Maize (Zea mays L.) is one of the most widely planted crop in the world and its production is increasing globally. Weed control has a major effect on the success of maize growth, because the competition ability of maize is relative low at early crop growth stages (Ghanizadeh et al., 2014). Weed control in maize largely depends on chemical methods. High input of herbicides results in environmental pollution, risks of residues carry-over and the development of weed resistance (Pannacci et al., 2006; Zhang et al., 2013). An effective way to reduce the side effects of the herbicides, according to the Integrated Weed Management System (IWMS), is to apply the lowest dose needed for biologically effective weed control (Swanton & Weise, 1991; Kudsk & Streibig, 2003; Blackshaw et al., 2006), although low doses can increase risk of polygenic resistance (Neve & Powles, 2005; Busi et al., 2011). Furthermore, several studies have demonstrated satisfactory weed control and acceptable crop yields, when herbicides are used at lower than normally recommended doses (Devlin et al., 1991; Zhang et al., 2000; Barros et al., 2009; Pannacci & Covarelli, 2009). Application doses of post-emergence herbicides can be, indeed, substantially reduced if the “minimum dose requirement for a satisfactory efficacy” (MDRE) is known with respect to the most common “herbicide-weed species” combinations (Dogan & Boz, 2005; Kudsk 2008; Pannacci et al., 2010). Furthermore, the knowledge of MDRE is one of the main factors in the implementation of the Decision Support Systems for Integrated Weed Management (Rydahl, 2004; Sønderskov et al., 2015), with the aim to decrease the dependence on herbicides that has become a distinct objective within the EU with the directive 2009/128/EC. The determination of MDRE requires dose-response studies for each “herbicide-weed species” under various environmental conditions (Kudsk & Kristensen, 1992; Pannacci & Covarelli, 2009).

Among post-emergence herbicides in maize, foramsulfuron is a sulfonylurea that exerts its herbicidal activity by inhibiting acetolactate synthase also known as acetohydroxy acid synthase and provides control of grass, perennial and some broadleaved weeds with a good selectivity to the maize (Bunting et al., 2004a, 2005). Furthermore, foramsulfuron can be applied in mixture with other herbicides increasing the control of some important key weeds, without risks of carry-over problems also in rotational vegetable crops (Bunting et al., 2005; Soltani et al., 2005).

In the past, several data were obtained on the efficacy of foramsulfuron used alone and in mixture with other herbicides against some problematic weeds (Bunting et al., 2004b; Nurse et al., 2007; McCullough et al., 2012), but few information is available about MDRE in the Mediterranean area conditions (Kir & Doğan, 2009). The objective of this study was to determine dose-response curves for foramsulfuron against several of the most important weeds in maize, and extrapolate the MDRE. Indeed, dose-response curve model applied in field herbicide efficacy trials was an important tool to estimate biologically meaningful parameters and to express weed control ability in terms of biological equivalent doses. Finally, the knowledge of MDRE for each “foramsulfuron-weed species” will allow to get the final goal to reduce foramsulfuron rates without losses of effectiveness against weeds.

The four field experiments showed that weed susceptibility against foramsulfuron can be quantified by dose-response curves, and that weeds can be classified with respect to their susceptibility via ED₉₀ and ED₉₅ values. The ranking among weed species based on their sensitivity to foramsulfuron was: redroot pigweed = green foxtail = wild mustard = black nightshade > velvetleaf = common lambsquarters = barnyardgrass > pale smartweed. Baghestani et al. (2007) evaluated the efficacy of reduced rates of foramsulfuron in Iran and reported a lower susceptibility of velvetleaf and redroot pigweed to foramsulfuron than that obtained in this study, while similar results were obtained for barnyardgrass. On the contrary, Kir & Doğan (2009) found similar results to this research in terms of sensitivity of redroot pigweed to foramsulfuron. In this study the lower efficacy of foramsulfuron against barnyardgrass in 2013 and 2014 than in 2011 and 2012 was probably due to the bigger growth stage of plants at treatment time in 2013 and 2014 (3-4 tiller visible) than in 2011 and 2012 (first tiller visible) (Table 2). Indeed, in 2011 and 2012 the early sowing of maize at the end of April reduced the total weed flora, and due to low temperatures, barnyardgrass plants grew more slowly than in 2013 and 2014. It is well known that annual weed species are generally more susceptible to herbicides at early growth stages (Kudsk, 2008). Similarly, growth stage at application could explain the lower efficacy of foramsulfuron against pale smartweed in 2013 and 2014 (pale smartweed plants at 6-8 true leaves at treatment time) than in 2011 and 2012 (pale smartweed plants at 2-4 true leaves at treatment time), and against common lambsquarters in 2013 (6-8 leaves at treatment time) than in 2011 and 2012 (2-4 true leaves at treatment time) (Table 2). Furthermore, in 2013 and 2014 the weather conditions, like water stress and high temperatures at the treatment time (21 June 2013 and 10 June 2014, see Figs. 1c and 1d, respectively) were more unfavourable to foliar-herbicides applications than in 2011 and 2012, especially against difficult-to-wet weed species, like barnyardgrass and common lambsquarters (Kudsk & Mathiassen 2004; Pannacci et al., 2010). This may be due to reduced herbicide availability caused by rapid drying of droplets to solid deposits in warm conditions (Ramsey et al., 2005). Furthermore, plants grown under high temperature, low humidity and soil water deficit tend to have leaves with thicker cuticles, more epicuticular wax and pubescence than unstressed plants, and this might retard interception, retention and penetration of the herbicide (Lundkvist, 1997). Indeed, common lambsquarters is characterised by very waxy leaves decreasing spray retention and uptake under hot and dry conditions (Steckel et al., 1997).

