Efficacy of organic herbicides in agronomic crops and improvement of soil biota with organic production practices

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI--COLUMBIA AT REQUEST OF AUTHOR.] CHAPTER I. Literature Review. CHAPTER II. Impact of Organic Herbicides in Corn (Zea mays). Abstract: Tillage is a weed control method in organic corn production that may negatively impact soil quality and increase erosion. Research was conducted to determine if post- directed herbicides could effectively control weeds in organic corn (Zea mays) while preserving crop yield. In central Missouri, corn was planted in 76 cm rows at one organic site in 2017 and two sites in 2018. As weeds reached 8 cm in height, repeated application of plant oils (manuka, clove + cinnamon, or d-limonene) and acids (acetic or caprylic + capric) were made at 374 L/ha between crop rows using a shielded sprayer. Five to seven applications were made between crop emergence and canopy closure. Following each application, symptomology on targeted weeds developed within 24 to 48 hours. At BREC in 2017, caprylic + capric acid and d-limonene were effective in reducing grass biomass on the second application by 55 and 58% at 9 days after treatment (DAT), when compared to the unweeded control. Similarly, both herbicides reduced grass biomass in 2018 by 91 and 96% (7 DAT), compared to the unweeded control. Hot weather conditions in 2018 contributed to more effective weed control compared to 2017. In 2018, caprylic + capric acid led to 68 and 73% grain yield increase in BREC and Busby Farm, respectively, versus the unweeded control. The utility of organic herbicides for weed control and preserving grain yield was evident. Post-directed, organic herbicides may provide a viable alternative to continuous tillage for weed management in corn. CHAPTER III. Postemergence Herbicides for Weed Control in Organic Soybeans (Glycine max). Abstract: Approximately 80% of organic soybean (Glycine max) supplying the United States are imported. Production of organic soybean to meet this need is limited by the lack of effective weed management. The objective of this research was to determine if post-directed organic herbicides could effectively control weeds and preserve crop yield. Soybean was planted in 76 cm rows at three locations in mid-Missouri during 2017 and 2018. As weeds reached 8 cm in height, repeated application of plant oils (manuka, clove + cinnamon, or d-limonene) and acids (acetic or caprylic + capric) were made at 374 L/ha between crop rows using a shielded sprayer. Following each application, symptomology on sensitive plants developed within 24 to 48 hours. Over the three locations, caprylic + capric acid was the most effective in reducing broadleaf (60 to 96%) and grass (78 to 97%) biomass after the fourth application, compared to the unweeded control. Cinnamon + clove oil was the least effective across all locations in reducing grass (27 to 82%) biomass after the fourth application, compared to the unweeded control. Efficacy of treatment varied due to hot and dry weather conditions in 2018, which contributed to more effective weed control than 2017. Soybean treated with caprylic + capric acid yielded 5 to 26% higher than the unweeded control across all locations and years. The utility of organic herbicides for weed control and preserving grain yield was evident. Post-directed, organic herbicides may provide a viable alternative to continued tillage for weed management in soybean. CHAPTER IV. Response of Soil Biota to Long-term Organic Weed Control Practices. Abstract: Current weed management practices in organic systems are primarily based on mechanical practices. Continuous tillage can negatively impact soil quality and microbial populations. The objective of this research was to determine if cultivation compared to alternative weed management methods, impacted important soil quality indicators. The four-year experiment consisted of five weed treatments (propane flaming, cultivation, between-row mowing, hot water spraying, and weed control) for two grain crops, soybean (Glycine max L.), and corn (Zea mays L.) grown in rotation. A third crop rotation consisted of winter wheat (Triticum aestivum L.), followed by soybean or summer cover crops. Hot water spraying and mowing were performed in area with crimped cover crops (no-till plots). Cultivation and flaming treatments were performed in areas where cover crops were terminated by tillage (conventional tilled plots). Soil quality indicators analyzed included aggregate stability, ?-glucosidase activity, and permanganate oxidizable carbon (active carbon). Between-row mowing resulted in having the highest values for all measurements (aggregate stability, ?-glucosidase, and active carbon) in 2017 and 2018 in both cropping systems; there was an increase in aggregate stability by 47 and 48% in soybean and corn, respectively compared to cultivation. No-till systems were significantly different from the other treatments, resulting in high enzymatic activity of ?-glucosidase in 2017 for soybean (294.0 ? PNP g soil-1 hr-1) and corn (286.1 ? PNP g soil-1 hr-1). Integrating new weed management practices, such as propane flaming and between-row mowing with the addition of crop rotation shows potential in organic cropping systems as both treatments sustained soil quality without detrimentally affecting microbial activity.