In the switchgrass crops, annual grass weed species were the most abundant, with a share of 17.2 % to 30.4 % in the total weed structure. Among the dicotyledonous weeds, the dominant were Amaranthus retroflexus (11.1−14.5 %), Chenopodium album (8.9−10.7), Galinsoga parviflora (9.4−12.5), Thlaspi avrense (9.9−11.3), Polygonum lapathifolium (3.6-9.0) and Sinapis avrensis (9.3−11.1 %).
The biological efficacy of the herbicide Master Power against the dicotyledonous weed complex at the application rate of 1.5 L/ha was the highest in the experiment (88.9 %). In addition, it destroyed 94.4 % of the weed shoots. On average, the efficacy of the formulation was 90.3 %. Biological efficacy of herbicides Dialen Super (0.5 L/ha), Prima (0.4 L/ha) and Esteron 60 (0.7 L/ha) against the complex of dicotyledonous species was approximately at the same level, from 83.6 to 86.9 %.
The peculiarities of biomass accumulation by both switchgrass and weed plants were directly influenced by the crop density. In particular, at a plant density of 50 plants/m2, switchgrass formed 23 g/m2 of fresh biomass, at 100 plants/m2 – 34.0 g/m2, 150 plants/m2 – 58.0 g/m2, and at the 200 plants/m2 (the highest in the experiment) – 73.0 g/m2. In the same treatments of the experiment, weeds formed 2288.8, 1088.7, 780.1 and 569.8 g/m2 of biomass, respectively. Thus, despite a significant increase in the competitiveness of cultivated plants, the complete dominance in the first year of vegetation was not observed even in the treatment with maximum plant density, where weeds formed almost eight times more biomass than switchgrass.
The highest amount of the mineral nutrients was removed from soil at the switchgrass plant density of 50 plants/m2, including 95 kg/ha of nitrogen, 57.7 kg of phosphorus, and 109 kg/ha of potassium. With the increase in the plant density to 100, 150 and 200 plants/m2, the nutrient removal by weeds significantly decreased: nitrogen up to 45, 32 and 24 kg/ha; phosphorus up to 27.8, 19.9 and 14.4 kg/ha; and potassium up to 51.8, 37.1 and 27.2 kg/ha, respectively.
More favourable for the growth and development of switchgrass were the treatments with the initial plant density 150 and 200 plants/m2, where in the second year of vegetation the yield of biomass reached 13.0 and 18.6 t/ha, yield of solid biofuel 7.2 and 10.3 t/ha, and energy yield 114.6 and 165.3 GJ/ha, respectively.
For the growth and development of switchgrass, the variants with the initial density of plants of 150 and 200 plants/m2, where the yield of crude mass at the level of 13.0 and 18.6 t/ha were obtained, the yield of solid biofuel was 7.2 and 10.3 t/ha, and the energy yield is 114.6 and 165.3 GJ/ha, respectively.
If in the first year of vegetation dicotyledonous weeds are predominant, the use of Prima herbicide is advisable, which provides high rates of economic efficiency of growing switchgrass (production costs of 1984 UAH/t at the profitability of 41.1 %), however, with mixed weed types (dicotyledonous + grass weed species), only Master Power herbicide will be efficient. In this treatment of the experiment, the cost of one ton of production was the lowest in the experiment (1952 UAH/ha) at the highest profitability in the experiment (43.4 %).
Keywords: switchgrass; weeds; seed productivity; weed structure; herbicides; efficiency; cost-efficiency.
