Effects of relay triple intercropping of Sugar Beet with Chickpea and Mung Bean on density, dry weight and biodiversity of weeds

One way to create sustainability and maintain the health of the agricultural ecosystems is to use intercropping. There are many reports on the effect of intercropping on population decline and increasing weeds diversity. In a study at all intercropping ratios, the dry weight of weeds was lower than the monoculture of two species of Sweet Corn and Mung Bean (Gholi Nejad et al., 2018). It was also reported that in intercropping, weed diversity increased compared to monoculture, but biomass decreased (Patience et al., 2013). Abadian et al, (2014) also showed that cowpea in additive intercropping with basil, reduced the population and biomass of weeds compared to monoculture of basil. This study was designed and conducted with the aim of studying the effect of relay triple intercropping of Sugar Beet with Chickpea and Mung Bean on density, dry weight and biodiversity of weeds in Mashhad weather conditions.
 
This experiment was conducted based on a randomized complete block design with three replications at the Research Farm of Faculty of Agriculture, Ferdowsi University of Mashhad during the growing season of 2015-2016. Treatments were 25% Chickpea (Mung Bean)+75% Sugar Beet, 50% Chickpea (Mung Bean)+50% Sugar Beet, 75% Chickpea (Mung Bean)+25%, Sugar Beet and monoculture of Chickpea (Mung Bean) and Sugar Beet. Legumes varied depending on the growth season, at the end of winter, Chickpea was intercropped with Sugar Beet and after harvesting Chickpea, Mung Bean was replaced by chickpea as a thermophilic plant in the late spring in the same rows of chickpea planting. During the growing season, composition of weed species together with weed density and dry matter were measure at 2 time before and after closing the canopy in randomly sampled 1×1 m2 quadrates. Using the species frequency Margalef richness index and several diversity indices including Shannon-Weiner and Simpson’s index were calculated for each treatment.
 
The highest range of weeds relative density was obtained in the first stage of sampling for Lamb's Quarters (29.81-68.1%) and in the second stage of sampling for Prostrate Pigweed (22.45-43.96%). The highest weed density at first and second sampling was observed in monoculture of sugar beet with 178 plants per square meter and the ratios of 75% chickpeas (Mung Bean)+25% sugar beet with 84 plants per square meter. Also, the highest biomass of weeds in the first and second stages of sampling was observed in monoculture of chickpeas (Mung Bean) with 48.72 g/m2 and the monoculture ratios of sugar beet with 40.49 g/m2. The highest Margalef species richness index was observed in 75% Chickpea (Mung Bean)+25% Sugar Beet and 25% Chickpea (Mung Bean)+75% Sugar Beet with 2.83 and 4.29 in the first and the second stages of sampling, respectively. The highest Shannon-Wiener diversity index was observed in 75% Chickpea (Mung Bean)+25% Sugar Beet and 50% Chickpea (Mung Bean)+50 % Sugar Beet with 0.6 and 0.76 in the first and the second stages of sampling, respectively. Finally, the highest Simpson’s diversity index was observed in 75% Chickpea (Mung Bean)+25% Sugar Beet and 50% Chickpea (Mung Bean)+50% Sugar Beet with 0.68 and 0.796 in the first and the second stages of sampling, respectively.
 
In relay triple intercropping ratios through more shading and space, the conditions for weed growth became more difficult and intercropping ratios reduced their growth. So that in total in two sampling dates, the highest density and biomass of weeds were observed in the monoculture ratios of sugar beet and the monoculture ratios of chickpeas (Mung Bean). In general, with the increase of plant diversity in the agricultural ecosystems, fewer ecological niches have been provided to weeds, which reduces the number and density of weed species. The best cultivation ratios to increase the diversity and species richness of weeds in the first sampling date was 75% chickpeas (Mung Bean)+25% sugar beet and in the second sampling date was 50% chickpeas (Mung Bean)+50% sugar beet. Based on this conclusion, intercropping can be mentioned as an effective tool to increase biodiversity and reduce the density and biomass of weeds.