The results of 2003–2006 research work showed that with the increase of weed number, their biological mass increased and bulb onion yield decreased. With annual broad-leaved weeds’ presence at the amount of 6–12 pcs/m2 onion yield decreased by 8.5–27.7%, at 18–35 pcs/m2 – 18.2–33.2% and under conditions of natural weed infestation onion yield losses were 55.2–81.6%. Coefficient of weed plant harmfulness depicting yield losses of onion, increased per one weed/m2, and amounted to 0.17–0.24%. A biological threshold of weed plant harmfulness in bulb onion crops was determined which made up 5.9–9.2 pcs/m2, that was a harmful effect of increased weeds.
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