In general, broadleaf weeds were dominant in the studied fields. An experiment in Ethiopia showed that broadleaf weeds accounted for more than 93% of the chickpea weed flora (Merga and Alemu, 2019). In this study, the Asteraceae had the highest relative density among weed families. Nath et al. (2018) in a study conducted in India reported that major weed flora in chickpea included broadleaf weeds. However, the composition of chickpea broadleaf weeds varies among different regions. Merga and Alemu (2019) categorized Solanum nigrum L., Medicago polymorpha L., Galinsoga ciliate, and Commelina benghalensis L. as high density species in their study. While in the study of Nath et al. (2018) Sonchus arvensis L.., Chenopodium album L., Euphorbia geniculate L.., Vicia hirsuta (L.) Gray., Physalis minima L.., Rumex dentatus L., Medicago denticulata L., and Cirsium arvense (L.) Scop. scored highest relative weed density. Chalechale et al. (2015) studied the weed population in the chickpea field of the Kermanshah and recorded 61 weed species which broadleaf weeds were most abundant. In the current study, the dominant broadleaf weeds included C. intybus, C. arvensis, G. tricornutum, C. oxyacantha, and cowcockle (Vaccaria pyramidata) and the dominant grass weeds consisted of H. spontarcum, wild oat (Avena fatua L.), bermudagrass [Cynodon dactylon (L.) pers.]. Mousavi et al. (2010) reported that russian knapweed [Acroptilon repens (L.) DC.], C. intybus, C. oxyacantha, G. glabra, prickly lettuce (Lactuca serriola L.), S. arvensis, and bitter bean (Sophora alopecuroides L.) are common weed species of chickpea fields in Kermanshah. Undrestanding the composition of weed populations is one of the most important components in weed management. The high abundance of broadleaf weeds indicates that these species are a great menace to chickpea production in the study area and proper control of such weeds should be considered in weed management programs.
The results showed that in the early growing season (four to seven-leaf stage of chickpea), weeds density which reflects the subsequent weed interference during the growing season, significantly affected chickpea yield. The critical period of weed control in chickpeas is between four-leaf to before flowering phenological stage (Mohammadi et al., 2005), so, weeds may have significant effect on chickpea yield during early growth stage. AL-Thahabi et al. (1994) reported a significant negative correlation between weed dry weight and chickpea grain yield. Slow seedling establishment, as well as limited vegetative growth and canopy expansion in the early growth stages make chickpea low competitive against weeds (Miller et al., 2002). Therefore, weed competition could decrease chickpea yield and yield component (Saxena et al., 1996). Accordingly, lack of weed control in the critical period of weed management can lead to a significant reduction in chickpea yield.
Weed canopy cover had a remarkable effect on chickpea grain yield, during the second sampling stage. In the field condition, plant species compete for solar radiation by enhancing access to the light and shading on neighbors (Tardy et al., 2015). In a chickpea field, due to the limited canopy expansion of chickpea, competition for light between the crop and weeds is unavoidable which seems determined by plant height and canopy cover (Duchene et al., 2017, Siebert and Stewart, 2006). In light competition, along with reducing the quantity of received light to a given plant, light quality could also be affected (Taiz et al., 2018). These two factors can ultimately lead to a reduction in the photosynthesis and dry matter production by plants (Tardy et al., 2015, Rao et al., 1991). Therefore, increasing the canopy cover of weeds in the podding phenological stage of chickpea seemed to reduce the photosynthesis and thereby grain filling of chickpea.
The weed species in the studied fields were mostly annual and broadleaf that most of them were listed in the dominant weed species list of the chickpea fields of Kermanshah province. In this case, C. intybus and C. oxyacantha were the most frequent weeds and also had significant negative correlation with chickpea yield. In western parts of Iran, C. intybus germination begins at the end of winter at the same time as chickpea germination (Chalechale et al., 2015). This species had good growth in low-input systems (Rumball, 1986). Thus, it able to have a good growth in the chickpea drylands of Kermanshah where almost no fertilizer operations are carried out (Chalechale et al., 2015). On the other hand, C. oxyacantha is a spinous weed that has a wide distribution in western parts of Iran (Chalechale et al., 2015). In addition to competition with chickpea, C. oxyacantha causes problems in harvesting operation and cause injuries to workers (Nosratti et al., 2020). This species has a high vegetative growth rate and completes its life cycle before exposing to water stress period in the Kermanshah weather condition (Chalechale et al., 2015). These attributes have made C. oxyacantha as a noxious weed in chickpea dryland fields of western parts of Iran.
