honey mesquite at most dates. Picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid), triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, and 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] were less effective than clopyralid. Honey mesquite control was correlated with Julian day of year sprayed, 15 plant, 11 environmental, and 8 herbicide concentration variables. Most effective control was negatively correlated with Julian day of year sprayed and new xylem ring thickness. Control was positively correlated with total daily apparent photosynthesis, upward methylene blue dye movement in the xylem, and herbicide concentration in the xylem 3 days after spraying. The only environmental variable of 11 measured that was significantly correlated with honey mesquite control was soil water and that was only with picloram. The concentration of herbicide in the stem was highest for clopyralid, followed by picloram, and then by 2,4,5-T and triclopyr, which were about equal. Variables most useful in regression equations for predicting control with herbicides were total daily apparent photosynthesis and rate of upward movement of methylene blue dye. Variables most incorporated into regression equations relating honey mesquite control with herbicide content were herbicide concentrations in the xylem 3 or 30 days after treatment.

Aerial applications of pelleted tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} at 0.5 kg/ha reduced creosotebush [Larrea tridentata (DC.) Coville] density by up to 86% and by 99% when applied at 1 kg/ha in February or March. Grass production 32 months after treatment was 174 to 305% greater in tebuthiuron-treated plots than on untreated ran gel and, but forb production in treated plots was reduced 38% or more and the poisonous half-shrub broom snakeweed [Gutierrezia sarothrae (Pursh) Britt. and Rusby] was reduced 94% or more.

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.

Invasion of rootplowed rangeland by blackbrush acacia (Acacia rigidula Benth. ♯ ACARI) is a problem in south Texas. Low-energy grubbing uprooted plants 1 to 2.5 m tall in densities of 50, 150, and 300 plants/ha at rates of 3.0, 1.6, and 1.0 ha/h, respectively, yielding an average plant kill of 85%. Plants grubbed above the second major lateral root sprouted. A grubbing depth of 20 to 30 cm was necessary to assure plant death. A grubbing blade modified by the addition of ripper teeth facilitated grubbing under dry soil conditions and extended the grubbing season. Contract cost for grubbing the average density of 169 plants/ha was $20.80/ha adjusted to 1983 dollars.

Triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} was aerially applied at four locations in Texas to determine effectiveness of the amine and ester formulations for the control of honey mesquite [Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell # PRCJG]. The ester formulation was superior to the amine in most trials. Honey mesquite control with triclopyr amine was comparable to that with 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] while control with triclopyr ester was similar to that attained with a 1:1 mixture of 2,4, 5-T and picloram (4-amino-3,5,6-trichloropicolinic acid). Honey mesquite control obtained with a 1:1 mixture of 2,4,5-T and dicamba (3,6-dichloro-o-anisic acid) was intermediate between that with the 2,4,5-T and picloram mixture and that with 2,4,5-T alone. Efficacy of triclopyr increased as rates were increased from 0.3 to 0.6 kg ae/ha and from 0.6 to 1.1 kg/ha. Triclopyr formulations were effective when applied in either water or diesel oil/water emulsions. Honey mesquite control from triclopyr in combination with picloram was generally comparable to that from the mixture of picloram and 2,4,5-T.

Field studies were conducted from 1985 through 1987 to evaluate control of wild jujube, a deciduous, thorny shrub that causes complete wheat and other crop loss by preventing harvest operations. Initial herbicide or sweep tillage treatments were applied in late June or July, as plants approached maximum foliar development, to wild jujube regrowth from plants cut at ground level the previous fall. Nearly 1 yr after treatment, over 97% reduction of wild jujube regrowth was achieved with initial treatments of 7.2 and 10.7 g ae/L of the isopropylamine salt of glyphosate, 2.4 and 3.6 g ae/L of the potassium salt of picloram, and mixtures of glyphosate plus picloram at half those concentrations, followed 2 to 3 mo later with a repeat herbicide treatment or sweep tillage. About 60% reduction in wild jujube regrowth was achieved with two sweep tillage operations 25 cm deep. Mixtures of half rates of glyphosate plus picloram were less costly than full rates of glyphosate alone and had less carryover to wheat than full rates of picloram alone.

Sand shinnery oak (Quercus havardii Rydb.) was controlled by both picloram (4-amino-3,5,6-trichloropicolinic acid) and tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea} in the Rolling Plains vegetational region of Texas. Tebuthiuron provided acceptable control of sand shinnery oak at rates of 0.6 to 1.1 kg/ha, but 2.2 kg/ha of picloram was required to give comparable results. Tebuthiuron pellets applied at 0.6 kg/ha controlled 79% of the sand shinnery oak within 30 to 67 months after application. Applied at 1.1 kg/ha, tebuthiuron controlled 91% (average for three locations). No differences were found between formulations of a given herbicide.

