Piperonyl Butoxide (PBO) pages

1. PBO - Introduction
2. PBO - Household
3. PBO - Resistance
4. PBO - Dust Mite
5. PBO - Herbicides

5. The potential use of PBO with herbicides

5.1. Introduction
Chemicals may be added to herbicides for three main reasons: to overcome resistance; to reduce the amount of herbicide used; or to reduce the phytotoxicity of the herbicide. There are over thirty technical publications citing PBO as a potential additive to either augment herbicides or overcome resistance to them. As a result of the recent commercial synthesis of safrole, the key part of the PBO molecule, it is likely that the use of PBO as an additive to herbicides will be seriously considered. This potential usage is further enhanced by the recent elucidation of the chemistry of PBO’s photostability (Harbach et al, 1998) that has indicated the possibility of using additives to increase the persistence of PBO on foliage. Since the most convenient method of application of PBO to overcome resistant insect pests has been as a tank mix to control particular problems, it is likely that this method will also be the most convenient when PBO is applied with herbicides.

5.2. Development
In 1977 Shell International was granted a German patent for their claim that PBO enhanced the effectiveness of their herbicide alanine and its derivatives against the wild oat (Avena fatua). They quoted the result of a trial in which alanine alone controlled 90% of wild oat at a rate of 1.35kg/ha whereas with PBO added at the ratio 1:4 the rate was 0.27kg/ha. Hatzious (1983a, 1983b) described glasshouse experiments undertaken in the USA which attempted to reduce the phytoxicity of herbicides such as EPTC to maize. He reported that EPTC at 6.7kg/ha and the herbicide metolachlor combined with PBO interacted synergistically.

Gaillardon et al (1985) working in France measured the growth response of wheat to combined treatments of four mixed function oxidase (MFO) inhibitors and the herbicide chlortoluron. The results indicated that PBO increased the toxicity of the herbicide. They suggested that compounds designed to inhibit plant cytochrome P-450 enzymes may interact with herbicide metabolism and are potential herbicide synergists. Gonneau et al (1988) studied the pathway of chlortoluron metabolism in excised leaves of tolerant plant species: Triticum aestivum, Bromus sterilis, Galium aparine and Veronica persica. The herbicide was mostly detoxified by hydroxylation of the methyl ring in Triticum and by N-dealkylation in Veronica. Both pathways were involved in Bromus and Galium. They suggested that the cytochrome P-450 could be involved in the methyl ring hydroxylating reaction.

Varsano and Rubin (1991) described studies of the synergism of triazine herbicides by PBO. They suggested the commercial use of PBO to synergise the activity of triazine herbicides in sensitive, tolerant and resistant plants. Varsano, Rabinowitch and Rubin (1992) undertook a detailed study on the mode of action of PBO as a herbicide synergist for atrazine and terbutryn in maize. They suggested the synergistic effect of PBO was due to enhanced herbicide penetration, induced membrane damage and/or inhibition of P-450-mediated herbicide metabolism.

Powles et al (1990) undertook research at the Waite Agricultural Research Institute, Australia on herbicide cross-resistance mechanisms in ryegrass (Lolium rigidum) and concluded that there was no evidence that resistance is the result of:

a. Any barrier to herbicide uptake.
b. Any differential translocation of herbicides within the plants.
c. Any major differences in the sensitivity of the enzyme target sites of the aryloxphenoxypropionate, cyclohexanedione or sulfonylurea herbicides.
d. Any differential activity of glutathione S-transferase.

Burnet et al (1993), also working at the Waite Institute, studied a mechanism of chlortoluron resistance in Lolium rigidum. They found that PBO, described as a cytochrome P-450 inhibitor, interacted with chlortoluron when applied to plants growing in the soil. Chlortoluron applied with PBO reduced plant dry weight to a greater extent than the herbicide alone.

Cottingham et al (1993) found that PBO synergised EPTC when applied to eight out of eleven hybrids of Zea mays and that PBO synergised metolachlor (chloroacetanilide) on only two Zea hybrids and to a lesser extent than EPTC. They suggested that tolerance to EPTC is more likely to be influenced by oxidative reactions than is tolerance to metolachlor.

Raja Rao et al (1995) evaluated cytochrome P-450 monooxygenases, including PBO, on the bioefficacy of thiazopyr. They claimed that PBO enhanced thiazopyr bioefficacy by effectively inhibiting its metabolism in plants.

Kolula-Syka and Hatzios (1996) studied the interactions of tribenuron with four chemical safeners and PBO on two Zea hybrids. On one hybrid, Pioneer 3180, PBO did not influence the activity of the herbicide but on the other hybrid, NK-9283, PBO synergised all three concentrations of tribenuron.

5.3 The use of UV stabilisers to increase the effectiveness of PBO when used outside.
Until the definitive study by Harbach et al (1998) it was believed that PBO was relatively stable in sunlight, but Harbach et al demonstrated that PBO degraded in sunlight when under aerobic conditions. However, in the absence of oxygen PBO could withstand prolonged irradiation for several weeks with no degradation.

In Australia significant amounts of PBO are now used to improve the performance of pyrethroid insecticides against the cotton bollworm. A research programme has indicated that the persistency of PBO on cotton leaves in sunlight is prolonged by the addition of various UV stabilisers at 1–4% w/v; the most active compounds were benzophenones and hindered amines. Hourly assays indicated that PBO penetrated the cotton leaves.

It is now believed that UV-stabilised PBO will prove more effective in suppressing resistance with herbicides than the standard technical product used over the past ten years. Endura is prepared to share its experimental data with interested parties working in this field.

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