This study showed that foramsulfuron, also at reduced doses, is an effective herbicide to control several weed species (i.e. redroot pigweed, black nightshade, common lambsquarters and barnyardgrass) characterized by high competitiveness to the crop (Zanin et al., 1994; Pannacci & Tei, 2014), high seedbank densities (Graziani et al., 2012) and low susceptibility to several pre-emergence herbicides (i.e. velvetleaf). The use of herbicides at reduced doses seems to be possible; however, a limit could be the presence of non-susceptible weeds, i.e. Fallopia convolvulus (L.) Holub, Polygonum aviculare L., Chenopodium polyspermum L. and Stellaria media (L.) Vill. for foramsulfuron (Tracchi et al., 2002), that may frustrate the overall effectiveness of weed control (Pannacci & Covarelli, 2009). In such cases a good strategy is to use two or more active ingredients in mixture (i.e. sulcotrione or dicamba for foramsulfuron) in order to enlarge the spectrum of controlled weeds, and also avoiding the increase of herbicide resistant weeds (Green & Streibig, 1993; Pannacci et al., 2007; Beckie & Reboud, 2009).

Foramsulfuron may be considered fairly safe to the crop, thanks to phytotoxicity symptoms always low and transitory as confirmed also by Tracchi et al. (2002), Nurse et al. (2007) and Zaremohazabieh & Ghadiri (2011) in other studies.

In general, lower crop yield in weed-free plots in 2014 than previous years can probably be due to the very high rainfall (113 mm in three consecutive days in the last decade of July) and low temperature (the daily average temperature decreased during the last decade of July until to 19 °C in these three days of rain) at flowering time (see end of July, Fig. 1d), that have affected pollination and growth of maize temporarily, reducing the potential yield of crop. The crop yield differences among untreated control and the increased foramsulfuron doses lower in 2011 and 2012 than in 2013 and 2014 were due to the high total weed control obtained also at low doses of foramsulfuron; indeed, the lower total weed pressure and younger growth stages at the treatment time increased their susceptibility to foramsulfuron (see Fig. 2). Otherwise, in 2013 and 2014, the low efficacy of foramsulfuron against barnyardgrass and pale smartweed (Fig. 2), together with their high ground covers, reduced crop yields values with no significant differences among the three doses (Table 4). This indicates that barnyardgrass and pale smartweed plants escaping the treatment were very competitive, resulting in high yield losses in the most infested plots. Similar yield losses in maize were obtained by Pannacci & Tei (2014) and by Pannacci & Onofri (2016), in the same area with similar weed infestations.

The results presented in this paper (i.e. ED₉₀ and ED₉₅ values) have shown that the post-emergence application dose of foramsulfuron can be reduced below recommended labelled rate depending on weed species and growth stage. In particular, foramsulfuron at 1/4 of the maximum labelled dose (≈ 20.3 g a.i./ha) provided 95% efficacy on redroot pigweed, green foxtail, wild mustard and black nightshade. The same efficacy was obtained against velvetleaf, common lambsquarters and barnyardgrass at 25-35 g/ha of foramsulfuron, corresponding to half of the maximum labelled dose. These results showed that foramsulfuron is an effective post-emergence herbicide to control the above mentioned weed species that are key weeds in maize and characterized by high competitiveness towards this crop. The MDRE’s estimated in this study for foramsulfuron to optimize chemical weed control in maize, especially ED₉₅ values, can be implemented in Decision Support Systems (Kudsk, 2008). Finally, foramsulfuron showed a good selectivity to the crop, without negative effects to crop yield.