DOI: 10.1111/j.1365-3180.2011.00862.x Herbicide hormesis – can it be useful in crop production? R G BELZ*, N CEDERGREEN  & S O DUKE à *Institute of Plant Production and Agroecology in the Tropics and Subtropics, Department of Agroecology, University of Hohenheim, Stuttgart, Germany,  Department of Basic Science and Environment, Faculty of Life Science, University of Copenhagen, Frederiksberg C, Denmark, and àUSDA, ARS, Natural Products Utilization Research Unit, University, MS, USA Received 23 November 2010 Revised version accepted 8 March 2011 Subject Editor: Per Kudsk, Flakkebjerg, Denmark Summary The yield-enhancing effects of some pesticides may change the focus of their use in crop production, from crop protection to crop enhancement. While such ben- eficial uses of pesticides are specifically en vogue for fungicides and seed treatments, the use of herbicides for crop enhancement has not yet been realised. The potential for improving crop production by low-dose, stimulatory effects of herbicides has been proposed, and reports of 10–25% efficiency of improving certain plant traits under field conditions seem promising. However, past attempts to make use of herbicide hormesis, the term for this effect, have been largely unsuccessful. The reasons for this may be manifold, but the lack of understanding of the principles and mechanisms of this low-dose phenomenon in plants may have contributed to the often claimed lack of adequate predictability for commercial use. Thanks to the research progress recently made in this area, we are now better able to understand the principles of herbicide hormesis and its potential for crop enhancement. Therefore, this review highlights the potential of phytotoxins to induce plant hormesis and the factors influencing its expression. Based on this, possible practical constraints and consequences for the portfolio of uses for herbicides are discussed, along with undesired but apparent hormetic side effects of herbicides. Keywords: allelochemical, biphasic, crop enhancement, dose–response, growth stimulation, phytotoxin. BELZ RG, CEDERGREEN N&DUKE SO (2011). Herbicide hormesis – can it be useful in crop production? Weed Research 51, 321–332. Introduction Some substances, although toxic at higher doses, can be stimulatory or even beneficial at low doses. This biphasic dose–response phenomenon is commonly termed hor- mesis and it is also characteristic for many herbicides and other phytotoxins (Duke et al., 2006; Cedergreen, 2008a). Among the herbicides causing hormesis is glyphosate, the currently most widely used herbicide (Schabenberger et al., 1999; Cedergreen, 2008a,b; Velini et al., 2008). Compared with the classical, monotonic relation between the dose of a toxin and the resulting response, hormetic or biphasic response patterns are characterised by an increase in response at low doses that changes to inhibition at higher doses. The numerous reports of phytotoxin-induced horme- sis in plants (e.g. Duke et al., 2006; Cedergreen et al., 2007; Calabrese & Blain, 2008; Cedergreen, 2008a) clearly document that hormesis is a reproducible phenomenon that might be translated into crop enhancement if causing a desired phenotypic change. However, hormetic effects are not necessarily beneficial for an organism, as for example, increased shoot elongation at the cost of stem robustness may lead to Correspondence: Regina Belz, Institute of Plant Production and Agroecology in the Tropics and Subtropics, Department of Agroecology 380b, University of Hohenheim, 70593 Stuttgart, Germany. Tel: (+49) 711 459 23681; Fax: (+49) 711 459 23629; E-mail: regina.belz@uni-hohenheim.de Ó 2011 The Authors Weed Research Ó 2011 European Weed Research Society Weed Research 51, 321–332