Peculiarities of weed infestation and yield formation in soybean under different terms of joint vegetation with weeds

Abstract

Purpose. To determine the level of weed harmfulness in soybean sowings under different periods of their joint vegetation.
 Methods. The research was carried out in the field of Agrofirma Kyivska LLC (Kyiv region) in the years 2018–2020. The field is located in the zone of unstable soil moisture of the Right-Bank Forest-Steppe of Ukraine. Ten variants of joint vegetation of soybean with various weeds were studied – from 3 to 30 days after sprouting soybean with an interval of three days. Until a certain time, weeds were removed from crops mechanically. The control variant was a plot clear of the weeds. The species composition, number and parameters of weed biomass accumulation, as well as soybean seed yield were determined.
 Results. During the study period, 13 species of weeds from nine families were identified in soybean crops. In the control variant of the experiment, the crops had a total of 145 plants/m2 of weeds, which totaled 1235 g/m2 of wet biomass. With the emergence of weed seedlings from 3 to 9 days after sprouting soybean, they are able to compete with cultivated plants. Under such conditions, their number was 59–83 plants/m2, and wet biomass 435–607 g/m2. At the same time, in the case of weeds presence in sowings 12 days after soybean germination, there was a significant reduction (75.8%) in their number compared to weed control. However, due to the large number of seedlings of highly competitive species, they are able to accumulate 299 g/m2 of biomass and successfully compete with soybean plants for nutrients. Only the appearance of weed seedlings in crops 18 days after crop emergence shows us the loss of the struggle of wild species for life factors in the soybean sowings their number decreased by 91.3% and the formation of biomass to 107 g/m2. The least intensive development of weeds was noted in the variants of their emergence on the 21st−30th day, where their number against the control decreased by 92.8–98.9%.
 Conclusions. If weeds appear in crops from 3 to 21 days after sprouting soybean, the yield of soybean seeds is from 1.23 to 1.79 t/ha, which ensure the conditions for the inexpediency of growing such crops. The lowest losses from the joint vegetation of soybean plants with weeds are experienced by crops where the emergence of weed seedlings is observed on the 24th day or even later after the soybean sprouting. Under such conditions, at least 2.0 t/ha of soybean seeds can be obtained without additional weed protection measures.