The absorption, translocation, and metabolism of 14C-picloram and 14C-2,4-D applied alone and together to leafy spurge was evaluated. Leafy spurge absorbed 34 and 24% of the 14C-2,4-D applied alone and with picloram, respectively, and 14 and 10% of the 14C-picloram applied alone and with 2,4-D, respectively. More 14C-2,4-D was translocated in leafy spurge than 14C-picloram, and adding picloram to 14C-2,4-D decreased 14C translocation to the roots. Adding 2,4-D to 14C-picloram increased the percentage of absorbed 14C that translocated in leafy spurge from 28 to 48%. Generally, 14C-picloram and 14C-2,4-D remained as the parent acid in leafy spurge whether applied alone or together. Of 14C-picloram recovered from the roots, 83% was unmetabolized picloram when applied alone compared to 95% when applied with 2,4-D, which probably is the reason for increased leafy spurge control when these herbicides are applied together.

Tebuthiuron, picloram, hexazinone, and bromacil were evaluated at 0.5, 1.0, and 2.0 g ai or ae per plant for sacahuista control on gravelly loam soils in western Texas. Tebuthiuron applied at 2.0 g per plant killed 87% of the sacahuista, which was significantly greater mortality than that achieved with the same rate of bromacil (62%) and hexazinone (44%). The 2.0 g plant-1 rate of tebuthiuron, bromacil, and hexazinone reduced live canopies of sacahuista 88 to 98%. Picloram did not control sacahuista. Sacahuista mortality increased curvilinearly with increasing rates of tebuthiuron and bromacil, whereas the response to hexazinone was linear.

Picloram uptake, translocation, and efficacy were studied using greenhouse-grown Russian knapweed plants. Uptake of 14C-picloram, applied as discrete droplets to adaxial leaf surfaces of well-watered plants, averaged less than 10% of that applied. Most uptake occurred within 30 min of application. Uptake increased linearly with external picloram concentrations from 6.2 to 74.5 mM and was proportional to picloram concentration. Only about 10% of absorbed picloram was translocated out of the treated leaf of well-watered plants within 96 h, with approximately equal acropetal and basipetal translocation. Water stress before, at the time of, and after picloram application did not affect picloram uptake, but reduced total translocation and increased the relative amount translocated basipetally. Water stress also reduced picloram efficacy. Although Silwett L-77 increased picloram uptake into Russian knapweed leaves, it did not increase efficacy under water stress conditions.

Fall applications of 0.14 kg/ha of picloram (4-amino-3,5,6-trichloropicolinic acid) to musk thistle (Carduus nutans L.) rosettes controlled musk thistle stems better and more consistently than fall applications of 2.2 kg/ha of 2,4-D amine [(2,4-dichlorophenoxy)acetic acid], 0.6 kg/ha of dicamba (3,6-dichloro-o-anisic acid), or a mixture of 0.3 kg/ha dicamba + 1.1 kg/ha of 2,4-D. Dry conditions in 1975 and cool conditions in November, 1976, hampered control with all herbicides except picloram. In spring applications when growing conditions were favorable, herbicide differences were usually not significant. For all herbicides, treatment of bolted stems did not control musk thistle satisfactorily. Picloram was superior for reducing stems 12 to 18 months after application in 2 of 3 yr. Residual control resulted principally because the herbicide killed many seedlings and first-year rosettes at the time of application.

Roller-applied picloram (4-amino-3,5,6-trichloropicolinic acid) at a concentration of 30 to 60 g ae/L gave similar control to leafy spurge (Euphorbia esula L. ♯ EPHES) as spray-applied picloram at 1.1 to 2.2 kg ae/ha. Leafy spurge control was most effective when the roller applicator was adjusted to treat a maximum amount of stem tissue with a 60 g/L picloram solution. Control was not improved by including a petroleum oil or surfactant additive. Fall applications of roller-applied picloram tended to provide better control than spring applications. When used on dense leafy spurge stands, the roller applicator applied only 40% as much picloram as spray treatments of 2.2 kg/ha. However, picloram residues in soil following roller application were similar to those from picloram at 0.3 to 1.1 kg/ha applied as a spray.

Foliar absorption of picloram by broom snakeweed, a rangeland shrub, was investigated. Picloram uptake into leaf, axillary bud, and stem tissues was similar. In addition, picloram uptake by leaf tissue from greenhouse- and field-grown broom snakeweed did not differ. Picloram accumulated rapidly and absorption saturated between 15 min and 1 h of application; no further absorption occurred through 72 h with maximum uptake ca. 15% of applied picloram. Picloram content increased linearly with increasing external picloram concentration, implying that movement of the herbicide across the cuticle is via diffusion. Absorption was dependent on relative humidity and temperature with the greatest uptake at 94% relative humidity and 35 C, respectively. Absorption was pH dependent; picloram absorption was greatest at pH 4 and least at pH 8. In addition, picloram absorption was less at pH 3 compared to pH 4. These results provide evidence that picloram is absorbed across the cuticle via simple diffusion and absorption is dependent on environmental conditions and solution pH at and following application.

