Journal of Agricultural Science, Cambridge (1993), 120, 219-224. © 1993 Cambridge University Press 219 The influence of spring nitrogen on yield, yield components and glucosinolate content of autumn-sown oilseed rape (Brassica napus) P. E. BILSBORROW, E. J. EVANS AND F. J. ZHAO Department of Agriculture, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK (Revised MS received II August 1992) SUMMARY The response of low glucosinolate, low erucic acid (double low) cultivars of oilseed rape to spring nitrogen application was examined at Cockle Park, Northumberland in three successive seasons (1987-90). Seed yield and protein content increased in response to increasing nitrogen application. In each season over 85% of the maximum recorded yield was obtained with an application of 150 kg N/ha. In the absence of spring nitrogen, seed yield varied considerably over the three seasons. The variation in seed yield and protein content between 1987/88 and 1988/89 can partly be explained by differences in the availability of soil nitrogen in the different seasons. Seed glucosinolate concentration was significantly increased with nitrogen applications between 0 and 150 kg/ha. Increasing nitrogen beyond 150 kg/ha had little or no effect on seed glucosinolate concentration. The results are examined with reference to the varying effects of increasing nitrogen application rates on seed glucosinolate concentrations reported previously. INTRODUCTION Autumn-sown oilseed rape (Brassica napus L.) responds well to spring applications of nitrogen with an optimum rate for seed yield of between 240 and 290 kg/ha (Chalmers 1989). Date of N application between late February and early April does not appear to have a critical effect on seed yield (Harris 1980; Archer & Vaidyanathan 1982). In pot and field experiments, the effects of N on seed glucosinolate content have not been consistent. Josefsson (1970) found that in soil-free culture, the glucosinolate content of rapeseed meal was lower at high levels of N. Similar effects have been observed in field experiments with spring rape (Wetter et al. 1970) although the decrease in glucosinolate content was generally small. In contrast, Forster (1978) found that for single low cultivars grown in pot trials, nitrogen increased seed glucosinolate levels. Similarly Chalmers (1989) observed small, but significant, increases in seed glucosinolate content with increasing rates of spring N on three sites cropped with the low glucosinolate cultivar, Ariana. Critical levels of photosynthate supplies have been identified during different stages of development which strongly influence the formation of both potential and actual yield of oilseed rape (Bilsborrow & Norton 1984). Changing the quantity of N fertilizer applied is a means whereby assimilate supply can be influenced. Amino acids are precursors of glucos- inolate biosynthesis (Josefsson 1970; Underhill et al. 1973; Underhill 1980) which suggests that alterations in amino acid supply induced by N supply may be expected to influence seed glucosinolate content. The aim of this study was to examine the effects of N rale on yield, yield components and glucosinolate content of low glucosinolate, low erucic acid cultivars of autumn-sown oilseed rape. MATERIALS AND METHODS In the 1987/88 and 1988/89 seasons, the effects of five levels of spring N (0, 75, 150, 210 and 280 kg/ha), applied as ammonium nitrate, were compared in a randomized block design replicated five and four times respectively (Table 1). The cultivar Ariana was grown in both seasons. Nitrogen was applied as a split-dressing; one-third at the start of spring growth (4 March 1988 and 28 February 1989) and the remaining two-thirds at the start of stem extension (7 April 1988 and 20 March 1989). In 1989/90, six rates of N (0, 50, 100, 150, 200 and 250 kg/ha) were replicated four times in a randomized block design using the cultivar Cobra. Nitrogen application was split, with 50 kg/ha applied on 16 March and the remainder on 27 March. All trials were desiccated