Competition between winter wheat and Veronica hederifolia: Influence of weed density and the amount and timing of nitrogen application

The effects of Veronica hederifolia L. densities on the yield of winter wheat (Triticum aestivum L.) were studied in field trials conducted at the same location in 1992 and 1993. In 1992, nitrogen at 60 kg ha(-1) was applied at tillering followed by a further 80 kg ha(-1) at the first node of stem elongation. In 1993, four regimes of nitrogen applications were compared: a total of 140 kg ha(-1) supplied at three dates, 60 kg ha(-1) supplied at tillering, 60 kg ha(-1) supplied at the first node of stem elongation and no nitrogen as the control. Competitive effects from V. hederifolia (ranging from 17 to 192 plants m(-2)), were greatest in 1993, the year with the best early growth development of this weed. In 1993, yield losses, as described with a non-linear model, changed for each regime of fertilization. In both years, V. hederifolia decreased wheat ear number per unit area for each nitrogen treatment. This is explained by an increase in tiller mortality and a nitrogen deficiency in wheat at the stem elongation and flowering stages. Nevertheless, with late application of nitrogen, individual grain weight increased and the effect of V. hederifolia on wheat yield loss was lowest. The results are discussed in relation to the effects on all yield components, and show the importance of choosing the right nitrogen fertilization for specific yield targets of wheat production when infested by weeds.