Browsing by Author "Mucheri, Tendai"

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    The efficacy of glufosinate ammonium on ryegrass as influenced by different plant growth stages and different temperatures
    (Stellenbosch : Stellenbosch University, 2016-12) Mucheri, Tendai; Pieterse, P. J.; Stellenbosch University. Faculty of Agrisciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Herbicide resistance in weeds is the ability of weeds to survive and reproduce following exposure to the recommended dosage rate of herbicide that is lethal to its wild type. There is a widespread concern in agriculture about weeds with high genetic diversity that have developed resistance to weed control, ryegrass (Lolium spp.) included. Ryegrass has developed resistance to commonly used herbicides which include paraquat and glyphosate. There is an opportunity of using glufosinate ammonium to alleviate ryegrass weed resistance problems. The herbicide not only has a unique mode of action but also has no ryegrass resistance proven to it yet. There are restrictive application timings with glufosinate ammonium since it is a contact herbicide. More specific recommended dosage rates of herbicides can therefore be developed by determining the contribution of environmental factors and growth stage of weeds to efficacy of glufosinate ammonium. The principle objective of the study was to determine the effective dosage of glufosinate ammonium for the control of ryegrass weed under different temperatures and ryegrass growth stages. Studies on the influence of temperature on glufosinate ammonium efficacy are reported in Chapter 3, 4 and 7. Influence of ryegrass growth stage on efficacy of glufosinate ammonium is dealt with in Chapters 5 and 6. Glufosinate ammonium dosage rates of 0, 1.5, 3, 4.5, 6 and 7.5 L ha-1 were used for a temperature study in glasshouses running at 10/15, 15/20, 20/25, and 20/30 °C night/day temperatures. In Chapter 3 the influence of temperature on efficacy of glufosinate ammonium on young and mature ryegrass is described. Mature ryegrass was sprayed at 6 weeks while young ryegrass was sprayed at 3 weeks. The study proved that a low temperature of 10/15 °C controlled approximately 95% of both young and mature ryegrasses with 3 L ha-1 while the trend observed at 15/20 and 20/25 °C was irregular. Temperatures of 25/30 °C resulted in poor control of ryegrass. There was a general increase in control of young ryegrass as compared to mature ryegrass. In Chapter 4 the effect of temperature on efficacy of glufosinate ammonium with the added adjuvant ammonium sulphate (AMS) on ryegrass is described. Applied glufosinate ammonium dosage rates were 1, 2 and 3 L ha-1 with added ammonium sulphate at rates 1, 2 and 3%. Glasshouses were set at 10/15, 15/20, 20/25, and 20/30 °C night/day temperatures. The findings of the study indicated that a dosage rate of 3 L ha-1 glufosinate ammonium with addition of 2 and 3% ammonium sulphate controlled ryegrass effectively. There was more effective control of ryegrass with all concentrations of AMS at lower temperatures compared to the control at higher temperatures. An increase in AMS concentration resulted in an increase of ryegrass control at lower temperatures but this was not evident with control at higher temperature. Glasshouse and field experiments as described in Chapter 5 were conducted to determine the influence of different ryegrass growth stages on glufosinate ammonium efficacy. Glasshouse experiments were conducted at Welgevallen experimental farm and the field experiments were conducted at Welgevallen, Roodebloem and Langgewens experimental farms. Growth stages of ryegrass were 2, 4, 6, 8 and 10 weeks. Applied dosage rates were 1.5, 3, 4.5, 6 and 7.5 L ha-1 for glasshouse experiments and 2.5, 5, 7.5 and 10 L ha-1 for field experiments. The findings of the study proved that growth stage of ryegrass has no influence on efficacy of glufosinate ammonium. However, differences in control were observed for different glufosinate ammonium dosage rates. The study also revealed better control of ryegrass in the glasshouse as compared to the field. The trials described in Chapter 6 were conducted at Welgevallen experimental farm to investigate the efficacy of glufosinate ammonium with different added adjuvants on different aged ryegrass in both the glasshouse and the field. Three different growth stages of ryegrass (3, 6, and 9 weeks) were obtained by planting the ryegrass at 3-week intervals from the date the experiment was initiated, whilst young and mature growth stages were obtained in the field by spraying wild ryegrass at average leaf numbers of 6 and 15 leaves plant-1, respectively. In the glasshouse, control of 6-week old ryegrass was more effictive regardless of the mixture applied. Glufosinate ammonium with the added adjuvant (Velocity®) controlled ryegrass more effectively than glufosinate ammonium applied alone or with another added adjuvant (Summit Super). Field experiment results showed that AMS added to glufosinate ammonium controlled young ryegrass better than glufosinate ammonium alone and with the adjuvant (Ballista®). A glasshouse trial to compare efficacy of glufosinate ammonium on ryegrass and bahia grass species as influenced by temperature is described in Chapter 7. Applied dosage rates were 1.5, 3, 4.5, 6 and 7.5 L ha-1. The glasshouse temperatures were set at 10/15, 15/20, 20/25 and 25/30 °C night/day temperatures. The findings of the study showed a similar trend in glufosinate ammonium control of both grasses; as temperature increased, control decreased. At 10/15 and 15/20 °C temperatures percentage control was significantly higher than at 20/25 and 25/30 °C temperatures. Even though the trend was similar, mortality of ryegrass at high temperatures was more apparent as compared to bahia grass.