Leafy spurge control with four herbicides was evaluated at nine sites in six Great Plains states. Sulfometuron alone did not control leafy spurge satisfactorily 12 mo after treatment (MAT). Sulfometuron plus dicamba at 105 plus 2240 g ai ha−1, when spring- or fall-applied, averaged 26 and 89% control and 31 and 86% grass injury, respectively, 12 MAT. Sulfometuron plus picloram at 105 plus 560 g ai ha−1, when spring- or fall-applied, averaged 63 and 92% control and 19 and 89% grass injury, respectively, 12 MAT.

Long-term control of downy brome with an integrated approach is needed in order to sustain range productivity. Studies were conducted to study the effectiveness of a combination of downy brome control practices. In two studies, glyphosate and paraquat were evaluated at various rates for up to three successive years for control of downy brome in rangeland. A third study evaluated the competitiveness of perennial cool-season grasses against downy brome in the absence of herbicides. Glyphosate, at 0.55 kg/ha, and 0.6 kg/ha paraquat provided selective downy brome control on rangeland when applications were combined with intensive grazing. Downy brome control was greater than 90% following two sequential years of 0.6 kg/ha paraquat at either the two- to eight-leaf stage or bloom stage at both study locations. At one study location, 0.55 kg/ha glyphosate provided 97% control after the first application at both growth stages. In the second study, control averaged greater than 92% following three sequential applications of glyphosate. When perennial cool-season grasses were seeded in the spring following fall tillage (no herbicides) and allowed to establish for three growing seasons, three of the five species were effective in reducing the reestablishment of downy brome. ‘Luna’ pubescent wheatgrass, ‘Hycrest’ crested wheatgrass, ‘Sodar’ streambank wheatgrass, ‘Bozoisky’ Russian wildrye, and ‘Critana’ thickspike wheatgrass controlled 100, 91, 85, 45, and 32% of the downy brome, respectively. Yields of perennial grass dry matter were 1,714, 1,596, 1,135, 900, and 792 kg/ha. Replacing noncompetitive annual grasses with competitive cool-season perennials will provide a longer term solution to a downy brome problem than the use of herbicides alone or with intensive grazing.

Twelve grass genotypes were evaluated for competitiveness with leafy spurge and herbage yield in two sites in North Dakota. ‘Rebound’ smooth brome, ‘Rodan’ western wheatgrass, ‘Bozoisky’ Russian wildrye, and ‘Arthur’ Dahurian wildrye reduced leafy spurge stem density an average of 63% after 3 yr in a silty clay soil at Fargo. ‘Reliant’ intermediate wheatgrass reduced leafy spurge stem density every year for 3 yr, including an 85% reduction the second year after planting, and consistently produced high herbage yields. Rebound smooth brome and Reliant intermediate wheatgrass averaged 72% leafy spurge reduction 3 yr after seeding in a loamy sand soil at Jamestown. Reliant intermediate wheatgrass and ‘Manska’ pubescent wheatgrass provided the most consistent herbage production, which averaged about 2,000 kg/ha annually for 3 yr.

Picloram was applied to leafy spurge using flexible and rigid ropewick applicators for 3 yr at each of two upland pasture sites. The ropewick applicators were a cost-effective way to apply picloram to control leafy spurge compared to broadcast application, but minimal advantage was demonstrated for the segmented, flexible unit compared to the rigid applicator. Picloram applied as an 80 g ae/L solution through ropewick applicators controlled leafy spurge equal to broadcast picloram at 1.1 kg/ha while using 36% as much picloram.

The locoweeds, woolly loco and silky crazyweed, contribute to livestock poisoning in the western United States. Differences in response of these two locoweeds to foliar-applied picloram and metsulfuron was investigated by evaluating differences in herbicide uptake, translocation, and metabolism. Silky crazyweed compared to woolly loco was more than 10 times as sensitive to increasing rates of either herbicide. The two species absorbed 8 to 15%, and 11 to 17% of applied picloram and metsulfuron, respectively. Translocation of picloram and metsulfuron out of treated leaflets of either species was less than 3% of that absorbed after 96 h. Approximately 70 and 100% of the absorbed herbicides remained as picloram and metsulfuron, respectively, in both species. Therefore, differences in picloram or metsulfuron uptake, translocation, and metabolism appear inadequate to account for the differential response to each herbicide by the two locoweed species. Selectivity differences between these locoweed genera to picloram and metsulfuron most likely are due to sensitivity differences at sites of action.

Leafy spurge control with picloram and various 2,4-D formulations was evaluated to determine the optimum treatment combination and carrier volume for aerial application using a microfoil boom. Leafy spurge control was similar when the same herbicide treatments were applied at 94 and 47 L/ha but declined at 23 L/ha. Leafy spurge control was better when similar herbicide treatments were applied with a one-row microfoil nozzle boom compared to a two-row nozzle. Leafy spurge control 12 months after treatment tended to be better with picloram at 0.28 and 0.56 kg ae/ha applied with 2,4-D mixed amine compared to picloram plus 2,4-D dimethylamine or oil-soluble amine formulations during average growing conditions. However, picloram plus 2,4-D oil-soluble amine was the best formulation in dry conditions.