Investigating the Effect of Mulch on Weed Control and Product Quality in Intercropping of Stevia (Stevia rebaudiana) and Iranian Borage (Echium amoenum)

Introduction Intercropping is one of the most economical and accessible methods for increasing crop production. Intercropping increases the quantity and quality of yield by reducing intraspecific competition and supporting plants mutually. Research on organic mulches relates to plant residues such as straw, leaves, and various types of manure. In sustainable agriculture, mulches can improve conditions for growth and environmental stress tolerance. The use of mulch and mixed cropping can affect environmental factors (such as temperature, humidity, water and soil salinity, and light intensity and quality) and biological factors (such as fauna and flora inside and outside the soil surface, pest, disease, and weed populations, and reduced intra- and inter-specific competition) at the microclimate scale, improving quantitative and qualitative Yield. Iranian borage (Echium amoenum) is a herbaceous, perennial, long-day plant, and stevia (Stevia rebaudiana) is a bushy, perennial plant sensitive to cold. In this regard, intercropping stevia and Iranian borage in new regions can create opportunities for employment and profitability. Materials and Methods The experiment was conducted as a two-year factorial with two factors based on a randomized complete block design with three replications at the Agricultural Research and Education Center of Arak during the 2020-2021 and 2021-2022 growing seasons. Treatments included wheat straw mulch at three levels (no mulch - zero, furrow mulch coverage - 3 tons per hectare, and complete furrow and ridge mulch coverage - six tons per hectare) and intercropping ratios of stevia and Iranian borage using replacement method (100% Iranian borage, 75% Iranian borage - 25% stevia, 50% Iranian borage - 50% stevia, 25% Iranian borage -75% stevia, and 100% stevia). Results and Discussion The lowest weed biomass (60 g/m²) and highest volumetric moisture content in the first year (12.07) and second year (12.7) were observed in pure Iranian borage cultivation with complete furrow and ridge mulch coverage. The highest weed biomass (296 g/m²) and lowest volumetric moisture content in the first year (2.5) and second year (2.03) were found in pure stevia cultivation without mulch. The highest canopy temperature in winter (3.86°C) was obtained with furrow and ridge mulch coverage, while the lowest summer canopy temperature (32°C) was observed in intercropping ratios of 50% stevia-50% borage, 75% borage-25% stevia, and pure Iranian borage cultivation with complete furrow and ridge mulch coverage. The highest stevioside content in stevia (3.74) was achieved with a 75% stevia-25% Iranian borage ratio and furrow mulch application. Analysis of Iranian borage fatty acids showed the highest gamma-linolenic acid percentage with furrow and ridge mulch coverage, differing by 1.2% from the lowest percentage observed without mulch. The highest unsaturated fatty acids, oleic acid (21.8%) and linoleic acid (24.7%), were obtained in the planting ratio (25% stevia and 75% Iranian borage and pure Iranian borage cultivation) with furrow and ridge mulch, while the lowest oleic acid (17.9%) and linoleic acid (18.8%) were found in the ratio (25% stevia and 75% Iranian borage without mulch). The highest saturated fatty acid, palmitic acid (10.5%), was found in the intercropping ratio (25% stevia and 75% borage) without mulch, and the highest stearic acid (4.95%) was obtained in the ratio (75% stevia and 25% Iranian borage and pure cultivation) without mulch. The lowest saturated fatty acid palmitic (9.92%), and the lowest stearic acid (4.05%) were obtained in the treatment (75% stevia and 25% Iranian borage with furrow and ridge mulch). The highest dry flower weight of Iranian borage (837 kg/ha) was obtained with the use of mulch in rows and ridges and an intercropping ratio of 50% stevia and 50% Iranian borage. The highest stevia plant height was observed in the 100% stevia and complete furrow and ridge mulch coverage. Results showed that applying mulch in-furrow and ridge and intercropping with a higher percentage of stevia leads to better quality in Iranian borage oil. Conclusion The furrow and ridge mulch coverage treatment resulted in the highest levels of measured traits in this study, including soil volumetric moisture content, unsaturated fatty acid linoleic acid content, the highest gamma-linolenic acid content, as well as maximum winter canopy temperature and lowest weed biomass. The planting ratio (75% stevia - 25% borage) with furrow mulch application achieved the highest stevioside content.