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    Factors influencing inhibition of glutamine synthetase enzyme in grass weeds by glufosinate ammonium under different temperatures
    (Stellenbosch : Stellenbosch University, 2019-12) Mucheri, Tendai; Pieterse, P. J.; Reinhardt, Carl Frederick; Kleinert, Aleysia; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: Evolution of weed resistance emphasized the need to implement integrated weed management strategies, however, farmers still immensely rely on chemical weed management. Glufosinate ammonium is an alternative herbicide that can replace or be used in rotations with herbicides such as glyphosate and paraquat, but it poses a problem due to its inconsistencies in controlling weeds. Studies in this dissertation aimed to investigate the influence of temperature on glufosinate ammonium efficacy. Chapter 3 of this dissertation investigated the influence of temperature on ryegrass cuticle thickness, phenolic acid concentration and calcium accumulation, and subsequently, the effect of the afore-mentioned factors on glufosinate ammonium efficacy. Ryegrass was grown at 10/15, 15/20, 20/25, and 20/30 °C (night/day) temperatures and treated with 0, 1.5, 3, 4.5, 6 and 7.5 L ha-1 glufosinate ammonium dosage rates. The grass was treated six weeks after planting and assessment was done four weeks after glufosinate application. Control of ryegrass decreased with increasing temperature. Results indicated that cuticle thickness and calcium content increased as temperatures increased, probably due to production of phenolic compounds responsible for plant defence mechanisms against herbicide stress, hence resulting in poor control of ryegrass under warmer temperatures. Chapter 4 investigated ammonia accumulation, glutamine synthetase, glutamate dehydrogenase, nitrate reductase activity and ryegrass photosynthesis in roots and leaves of control (0 L ha-1) and treated ryegrass (4.5 L ha-1) harvested 24 hours after glufosinate ammonium application. There was a significant increase in glutamine synthetase enzyme activity with increasing temperature after glufosinate ammonium application. Better control of ryegrass under cooler temperatures with glufosinate ammonium was mainly attributed to the plants’ inability to produce adequate glutamate and α-ketoglutarate, which form the carbon skeleton for transamination processes. Response of glutamine synthetase to glufosinate ammonium was significantly higher at warmer temperatures, such that the use of an alternative glutamate dehydrogenase pathway was not vital. The significant increase in glutamine synthetase activity in ryegrass under warm temperatures was able to circumvent photosynthetic inhibition. A comparative study to investigate the response of different grass weed species to glufosinate ammonium was conducted on ryegrass (Lolium spp.), ripgut brome (Bromus diandrus L.) and wild oats (Avena fatua L) in Chapter 5. The grasses were grown at 10/15, 15/20, 20/25, and 20/30 °C (night/day) temperatures and treated with 0, 1.5, 3, 4.5, 6 and 7.5 L ha-1 glufosinate ammonium dosage rates. The study observed that glufosinate ammonium control differed among weed species. Control of ryegrass increased with decreasing temperature. Temperature had no effect on wild oats. Control of ripgut brome was initially poor at 10/15 °C then increased at 15/20 and 20/25 °C and finally decreased again at 25/30 °C. Such differences in the grass response to glufosinate ammonium, even after being grown under the same conditions, was attributed to their differences in morphological characteristics such as cuticle thickness, calcium accumulation and photosynthesis after herbicide application. Increase in cuticle thickness decreased mortality of all grasses. The study perceived that negative effects of calcium on mortality can only be noticed if the cytosolic and mitochondrial calcium is mobile and active, thus, allowing it to react with glufosinate ammonium. A possible solution to mitigate problems arising from calcium level, cuticle thickness and phenolic compounds was investigated in Chapter 6. The study investigated the role of adjuvants in increasing glufosinate ammonium efficacy. Ryegrass was grown at 20/25 °C and treated with 0, 1, 2, 3 and 4 L ha-1 glufosinate ammonium. Glufosinate ammonium was applied solo and in tank mixtures with Velocity Super™ (ammonium sulfate, L 9603), Summit Super (nitrogen solution/non-ionic surfactant, L 8539) and Class act NG™ (ammonium sulfate plus a non-ionic surfactant, L 10477). Better control of ryegrass was observed when treated with glufosinate ammonium in a tank mixture with Class act NG™ and Velocity Super™ than its solo application as well as in a tank mixture with Summit Super. Ammonium sulfate exhibits surfactant and humectant properties and it facilitates movement of glufosinate ammonium into the plant while non-ionic surfactants aim to reduce water surface tension only. This explains better control observed with glufosinate ammonium in tank mixture with adjuvants containing ammonium sulfate than with Summit Super. The study suggests that adjuvant Class act NG™ and Velocity Super™ can be used to mitigate the defensive response of phenolic compounds after glufosinate ammonium application, hence, increasing its efficacy. The practical relevance of glasshouse observations in Chapter 3, 4, 5 and 6 was confirmed in Chapter 7. The study was conducted under rainfed conditions at Langgewens and Roodebloem farms in 2018 and 2019. Glufosinate ammonium was applied at different times of the day (8:00 am, 12:00 pm and 5:00 pm). The dosage rates applied were 0, 2.5, 5 and 7.5 L ha-1. The study observed that morning (8:00 am) and evening (5:00 pm) applications showed better control of ryegrass than mid-day application provided relative humidity during application time was greater than 75%. Application at mid-day (when temperatures were higher than morning temperatures) showed good control only if relative humidity was recorded above 80%, however, higher dosage rates of 5 or 7.5 L ha-1 were required to achieve greater than 90% control.