Effect of Purple Nutsedge (Cyperus rotundus) Population Densities on Onion (Allium cepa) as Influenced by Nitrogen in the Eastern Province of Sri Lanka

Fertilizer management is an imperative element of integrated weed control that protects crop yields and reduces weed populations over time. This experiment was conducted under field conditions to determine the effect of population density of purple nutsedge (Cyperus rotundus) on the yield of onion (Allium cepa L.) at different N rates. Initial densities of Purple nutsedge were 0, 10, 20 and 40 plants/m2. Nitrogen was applied as 100 kg/ha, 150 kg/ha and 200 kg/ha. Purple nutsedge and onion (variety Vethalan) were grown together for 12 weeks. There was a significant interaction (p<0.05) between Purple nutsedge population densities and N rate on onion bulb yield, total dry weight and bulb diameter. At 100 kg N/ha and 200 kg N/ha, total dry weight of onion was not significantly (p>0.05) affected by purple nutsedge population densities. In contrast, at 150 kg N/ha, effect of weed population density on onion total dry weight was significant (p<0.05). The highest onion yield loss of 49% was observed at this intermediate nitrogen level with 40 weed plants/m2 . As purple nutsedge densities increased, onion yield decreased. Purple nutsedge biomass, shoot height and average tuber weight were significantly (p<0.05) reduced by increased N levels. The results indicate that as N increased above 150 kg/ha, the negative effect of the weed on the crop was minimized. Purple nutsedge and onion have similar optimal rate of N fertilizer. Therefore, reducing the effect of purple nutsedge interference on onion is difficult and complex. Key words: Average tuber weight; Bulb yield; Nitrogen rates; Nutsedge biomass;, Nutsedge interference. DOI: http://dx.doi.org/10.4038/tar.v22i4.3785 TAR 2011; 22(4): 348-355