Relationships among planting time, chemical weed control and weed cover in onion (Allium cepa L.)

With the aim to study the influence of planting time and chemical weed control on weed flora development and on yield and crop growth in onion, research was carried out in 1994 and 1995 in Sele Plain (SA) on a clay-loam soil. The trial was planned so as to compare three planting times (March 15(th), April 21(st), May 29(th) in 1994; April 6(th), May 9(th), June 5(th) in 1995) in factorial combination with two treatments concerning weed control (pre-transplanting chemical application with chlorthal-dimethyl + propachlor and no weed control). A split-plot design, with three replications, was arranged. Compared with the highest yield attained with the first planting time (36.8 t.ha(-1) as an average), reductions of 28.6% and 57.4% were recorded for the second and the third planting time respectively; no weed control caused a mean yield reduction of 39.3 % in comparison with herbicide treatment. Highest crop biomass and LAI values were reached with the first planting time and chemical weed treatment (682.4 g.m(-2) and 4.4 m(2).m(-2), respectively), just before decrease caused by leaves drying till harvest. CGR showed an increasing trend in the first phases of crop cycle (top value of 29.8 g.m(-2).d(-1) with the third planting time and herbicide treatment) and subsequently a decrease. Shannon-Wiener diversity index (H'), calculated on the base of weed species biomass, displayed at harvest a decreasing trend from the first to the third planting time (respectively 1.10 and 0.78) corresponding to herbicide treatment but an opposite trend with no weed control (1.34 with the first planting time and 1.44 with the third one). The greatest mean influence to diversity index was given by Amaranthus retroflexus, Cynodon dactylon, Cyperus esculentus and Portulaca oleracea.