Split N application on rapeseed (Brassica napus L. sp oleifera forma annua)). Effects on seed yield and seed quality

The higher nitrogen demand by rapeseed crop needs an adequate supply to maintain a sustainable production, which is highly dependent on the moment of N availability for the crop. The effect of splitting N application was evaluated during two years 1994, and 1995 in a field experiment in La Plata on two spring rapeseed cultivars (Iciola 41 and Global). Fertilisation treatments were: N0: (control) and the application of 90Kg de N.ha-1 (as urea) in N1: all at sowing, N2: half at sowing and half at the end of rosette stage and N3: 1/3 at sowing, 1/3 at the end of rosette stage and 1/3 at flowering onset. Biomass production, seed yield and its components were evaluated at ripeness and seed N and oil seed content were also measured. During 1995 biomass production and N accumulation at different phenological stages of crop development were also evaluated. Leaf area index was evaluated during different phenological stages on Global cultivar. No interactions were found between cultivars and fertilisation treatments for seed yield and its components. In 1994, N application, mainly as split dose, increased seed yield and biomass production. Significative differences were found between treatments N0 vs. N2 and N3 (94, 196 y 212 g.m-2 respectively). The higher seed production in this year was related to more number of seeds and siliques.m-2 but not to a higher seed weight. Seed N and oil content were influenced mainly by year. A significative fertilisation x cultivar interaction was observed. During 1995 no significative differences were observed between fertiliser treatments for all the variables evaluated probably due to a severe water deficit observed during the most sensitive crop period. Our results suggest that fertilisation response is highly dependent on year characteristics, specially on water availability during critic stages of crop development. When weather conditions are favourables for N response, application as split form results a more effective way to improve N agronomic efficiency for seed yield due to its effect on those components associated to seed number formation.

Key words: fertilisation, apparent N recovery, agronomic N use efficiency, nitrogen nutrition index (NNI)