Recent reports of damage to maize seedlings where atrazine or atrazine/terbuthylazine was applied post-emergence again raised the issue of maize tolerance to triazines. The potential of atrazine or atrazine/terbuthylazine to injure maize when applied post¬emergence was investigated under controlled conditions. Factors considered were growth stage, surfactants, cultivar tolerance, temperature and crop recovery from herbicide injury. The first experiment was conducted to determine the effect of atrazine and terbuthylazine on maize seedlings, and the second one was carried out to identify whether stage of maize development has an influence on atrazine or terbuthylazine phytotoxicity. In Experiment 1, herbicide treatments were applied at the 2-leaf stage of cultivar PAN 6043, and in Experiment 2 at the 2-leaf and 4-leaf stages. The third experiment was carried out to determine cultivar tolerance to atrazine or atrazine/terbuthylazine. The three cultivars used were CG 4526, PAN 6043 and PAN 6140. The fourth and fifth experiments were carried out in growth chambers where the role of temperature and cultivars were investigated. The temperature levels were 20/15 and 30/25°C (day/night). Herbicide and temperature treatments were applied at the 2-leaf stage of cultivars CG 4526, CAN 3891, PAN 6043 and PAN 6140. Finally, an experiment was conducted to investigate the recovery of maize seedlings from the initial atrazine/terbuthylazine phytotoxicity. The cultivar PAN 6043 was used and the herbicide rates were the recommended and double the recommended rate. The rates of the herbicides and associated activity enhancers were 2.5 L ha-1and 1 L ha-1 respectively. The following products were used: Atrazine 500 SC (485 g L-1 atrazine) + BP Agripon (940 g L-1 mineral oil); Gesaprim Super 600 SC (291 g L-1 atrazine and 291 g L-1 terbuthylazine) + AG Penetrex (363 g L-1); Gesaprim Super 600 SC (291 g L-1 atrazine and 291 g L-1 terbuthylazine) + Complement (363 g L-1). Maize seedlings were tolerant when atrazine or atrazine/terbuthylazine was applied at the 4-leaf stage, but significant damage was caused when application was done at the 2-leaf stage. No significant differences were found between surfactants. Significant differences occurred with cultivars at specific temperatures. At the low temperature regime (20/15°C), CG 4526 showed shoot dry mass (SDM) reduction of > 40 %, while the growth of PAN 6140 was reduced by less than 10 % by both herbicides. However, with an increase in temperature, CG 4526 was the most tolerant. It showed <17 % reduction in SDM while others showed >30 % reduction. It is suggested that the tolerance of maize (specifically at the two-leaf stage) to atrazine or atrazine/terbuthy1azine is dependent on cultivar and temperature. Although initial phytotoxic symptoms were visible several weeks after spraying, the growth inhibition of the atrazine/terbuthylazine-treated maize plants was transient. At five weeks after application, leaf area index (LAI) had recovered 100% and 95% at the recommended and double the recommended rate, respectively. Recovery in SDM and root dry mass (RDM) were similar, with 100% recovery reached after eight weeks following application. It was found that although growth stage plays an important role in the tolerance of maize seedlings when atrazine or atrazine/terbuthylazine is applied post-emergence, surfactants, cultivars, and temperature also have an influence on the tolerance of maize seedlings towards these herbicides. / Dissertation (M Inst Agrar (Weed Science))--University of Pretoria, 2006. / Plant Production and Soil Science / unrestricted

La presenza di pesticidi nelle acque di falda è emersa come un importante problema ambientale negli ultimi decenni. In base a “Rapporto nazionale pesticidi nelle acque - Dati 2015-2016” (ISPRA, 2018), in Italia, gli erbicidi triazinici e i loro metaboliti sono tra le sostanze identificate con più frequenza nelle acque, sebbene l’atrazina sia stata bandita nel 1992 e l’utilizzo di terbutilazina sia sottoposto a severe restrizioni. Lo scopo di questo progetto è valutare e quantificare la presenza di atrazina, terbutilazina e dei loro metaboliti nelle acque di falda in aree maidicole italiane. I risultati dimostrano come le molecole oggetto di studio siano state frequentemente individuate nei campioni di acqua di falda, anche se a concentrazioni inferiori al limite stabilito dalla Commissione Europea. Considerando che l’utilizzo di questi erbicidi è proibito o fortemente regolamentato, è interessante valutare il loro comportamento in una scala temporale pluriennale al fine di mettere in evidenza la contaminazione delle acque di falda dovute all’uso diffuso e alla persistenza ambientale di tali molecole. / The presence of pesticides in groundwater has emerged as an important environmental issue in the last decades. According to “Rapporto nazionale pesticidi nelle acque - Dati 2015-2016” (ISPRA, 2018), in Italy triazine herbicides and their metabolites are among the substances most frequently detected in water. While terbuthylazine is still used under restriction, atrazine was banned in 1992. The aim of research project is to investigate the presence of atrazine, terbuthylazine and their metabolites in groundwater in Italian maize-growing areas in order to supply data and information on the groundwater quality. The results show that atrazine, terbuthylazine and their metabolites were often detected in groundwater, even if their concentrations were below the limit established by European Commission. Considering these molecules are banned or heavily regulated, it is interesting to evaluate their behavior on a multiannual time scale in order to highlight the groundwater contamination due to the widespread use and to the environmental persistence of these substances.