The study was carried out at Wondo Genet to determine the critical weed competition period for growth, yield and yield components of chamomile plant. The weed competition duration had significant influence on plant height, number of primary branches/plants, fresh stem yield/plant, fresh leaves yield/plant, fresh and dry flower/plant, fresh and dry flower yield/ha, essential oil content (v/w) and yield kg/ha. The highest fresh and dried flower yield (3780.10 kg/ha) and (798.32 kg/ha) was recorded from weed-free check followed by weed-free for 60 DACE (Days After Crop Emergence) (3652.10 kg/ha) and (789.43 kg/ha), respectively. The maximum essential oil content and yield was recorded from the weed-free check (0.79v/w) and (6.25 kg/ha) whereas the minimum was in the weedy-check (0.10 v/w) and (0.15kg/ha), respectively. The yield losses of chamomile were estimated based on the fresh and dry flower and essential oil yield per hectare. Thus, the highest fresh and dry flower yield losses were recorded from the weedy-check (83.58%) and (81.05%) while the lowest was from weed-free check (0.00%), respectively. The highest essential oil yield loss was also recorded from the weedy for 60DACE (97.92%) followed by weedy-check (97.60%). To determine the beginning and the end of the critical period of crop-weed competition 5 and 0% acceptable yield loss levels were used. Therefore, to reduce the yield losses of chamomile by more than 10% and higher economic return, plants must be kept weed free within 40 to 60DACE to reduce the risk of economic yield losses as it has been found to be the critical period of weed-crop competition in chamomile plants.

Keywords: Critical period, essential oil content, yield, flower yield, weed species, yield losses