Evaluation of the effect of weed diversity and evenness on lentil (Lens culinaris L.) yield

Lentil is one of the most important legumes in rainfed and irrigated cultures, which is sensitive to weeds due to its low density and growth rate in early stages of growth. Therefore, lentil weed control is essential to prevent crop loss. Most studies of weed and crop competition have focused on mono-species competition, while generally there is not observed and the multispecies competition effects of weed populations are important. On the other hand, studies have shown that the presence of weeds in agricultural ecosystems is not always negative, even the presence of different weed species along the crop with increased diversity and nutrient cycling can lead to positive effects. Therefore, the knowledge of the effects of presence of different weed species on a field and the relationship between them is important. In this study, the relationships between weed communities in a lentil field and the effect of diversity indices on lentil yield were investigated. 

This experiment was conducted on one of rainfed lentil fields of the Agricultural and NaturalResources Campus of Razi University in Kermanshah during 2016 growing season. Systematic sampling was performed in two stages (before flowering and before physiologic maturity) to record weed density, height and dry matter as well as lentil grain yield. In addition, weed diversity and evenness indices (Shannon Weiner, Simpson, Smith and Wilson and Camargo Indices) were calculated using weed record data. The relationships between weeds and lentils were extracted by stepwise regression using SPSS statistical software V.20 and the spatial maps of weeds and lentil diversity were drawn using ArcGIS 10.2.2 software. 

The results of this study showed that a total of 45 weed species were recorded in the first stage of sampling and 28 species in the second stage of sampling. Recorded weed species in the studied lentil field were according to the important weed species of lentil farms in the west of Iran. So that, Brassicaceae, Asteraceae and Convolvulaceae were most frequented families, in addition, the most of the recorded species were broadleaves and the narrow leaves did not have much frequency. The results also showed that lentil yield was significantly affected by weed density and dry matter in the first and second stages of sampling, respectively. It has been stated that the high density of weeds during the vegetative period decreases biological yield and subsequently grain yield of the crops. Researchers also reported a negative correlation between yield and dry weight of weeds. The results of the effect of weed population evenness and diversity on lentil yield in the first sampling stage showed that Smith and Wilson evenness index and Simpson diversity index had a significant positive effect on lentil yield. In the study of, also there was a significant positive relationship between yield and weed diversity indices. However, in the second stage of sampling weed evenness and diversity did not have a significant effect on lentil yield. The study of spatial maps of lentil yield, Smith and Wilson evenness index and Simpson diversity index showed that the lentil yield was spatially correlated with weed evenness and diversity. In areas with low lentil yield, weed evenness and diversity were also low and in areas where lentil yield was high weed evenness and diversity were also high. This suggests that increased diversity and evenness in the fields can reduce the negative effects of weeds on crop yield. 

It can be concluded that the presence of weeds in the first and second stages of sampling led to decreased lentil yield, while the weed diversity and evenness had a positive and significant effect on lentil yield. In fact, by increasing the diversity and evenness of weeds and increasing the positive and negative interactions between them with other organisms, it seems that there was a positive effect on the lentil yield. In fact, increasing the weed diversity and evenness in farms could results in a fairer division of resources and less damages of some weed species on yield. The spatial distribution of the weed diversity and evenness, and its comparison with lentil yield showed a positive spatial relationship between increasing the weed diversity and evenness and lentil yield. Hence, increasing knowledge about weeds as well as their relationship with crops can reveal the positive aspects of weed presence in farms and, consequently, better weed management by increasing awareness in this field.