Broccoli growth in response to increasing rates of pre-plant nitrogen. I. Yield and quality

Nitrogen management is critical to the production of broccoli (Brassica oleracea L. italica Plenck). Field trials were conducted in 2001 and 2002 to determine the rate of pre-plant nitrogen required to optimize broccoli yield and quality. Seven rates of nitrogen (0, 50, 100, 150, 200, 300, 400 kg N ha-1) as ammonium nitrate were broadcast and incorporated before transplanting two broccoli cultivars, Captain and Decathlon. Maturity of the heads was delayed by 5 d at 0 kg N ha-1 compared with the other rates of applied N. Marketable yield was maximized at 243 to 272 kg N ha-1 for yield expressed in t ha-1 and 171 to 187 kg N ha-1 for yield expressed as cases ha-1. Averaged over cultivar and year the most economical rate of nitrogen (MERN) ranged from 298 to 309 kg ha-1, 50 kg higher than estimates for the maximum marketable yield derived from quadratic plateau models. The incidence of misshapen heads decreased and floret color improved as nitrogen rate increased, but hollow stem and head rot also increased with high rates of nitrogen. Floret NO3--N concentration increased and vitamin C concentration decreased at high nitrogen rates. Applying the rates of nitrogen required to maximize yield may have negative economic and environmental consequences. However, restricting nitrogen also jeopardizes both yield and quality. Hence, the optimum amount of pre-plant nitrogen to apply to broccoli that balances yield, quality, economics and environmental concerns remains a complex issue. Key words: Brassica oleracea L. italica Plenck, color, postharvest, nutrition, hollow stem, vitamin C