The interactions among weeds and chickpeas reflected the complex positive and negative relationships. It showed that not all weed species are detrimental to crop yield. We found that, the negative correlations among chickpea yield and most of weed species were not significant and even some weed species, such as G. glabra positively correlated with chickpea yield (Tables 6 and 7). The positive association between G. glabra and chickpea may indicate a complementary phenomenon. Mutualistic or negative interactions may occur within plant communities in agricultural systems (Callaway, 1995, Radosevich, 1987, Radosevich et al., 2007). In plant communities sometimes the impact of complementarity is stronger than the competition, in which case the negative effects of competition are compensated or eliminated and the overall use of resources will be improved through the positive interactions (Bedoussac et al., 2015). For instance, the activity of fabaceae plants in the soil can case acidification of the rhizosphere and increase phosphorus mobilization through the dissolution of it (Duchene et al., 2017, Hinsinger et al., 2011, Weisser et al., 2017). This phenomenon can increase efficient phosphorus uptake from the soil. Pivato et al. (2007) showed that the legume enhanced the abundance and diversity of arbuscular mycorrhizal fungi.
On the basis of indirect facilitation phenomenon, the competitive suppression of species A on species B can decrease the competitive effect of species B on the third species (C) (rooker et al., 2008). In our study, the negative correlations of G. glabra with most prominent weeds (e.g. C. intybus, C. oxyacantha, and C. orientalis) were observed. Interspecific competition of G. glabra with other weed species may have led to this negative correlation. In the study of Toler et al. (1996) the individual competitive effects of Sorghum halepense and Amaranthus hybridus on soybean yield in the mono-species competition was more than the two-species competition. This was due to the negative interaction of the two weeds in the two-species competitive conditions. Furthermore, G. glabra can have allopathic effects on other weed species. In a study, aqueous extract of G. glabra root reduced the growth of A. fatua, Silybum marianum L. and wheat (Mustafa et al., 2018). However, in other study on the allopathic effect of S. halepense C. album, G. glabra, C. arvensis and Polygonum aviculare L. on germination and seedling growth of chickpea, The results indicated that, shoots, the soil of root zone and extracted water from rizospher of S. halepense, C. album and C. arvensis significantly decreased germination and seedling growth of chickpea as compared to control. While, G. glabra did not affect these traits (Mohammadi et al., 2004).
In this study, weed evenness and diversity had a positive effects on chickpea yield. This finding is consistent with observations in other studies on the effect of weed evenness and diversity on crop performance (Adeux et al., 2019, Cierjacks et al., 2016). Evenness represents the distribution of mean values of species frequencies within plant communities (Mason et al., 2005). Higher levels of weed evenness and diversity indicates more equitable sharing of resources among the plant species within occupied niche space (Ejtehadi et al., 2009). the phenotypic differentiation between species results in contrasting ability to capture the resources which this co-existence is supported by spatio- temporal heterogeneity in resource supply, therefore, it leads to decreased crop competition and limits the probability of occurrence of dominant and competitive weed species by increasing inter-species interference (Adeux et al., 2019, Storkey and Neve, 2018). Accordingly, in the present study the most dominant weed species (C. intybus and C. oxyacantha) had the least association with evenness and diversity and thereby chickpea yield. Existence of different plant species in a high-diversity plant community can support complementary or facilitation processes (Weisser et al., 2017). Ferrero et al. (2017) found that weed diversity was positively associated with rates of crop yield of maize and soybeans. In a study of weed species diversity association with coconut and banana yield, weed diversity and crops yield had a positive and significant correlation (Cierjacks et al., 2016). Yang et al. (2019) also reported a significant positive effect of weed species diversity on aboveground biomass which was mainly contributed by complementarity effect.