The herbicide 3,6-dichloropicolinic acid was more effective than 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid] or equal-ratio combinations of 2,4,5-T with picloram (4-amino-3,5,6-trichloropicolinic acid) or dicamba (3,6-dichloro-o-anisic acid) when aerially applied as broadcast sprays for control of honey mesquite [Prosopis juliflora (Swartz) DC. var. glandulosa (Torr.) Cockerell]. Applied at 0.56 kg/ha, 3,6-dichloropicolinic acid controlled 60 and 68% of the honey mesquite at two locations; whereas, the mixture of 2,4,5-T and picloram controlled only 21 and 30% of the honey mesquite at the same rate of application. The herbicide 3,6-dichloropicolinic acid was equally effective when applied alone or in 1:1 mixtures with 2,4,5-T, picloram, or triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} at equivalent rates. Applying 3,6-dichloropicolinic acid at 1.12 kg/ha resulted in an absolute increase in mortality of 26% over the 0.56-kg/ha rate.

Brush control and woody plant community structure in the Cross Timbers of Oklahoma resulting from treatments with herbicides and fire were compared. Tebuthiuron and triclopyr were applied alone and in combination with burning at 2.2 kg ai ha-1 in March and June of 1983, respectively. The burned pastures were burned with strip headfires in late spring of 1985, 1986, and 1987. Both herbicides were effective on the dominant overstory brush species, blackjack oak and post oak, and this resulted in good reduction of canopy cover of brush initially. However, effects of triclopyr were short-lived because of ineffectiveness on many of the other hardwood species (American elm, gum bumelia, hackberry, roughleaf dogwood, and buckbrush). Crown reduction and tree kill of these hardwood species was usually better with tebuthiuron than with triclopyr. Neither herbicide was effective on eastern redcedar. Better brush control, associated with tebuthiuron, resulted in better fine fuel release and by 1988, burning was having a significant effect on woody plants in the tebuthiuron-treated plots.

Solutions containing 24 or 120 g ae/L of amine or salt formulations of glyphosate [N-(phosphonomethyl) glycine], triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, and picloram (4-amino-3,5,6-trichloropicolinic acid) were applied to small shrubs with a carpeted roller. Glyphosate reduced the live canopy cover of false broomweed (Ericameria austrotexana M.C. Johnston) by 83 to 87% at one site but only by 37 to 58% at another location. The lower concentration of picloram reduced false broomweed cover by at least 92%, and the higher concentration completely controlled the weed at both study sites. Effectiveness of triclopyr was usually intermediate between glyphosate and picloram. Removing a scraper designed to abrade stems prior to herbicide deposition reduced the effectiveness of picloram but not other herbicides. The herbicides reduced canopy cover of common goldenweed [Isocoma coronopifolia (Gray) Greene ♯3 IOCCO] by 92 to 100%. Common goldenweed was also controlled by 2,4-D [(2,4-dichlorophenoxy)acetic acid] amine applied with the carpeted roller. In glasshouse experiments, 1.2 to 60 g ae/L of 2,4-D amine reduced live canopy dry weight of common goldenweed by 73 to 88% at 5 months after application with a carpeted roller.

Pelleted tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl] -N,N′-dimethylurea} was applied aerially on duplicate plots at 2.2 and 4.4 kg/ha in spring, summer, fall, and winter of 1978 and 1979. Treatments were made near Bryan in the Claypan Resource Area of Texas on an area supporting a stand of mixed brush consisting mostly of oak (Quercus) species. Soils were sampled at eight locations in each plot at depths of 0 to 15 and 15 to 30 cm in March 1980. Bioassays using ‘Tamcot’ cotton (Gossypium hirsutum L.) and ‘Caddo’ wheat (Triticum aestivum L.) were done in the greenhouse to detect levels of tebuthiuron residues in the soil. A standard curve was developed to compare known concentrations of tebuthiuron to the unknown content from treated plots. Tebuthiuron persisted in all treated soils and ranged from 0.08 to 0.49 μg/g. Concentrations were usually greater in soil treated in 1979 than in 1978 and in soil treated with 4.4 kg/ha of tebuthiuron than 2.2 kg/ha. No consistent differences in tebuthiuron residues existed either between soil depths or among seasons of application using the bioassay.

Yaupon (Ilex vomitoria Ait. ♯ ILEVO) in the Post Oak Savannah of Texas was treated with liquid and dry formulations of herbicides. Sprays of Bay Met 1486 {N-[5-(ethylsulfonyl)-1,3,4-thiadiazol-2-yl]-N, N′-dimethylurea} and picloram (4-amino-3,5,6-trichloropicolinic acid), sprays and pellets of tebuthiuron {N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N′-dimethylurea}, and boluses of hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4(1N,3H)-dione] at 4.5 kg ae or ai/ha were the most effective treatments, reducing the canopy 87 to 99% and killing 68 to 93% of the yaupon after 2 yr. The percentage of dead yaupon after 1 yr was similar to that found 2 or 3 yr after treatment. Most herbicides, except clopyralid (3,6-dichloro-2-pyridinecarboxylic acid), reduced the yaupon canopy sufficiently to allow an increase in understory grass and broadleaf vegetation, particularly 1 yr after treatment.

