
Research location and soil conditions
The results of the study come from a three-year field experiment conducted in 2012–2014 in production fields of multi-branch Soleks company in Wojnów, the district of Siedlce, in Mazovia Voivodeship, in Poland (52° 12′ 59″ N, 22° 34′ 37″ E). The research was conducted on soil classified according to the World Reference Base for Soil Resources15 belonging to Haplic Luvisol (LV-ha) with a slightly acid reaction (pH in KCl 5.60–6.35). The content of available forms of macronutrients in soil level 0–30 cm in mg kg−1 was as follows: phosphorus high to very high (68.6–110.0), potassium medium to very high (129.0–149.4) and magnesium high (50.0-56.0). The soil was analysed at the Chemical and Agricultural Station in Wesoła, near Warsaw, in Poland. The soil analysis are in accordance with the tables of Chemical and Agricultural Station in Poland.
The factors of the experiment
The field experiment was set up by split-plot design in triplicate as a two-factor experiment. The factors of the experiment were:
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I
three edible potato cultivars—Bartek, Gawin, Honorata (Table 1).
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II
five potato treatment objects with herbicides and biostimulants (Table 2):
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1
control object—without chemical protection,
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2
herbicide Harrier 295 ZC (linuron and clomazone)—single spraying, 1.5 l ha−1, BBCH 00-08,
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3
herbicide Harrier 295 ZC (linuron and clomazone)—single spraying 1,5 l ha−1, BBCH 00-08 + biostimulant Kelpak SL—double spraying, 1.0 l ha−1, BBCH 13–19 and 0.5 l ha−1, BBCH 31–35,
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4
herbicide Sencor 70 WG (metribuzin)—single spraying, 1.5 l ha−1, BBCH 00-08,
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5
– herbicide Sencor 70 WG (metribuzin)—single spraying 1.5 l ha−1, BBCH 00-08 + biostimulant Asahi SL—double spraying, 1.0 l ha−1, BBCH 13–19 and 0.5 l ha−1, BBCH 31–35.
The timing of the treatments, as well as tuber planting and potato harvesting, is shown in Table 3.
The field experiment included 45 plots. The area of one experimental plot was 18.73 m2 = 5.55 m × 3.375 m, i.e. 15 plants every 37 cm × 5 ridges every 67.5 cm = 75 plants.
Fertilization and chemical protection
In each year of the study, the forecrop was winter wheat. All plots in the experiment were fertilized with the same dose of manure and mineral fertilizers. In autumn, manure was applied at a rate of 25.0 t ha−1 and phosphorus and potassium fertilizers at rates of—44.0 kg ha−1 P (46% triple superphosphate) and 124.5 kg ha−1 K (60% potassium salt), which were covered with pre-winter plowing. Nitrogen was applied in the spring, before planting the tubers, at a rate of 100 kg ha−1 N (34% ammonium salt). The fertilizer was introduced into the soil at a depth of 15–20 cm. Potato protection against diseases and pests was applied according to the recommendations of the Institute of Plant Protection—National Research Institute18. During the growing seasons, fungicides were used against the potato blight: Ridomil Gold MZ 68 WG (mancozeb and metalaxyl-M) and Altima 500 SC (fluazinam). Colorado potato beetle was inspected by using the insecticides: Apacz 50 WG (clothianidin) and Fastac 100 EC (alpha and cypermethrin).
Determination of weed infestation, non-commercial yield and structure tubers
Analysis of weed infestation during potato vegetation was carried out by the quantitative-weight method twice: before potato row closing (phase BBCH 34-35)19 and before harvest of the potato (phase BBCH 97). The BBCH-scale is a system for a uniform coding of phenologically similar growth stages of all mono—and dicotyledonous plant species. The abbreviation BBCH derivers from Biologische Bundesanstalt, Bundessortenamt and Chemical industry. The scale is used in the European Union countries to make characteristic of the development phases of plants. Weeds were determined in an area of 1.0 m2, defined by a 33.4 × 150 cm (5010 cm2) frame. The frame was randomly thrown at three consecutive locations in the plot diagonally through the ridges. Weed control efficacy was expressed as a percentage of weed number destruction relative to a control plot tended only mechanically, according to the methodology of Badowski19. Each year, just before tuber harvest, 10 plants from each plot were randomly dug up. The number and weight of tubers with diameters < 35, 36–50, 51–60 and > 60 mm were determined. Non-commercial and commercial yield was determined. The weight of tubers of the < 35 mm diameter fraction and the weight of tubers with external and internal defects present in the other sample fractions (greened tubers, tubers damaged by soil pests, severely deformed tubers, disease-infested tubers and severely physiologically cracked tubers) were taken as the non-commercial yield of tubers20.
Statistical analysis
The results of weed evaluation—total number of weeds determined before the rows were short-circuited and before the tuber was harvested, and the non-commercial yield of potato were subjected to analysis of variance. The significance of the sources of variation was tested with the Fischler-Snedecor ‘F’ test, and the evaluation of the significance of differences at a significance level of P ≤ 0.05 between the compared averages was performed using Tukey’s multiple intervals21. The relationship between weed infestation and non-commercial tuber yield was determined using linear correlation coefficients. All calculations were made in Excel 2016 using the authors’ algorithm by using the mathematical model:
$${\text{Yijl }} = {\text{ m}} + {\text{ai}} + {\text{gl}} + {\text{e}}/{1}/{\text{il}} + {\text{bj}} + {\text{abij}} + {\text{e}}/{2}/{\text{ijl}},$$
Indications in the model
Yijl means value of characteristic researched: I means the level of A (cultivars) j means the level of B (cultivars) in the first replication, m means the experimental average, ai means the effect of i-level of A (cultivars), gl means the first replication effect, e/1/il means the random effect of a (cultivars) with replications, bj means the effect of j-level of B (objects), abij means the interaction effect of A (cultivars) and B (objects), e/2/ijl means random error.
Weather conditions
The course of weather conditions in the years of the study differed significantly from the air temperature and precipitation of the 1980–2009 multi-year period (Table 4, Figs. 1 and 2). The Meteorological Station in Zawady is located about 8.0 km from the experimental field.
Air temperature during the vegetative growth periods of potato (Zawady Meteorogical Station in Poland).

Precipitation during the vegetative growth periods of potato (Zawady Meteorogical Station in Poland).
In 2012, precipitation was lower than in the multi-year period, and temperatures were higher; it was a dry year (K = 0.95). The months of July and August, which determine tuber formation and yield accumulation, were very dry (K = 0.70) and dry (K = 0.90). The year 2013 was warmer and more abundant in precipitation (K = 1.60—optimal) than in the perennial period, which was favorable for potato yield, and the non-commercial yield was the lowest. In 2014, precipitation was unevenly distributed, July was extremely dry (K = 0.16), and the growing season was relatively dry (K=1.20).
Ethical approval
All methods of experimental research and field studies on cultivated plants, including the collection of plant material were carried out with the relevant institutional, national guidelines and legislation.