Assessment of weed species composition and diversity in tomato (Solanum lycopersicum L.) farms in Ethiopia

Ethical considerations

This study was approved by the review committee of the Ethiopian Institute of Agricultural Research, Ambo Agricultural Research Center, Ambo, Ethiopia. Ethical approval was obtained from review committee on January 10, 2018, before the implementation of the study. Verbal consent was obtained from participants through interviewing because all respondents were illiterate.

Description of study areas

The weed survey was conducted at major tomato-growing areas of the country (Table 1), such as East Shewa, Wollega, West Shewa, Arsi, and Sidama zones, in the 2018–2019 cropping season during the off-season (Figure 1).

Figure 1. Map of weed flora surveyed areas in major tomato growing zones.

Weed species sampling techniques

Weed assessment was done at 5–10 km intervals on the main roadside and gravel roads of major tomato growing areas. In each field, a 1 m × 1 m quadrat was used, following an inverted X pattern.

A quadrat was randomly thrown along transects in each field, and the weed types and extent in a quadrat were recorded according to the methodology (Thomas, 1985; Kevine McCully et al., 1991). The first quadrat sample was taken following where the surveyor walks 50 paces along the edge of the field, turns right and walks 50 paces into the field, throws the quadrat, and starts taking the sample. A new specimen that was difficult to distinguish was brought to the Ambo Agricultural Research Center (Ambo ARC) and identified by experienced researchers using identification manuals. The Global Positioning System (GPS) coordinates of the sampling points were also collected. About 39 tomato fields and 195 weed samples were taken in the studied areas.

The nomenclature of weed flora was done by using the flora of Ethiopia and Eritrea as a weed identification guide (Stroud and Parker, 1989). Information on cropping systems, crop varieties used, crop management practices, adopted weed control methods, and herbicide use history was collected simultaneously during weed flora assessment from farmers and other tomato growers through interviews (Dereje, 2023b). At each surveyed site, 2–3 voluntary respondents were randomly selected and interviewed.

Field data on weed species composition and similarity index (SI) were collected at every sampling point. At each site of data collection, weed species were recorded on a data sheet and later weed species composition (frequency, abundance and dominance) and similarity index was calculated using standard formulas.

Data analysis

Data on weed species was summarized using the formula described by Tessema et al. (1999) and Thomas (1985).

Frequency: is the percentage of sampling plots in which a particular weed species is found in a field. It explains how often a weed species occurs in the survey area (Marshall, 1988).

Where, F = Frequency of particular weed species, X = number of samples in which a particular weed species occurs, N = total number of samples.

Abundance: is the population density of a weed species expressed as the number of individuals of weed plants per unit area (Tessema et al., 1999).

Where, A = Abundance, $\sum \mathrm{W}$ = Sum of individuals of a particular weed species across all samples, N = total number of samples in a field.

Dominance: abundance of an individual weed species in relation to total weed abundance in a field (Thomas, 1985; Tessema et al., 1999).

Where, D = Dominance of a particular species, A = Abundance of the same species, $\sum$W = Total abundance of all weed species.

Similarity index (community index): is the similarity of weed communities between different locations or crops. If the index of similarity is >60%, it is assumed that the two locations are similar in species composition and hence the same control method can be applied (Tessema et al., 1999).

Where, SI = Similarity index; Epg = number of species found in both locations; Epa = number of species found in location I; Epb = number of species found in locations II.

Data on the farmers perceptions of weed management practices in tomato fields were calculated through descriptive statistics using Statistical Package for Social Sciences (SPSS) (RRID:SCR_002865) version 26.

Classification of weed species in tomato fields

A field survey on weed species in major tomato growing areas showed that 63 weed species from 23 families were identified in the surveyed areas (Dereje, 2023a). Based on number of taxa, the Asteraceae family contained 13 weed species, followed by the Poaceae family with 11 weed species (Table 2). Among weed species, 50 of them were categorized as broad-leaved, whereas 13 were classified as grasses and sedges.

Weed species composition and distribution

Portulaca oleracea and Cyperus rotundus were the most dominant weed species in East Shewa and West Arsi, with 50 and 47.25%, respectively (Tables 3 and 4). The lowest weed species dominance was obtained by Sonchus asper at East Shewa, Bideance pilosa at East and West Shewa (Table 3), Oplismenus hirtellus, Sonchus oleraceus, and Setaria pumila at Sidama Zones.

The highest weed frequency was also recorded by Portulaca oleracea (95%) and Cyperus rotendus (70%) in the West Arsi Zone (Table 4). A similar finding reported that Cyperus species, Portulaca oleracea, and Amaranth species were the most frequent weed species at Rift Valley areas in the country (Firehun and Tamado, 2006).

Similarity index of weed flora

The result revealed that the highest weed community index was obtained between East Shewa and Wollega zones, with 58.62% similarity (Table 5). Whereas the lowest weed species similarity index value (28.57) was registered between West Shewa and Arsi zones.

This diversity and distributions in weed species composition may be due to variation in climatic conditions, soil types, and farmer practices (Saavedra et al., 1990; Mennan and Isik, 2003). Karar et al. (2005) also stated that crop husbandry, the use of the same herbicide, and the entrance of new invasive weed species into the areas may cause the diversification and distribution of weed flora in a given habitat.

Farmers perceptions and practices on weed management in tomato fields

In this study, 61% of the respondents used hoeing, followed by hand weeding (23%), and hoeing plus hand weeding (15%) for the management of weeds in the tomato fields (Figure 2). About 48% of tomato growers rotated maize after tomato, and 25% of them planted onion after tomato to reduce the weed seed bank in the soil by interrupting germination and suppressing weed growth.

Figure 2. Farmers weed management practices in tomato fields.

On the other hand, amongst tomato growers, 48% used an improved tomato variety named “Galilea,” followed by Koshoro (17%), whereas the remaining used local and unknown varieties (Table 6). Furthermore, a significant proportion of growers (69%) used recommended fertilizer rates, whereas the others used above and below recommendation rates.

About 46% of respondents stated that Tuta absoluta was one of the biggest obstacles to tomato production in the surveyed locations, along with weeds. This could be as a result of the insect’s resistance to numerous insecticide classes and a lack of knowledge about integrated pest management.