Understory vegetation was studied following treatment by herbicides and fire on grazed pastures within the Cross Timbers vegetation type. Tebuthiuron and triclopyr at 2.2 kg ha-1 were applied in March and June of 1983, respectively. The pastures were burned in late spring of 1985, 1986 and 1987. Frequency of horseweed, rosette panicgrass, and little bluestem increased with both herbicides, but the increase was usually larger with tebuthiuron than triclopyr. Burning as a follow-up to either herbicide had little effect on frequency of rosette panicgrass and little bluestem. Horseweed increased following burning in 1985 and 1987. Standing crop of grasses and forbs increased dramatically following herbicide treatments. Grass production was greater with tebuthiuron, whereas production of forbs and browse was generally greater with triclopyr.

Aerial applications of 20% tebuthiuron JA/-[5-(1,1- dimethylethyl) - 1, 3, 4 - thiadiazol-2 -yl] -N, N' dimethylurea pellets at 2.2 or 4.4 kg/ha in the spring to heavy brush cover in Texas Post Oak Savannah did not increase the amount of grass, compared to that of untreated pastures, until the growing season after application. By the second growing season after tebuthiuron application, however, native grass stands were composed of a higher proportion of perennial species of good-to-excellent grazing value than were stands on adjacent, untreated rangeland. Tebuthiuron at 4.4 kg/ha did not improve botanical composition of grass stands, but increased the amount of grass during the second and third growing season after application compared to those where 2.2 kg/ha were applied. Daily steer gains were increased in one experiment in the fall after application of 2.2 kg/ha of tebuthiuron in the spring, but were unchanged after two growing seasons in another. Days of available grazing were increased, however, by the second or third growing season after treatment with 2.2 kg/ha of the herbicide in both experiments.

Experiments were conducted in south Texas from 1983 to 1987 to evaluate clopyralid for huisache control using a carpeted roller. Huisache control with clopyralid applied as broadcast sprays or with a carpeted roller generally equaled that from the same rates of picloram or of 1:1 mixtures of clopyralid plus picloram. Applications of herbicides in the fall generally were more effective than in the spring, regardless of application method. In most cases, the herbicides at 48 to 60 g ae/L fall applied with a carpeted roller killed 70% or more of the huisache. Herbicide efficacy was not reduced when all huisache top growth was removed within 1 week after roller application of a 1:1 mixture of clopyralid and picloram at 60 g ae/L. The treat-and-cut sequence should improve the quality of hay from huisache-infested pastures.

In Alaska Conservation Reserve Program (CRP) lands, succession of fields planted with grass and clover to shrubs and small trees is resulting in program compliance problems related to ease of reconversion to crop lands. Standard practice for slowing this succession is mowing every 2 to 3 yr, which does not kill the woody vegetation. A field study was conducted at three sites over 2 yr to determine if 2,4-D (2.2 kg ae ha−1 2-ethylhexyl ester) or triclopyr (2.2 kg ae ha−1 butoxyethyl ester) applied broadcast or 2,4-D (2.2 kg ae ha−1 2,4-D dimethylamine salt) or triclopyr (1.7 kg ae ha−1 triclopyr triethylamine salt) applied with a Diamond Wet Blade™ mower (DWB) would result in longer shrub control compared to mowing. Mowing was conducted at both 15 and 45 cm above ground level and herbicides were applied with the DWB at three rates. Measurements 2 yr after treatment (YAT) confirmed that both herbicides reduced shrub cover about 50% compared to controls. Reduced rates of the herbicides applied with the DWB did not result in decreased shrub control. Grass cover was negatively correlated with shrub cover. Typically, mower height did not alter treatment effects. Treatments had little impact on forb cover and composition 2 YAT, with the exception of fireweed, which was generally reduced where herbicides were applied. Application of 2,4-D and triclopyr does not decrease the frequency of shrub control in CRP lands in Alaska. Use of 2,4-D and triclopyr with or without mowing resulted in no widespread improvement over the current practice of mowing to 15 cm every 2 to 3 yr.

Southern wax myrtle is a pernicious weed in south Florida pastures and this plant can eliminate all forage production under high densities. Previous work has shown that triclopyr at 1.12 kg/ha is the most effective herbicide on this species. The introduction of the Burch Wet Blade (BWB) mowing system provides an alternative application method to traditional broadcast herbicide applications. The objective of this experiment was to compare the efficacy of triclopyr and other herbicides using both broadcast and the BWB application systems. In general, broadcast applications of at least 1.1 kg/ha triclopyr provided better control than the same treatments applied with the BWB system in 1998. In 1999, control was lower overall compared to 1998, but the same trend was observed. Broadcast applications of triclopyr reduced wax myrtle densities better than when herbicides were applied with the BWB system in 1998. However, in 1999, dicamba + triclopyr, and at least 1.1 kg/ha triclopyr reduced wax myrtle densities compared to the mow-only treatment, regardless of application method. Although the BWB system provided an initial overall reduction in plant height, recovery of plants was sufficient and often outgrew those receiving broadcast applications of herbicides. Regardless of application method, retreatment of wax myrtle plants 1 yr after the initial application will likely be needed to obtain adequate control.