Species composition and number of weeds in phacelia seed in Western Siberia

S. I. Mikhailova 1, 2, T. V. Ebel 1

Introduction. Phacelia tanacetifolia, an annual plant of the Boraginaceae family, is characterized by early maturity, ecological plasticity, and high nectar productivity, which makes it popular as a valuable honey plant in many countries around the world. In recent years, interest in the use of phacelia in medicine has increased.

The introduction of phacelia into crop rotations as a cover and green manure crop contributes to increasing crop yield and improving soil structure. The importance of phacelia is growing in the context of the development of organic farming, where it can provide a reliable honey flow conveyor and preserve beneficial entomofauna in the agrocenosis, as well as serve as a phytomeliorant for developing a forage base for beekeeping on heavily eroded lands.

Many authors point to the ability of phacelia to suppress weeds in crops, including through allelopathic activity. However, this does not exclude high weed infestation in commercial phacelia crops, which requires the improvement of chemical and agrotechnical measures for regulating weed vegetation.

In the Siberian Federal District (SFD), phacelia is among the most popular and in-demand green manure and honey plants, the demand for whose seed is constantly growing. However, as the results of long-term monitoring have shown, the quality of seed of green manure and honey crops entering the retail networks of the SFD does not always meet the standards of the Russian Federation.

The purpose of this article is to identify the species composition and number of weed diaspores accompanying the seed of Phacelia tanacetifolia grown in the conditions of Western Siberia.

Materials and methods of research. Herbological analyses of Phacelia tanacetifolia seed lots were carried out in the testing laboratory of the Tomsk branch of the Russian Plant Quarantine Center. The material for the studies consisted of seed samples of phacelia grown in the regions of Western Siberia in the period 2018–2023. In order to identify the species composition of weeds contaminating phacelia seed, 30 seed lots supplied to Tomsk retail networks from different cities were analyzed. To determine the number of weed diaspores and identify the predominant species, seeds of all weed species were separated from each 0.5–1 kg sample. Identification of weed seeds, taking into account the main morphological features, was carried out using a Stemi 305 binocular microscope, guided by the main methodological manuals, and also using the carpological collection of the Tomsk branch of the Russian Plant Quarantine Center, which includes over 1,000 samples of seeds and fruits belonging to more than 370 species of mainly weeds. To account for the number of weed seeds, 10 samples weighing 5–10 g each (depending on the degree of infestation) were taken from each sample, and all weed diaspores were counted and recalculated per 1 kg of seed.

Results and discussion. During the monitoring of the contamination of phacelia seed lots, diaspores of 53 weed species belonging to 19 families were identified.

More than half (55.5%) of the identified weeds are adventive species, five of which are invasive species in the territory of Siberia.

Table 1 – Species composition of plants found in the seed of phacelia grown in Western Siberia in the period 2018–2023.

Occurrence, % No. Species Group of total number of samples

4 Adv 13.0

Species Group of total number of samples 39 Adv 4.3 40 Adv 13.0 41 Adv 4.3 42 Ap 8.7 43 Ap 21.7 44 Adv 8.7 45 Adv 13.0 46 Adv 17.4 47 Ap 39.1 48 Ap 8.7 49 Ap 34.8 50 Ap 13.0 51 Adv (I) 13.0 52 Adv (I) 17.4 53 Adv 13.0 Note – Adv – adventive species, Ap – apophyte, I – invasive species.

An analysis of the studied seed lots showed that the trend we identified earlier remains relevant: phacelia seed is characterized by high weed infestation and a diverse species composition of weeds. Table 2 presents the results of several herbological analyses of phacelia seed lots performed in 2020–2023.

In the studied samples, from 6 to 24 weed species were found. Most often, diaspores of 11–15 species are detected. Almost all of them are easily identified, except for some species. For instance, the ereme-fruits of two species of the genus Galeopsis hardly differ in morphology, although the plants themselves in the flowering phase are well distinguished by their flowers.

According to the 2020–2023 survey, the total weed infestation of phacelia lots varies from 2140 to 131600 units/kg and remains at the same high level as in previous years of monitoring. Predominant weed contaminants in phacelia seed include small-seeded dicotyledonous species (common lambsquarters, false flax, redroot pigweed, tumble pigweed) and monocotyledonous weeds (barnyard grass, wild millet, green foxtail, and yellow foxtail). Moreover, diaspores of millet-like grasses are detected regularly with high frequency (64–100%) and often with high abundance.

Table 2 – Content of weed diaspores in phacelia seed lots based on herbological analyses conducted in 2020–2023.

Origin Date of Sample Analysis Origin Number of weed species Dominating weed species (number of seeds, pcs/kg) 04.04.2020 Tomsk 12 13840 4320)

3960) 06.08.2020 Tomsk 9 5490 5090) 19.08.2020 Novosibirsk 6 26340 25620) 17.09.2020 Barnaul 11 12500

1950) 03.02.2021 Barnaul 15 21360 13200)

5680) 04.03.2021 Tomsk 16 59550 31600)

5860) 20.07.2021 Novosibirsk 24 146000 88000) 31000) 18.03.2022 Barnaul 12 35300 28600)

3200) 04.10.2022 Tomsk 12 2140 1500) 20.01.2023 Novosibirsk 22 131600 38800) 38800)

13600) 27.01.2023 Kemerovo 11 14440 10400) 10.12.2023 Novosibirsk 14 23500 10760) 4600)

Main weed impurities in tansy phacelia seeds and challenges of seed processing

Obtaining clean phacelia planting material remains a complex practical task. Innovative methods of seed cleaning, in particular optical sorting, are not yet used on farms. Because of this, it is rare to produce seed material with high purity levels, and the examined seed lots (including those analyzed in 2018–2019) show a consistently high number of weeds.

• Minimum lot infestation — 2140 pcs/kg
• Maximum lot infestation — 131600 pcs/kg
• Identified weed species — 53 species
• Represented weed families — 19 families

According to herbological analyses conducted in 2018–2023, the species composition of weed impurities in phacelia seeds includes 53 weed species from 19 families. Infestation of individual lots varies from 2140 to 131600 pcs/kg. The dominant weed impurities in seed material are small-seeded species:

• Dicotyledonous weeds: fat-hen (common lambsquarters), false flax, redroot pigweed, tumble pigweed;
• Monocotyledonous weeds: barnyardgrass, common millet (weed), green foxtail, yellow foxtail.

The high frequency and abundance of weeds from the Paniceae tribe in phacelia seeds are due to the general trend of increasing spread of millet-like grasses in crops. Due to similar size characteristics, it is extremely difficult to clean phacelia seed material from such weeds using standard equipment.