Relative humidity after application, spray additives, and solution pH affected both foliar absorption and translocation of 14C-picloram to leafy spurge roots. 14C-picloram absorption increased from 11 to 34% and translocation increased from 5 to 21% as time at posttreatment humidity increased from 0 to 48 h. Absorption and translocation were not different when pre- or posttreatment temperatures were 30/18 or 18/10 C (day/night). 14C-picloram absorption and translocation to the roots were 18 and 6%, respectively, when applied alone, and increased to 46 and 12%, respectively, when applied with ammonium sulfate at 2.5 kg/ha. Absorption and translocation were unaffected by ammonium nitrate. Foliar absorption and translocation of 14C-picloram in leafy spurge were unaffected by pH of unbuffered spray solution but increased at least 50% when applied in a solution buffered at pH 4.8 with trisodium citrate. Foliar absorption in detached leafy spurge leaves increased from 26 to 51% of applied 14C as the citrate buffer concentration increased from 0.01 to 0.1 mM, respectively.

Field and greenhouse studies were conducted to evaluate the following growth regulators: ABG-3034, a cytokinin, benzylaminopurine (BAP); alkanolamine salt of 2,4-D; gibberellic acid (GA); isopropylamine salt of glyphosate; kinetin; napthaleneacetic acid (NAA); and PP333, an experimental antigibberellin compound, and the herbicides: dimethylamine salt of dicamba and potassium salt of picloram alone and combined on the growth and control of leafy spurge. The results of greenhouse and field studies show that growth regulators did not enhance the performance of herbicides in controlling leafy spurge.

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.

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.

Picloram release by leafy spurge roots, as affected by picloram rate, plant growth stage, and time intervals after treatment, was quantified under field conditions. Picloram was pipe-wick applied to leafy spurge in the vegetative, flowering, and seed-filling growth stages. Percent leafy spurge control was evaluated and picloram residues were determined in soil samples from 0- to 13-, 13- to 26-, and 26- to 39-cm depths taken 1, 2, and 3 weeks after treatment. Leafy spurge was controlled (frequently >85%) by all picloram concentrations applied, although control tended to increase as solution concentration increased. Picloram release from roots was greater from plants treated in the flowering and seed-filling stages than from plants in the vegetative stage. Picloram release from roots generally was correlated with application rate, averaging 490, 820, and 1420 ppbw in soil for the 30, 60, and 120 g ae/L application rates, respectively. Picloram release from roots occurred rapidly with 86% of the picloram detected in the 0- to 13-cm soil depth present by 1 week after treatment. Picloram was detected at all soil depths sampled, but over 84% was in the upper 13 cm and 8% was in both the 13- to 26- and 26- to 39-cm depths. Leafy spurge shoots emerged through a 7.5- and 15-cm depth of picloram-treated soil at concentrations up to 1000 ppbw within 14 to 21 days after the untreated control. Picloram soil residue had little effect on leafy spurge root growth.