Effect of glyphosate applications on growth responses of various glyphosate tolerant maize hybrids
Odendaal, Heinrich Benjamin
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Glyphosate is a highly effective non-selective, broad-spectrum, post-emergence herbicide used to control a broad spectrum of grasses and broad-leaf weeds. Glyphosate was initially largely used as a burn-down herbicide to eliminate weeds prior to planting, but since glyphosate resistant crop cultivars became available, applications can be done after planting as well. To prevent effects on ear initiation and subsequent yield loss, glyphosate should not be applied to glyphosate resistant (GR) maize cultivars after the V6 growth stage. Since late application and high application rates of glyphosate may play a role in reducing maize yield the need existed to study the possible effect of glyphosate application on plants. Field and glasshouse trials were conducted to elucidate timing of glyphosate application and different application rates on plant physiological parameters. While the majority of trials were conducted in South Africa, one glasshouse trial was conducted in Brazil. The study was done with five glyphosate resistant maize cultivars (BG5685RR, DKC73-76R, DKC75-35R, DKC80-30R and KKS4479R) using two glyphosate products (glyphosate and generic glyphosate, both 540 g a.e.l-1). Glyphosate was applied at different growth stages (V4, V4 followed up at V6 (V4/V6), V6 and V8) of GR maize in field trials over two seasons (2013/14; 2014/15). The experimental design was a split-plot with cultivars as main factor and treatments as subplots with three replicates per treatment. Plant height was significantly affected at the different application times of glyphosate and slight stunting of between 5 and 10% was recorded for both seasons. Shoot mass was reduced with more than 10% during 2014 where glyphosate was applied at V6 growth stage. During 2015, shoot mass was reduced between 2 and 15% across cultivars when glyphosate was applied at the V4, V4/V6 and V6 growth stages. The crop growth rate of two of the cultivars were lower when glyphosate was applied at V8 during 2015. Maize yield of BG5685RR and KKS4479R was reduced with 20 and 21% respectively, when glyphosate was applied at the V4, V6 and V4/V6 growth stages. Plant height increased overall between 4 to 20% when generic glyphosate was applied to GR cultivars during 2014. Slight stunting was observed for KKS4479R where generic glyphosate was applied at V4 (5%) and V4/V6 (6%) during 2015. Similar results were obtained with shoot mass during 2014. Crop growth rate differed significantly between GR cultivars where generic glyphosate was only at the V8 growth stage. Yield of BG5685RR was reduced with >20% where generic glyphosate was applied at V6 and V8 growth stages during 2014. The effect of above-mentioned products, applied at different application rates, on the physiology and growth of GR cultivars was tested in a glasshouse (controlled conditions) trial at Potchefstroom. This study consisted of six different glyphosate treatments for each product: (i) control, (ii) 0.75, (iii) 1.3, (iv) 1.7, (v) 2.0 and (v) 4.0 l.ha-1). Photosynthetic efficiency and plant productivity was evaluated by taking chlorophyll a fluorescence measurements from dark adapted plants to determine the PITOTAL parameter which is an indication of a plant’s overall vitality. Contradictory results were obtained with regard to PITotal values since KKS4479R showed a decrease of 20% across application rates except at the 1.3 l.ha-1, and DKC73-76R showed a reduction of 28% only in 1.3 l.ha-1 application rates where glyphosate was applied. PITotal values of DKC78-35R and DKC80-30 decreased between 10 and 25% where generic glyphosate was applied at 0.75, 1.7 and 2.0 l.ha-1 respectively. Cultivars had a greater effect on root, shoot and total dry mass than the different application rates of both glyphosate and generic glyphosate. Biomass of DKC73-76R and KKS4479R was reduced between 5-15% where glyphosate and generic glyphosate was applied. A similar trial was conducted under greenhouse conditions in Florianópolis, Brazil where the above mentioned application rates of similar products were applied on five GR maize cultivars (DK245R2, AG-8025 PRO2, AG-8025 RR, AG-9045 RR, AG-9045 PRO2). In the latter experiment, cultivars also affected all parameters more than different application rates of both glyphosate products. The maximum quantum efficiency of photosystem II (PSII) was measured as the photochemistry FV/FM in the GR maize cultivars to quantify glyphosate applications on the photosynthesis process. AG8025PRO2 was most sensitive for glyphosate applied at 4 l.ha-1 compared to DK245R2 which was more sensitive to generic glyphosate applied at 0.75 l.ha-1. Stunting of 10% was observed across GR cultivars where both products were applied at 4 l.ha-1. Inconsistent results across cultivars, time of glyphosate application and different products applied were obtained in this study indicating the complexity of glyphosate application on growth parameters of GR maize hybrids. Some cultivars were, however, more sensitive to glyphosate application with subsequent decreases in plant growth, physiological processes and yield.