Effects of residual nitrogen, nitrogen fertilizer, sowing date and harvest time on yield and nutritive value of forage rape

The main objective was to investigate effects of residual N from the previous maize crop, N fertilizer and sowing dates on yield, nutritive value and gas production of forage rape over three subsequent harvest times. Forage rape (cv. Goliath) was sown at Camden, NSW, Australia either in February (as Early) or March (as Late) in 2 m × 2 m plots where maize had been grown immediately before forage rape with pre- (0, 135 kg N/ha) and post- (0, 79, 158 kg N/ha) sown N fertilizer. All treatments received either 0 or 230 kg N/ha as urea applied in three splits: at sowing, after harvest 1 (H1) and harvest 2 (H2) at 70, 90 and 70 kg N/ha, respectively. On average, for each sowing date, H1, H2 and H3 were harvested 68, 133 and 164 d after sowing, respectively. Representative biomass samples of each treatment were assayed for chemical composition and in vitro digestibility and gas production. Application of N fertilizer increased (P<0.001) dry matter (DM) yield (1.8–3.7 t/ha), plant height (52–71 cm), crude protein (CP; 219–246 g/kg DM) and both neutral detergent fiber (aNDF; 203–220 g/kg DM) and acid detergent fiber (ADF; 175–191 g/kg DM) contents compared to 0 N. Residual N had no effect on DM yield, indicating the need of applying N directly to the forage crop. Sowing date had no effect on DM yield, but Late sowing increased (P<0.001) CP (227–238 g/kg DM), and decreased (P<0.001) aNDF (219–203 g/kg DM), lignin(sa) (26–16 g/kg DM), water soluble carbohydrate (WSC; 129–117 g/kg DM) and nitrate N (9–6 g/kg DM) contents compared to Early sowing. With the progress in harvest time from H1 to H3, aNDF, ADF, WSC and nitrate N contents decreased (P<0.001). Plant height was positively related to DM yield (R2 = 0.76; P<0.001) and ADF content (R2 = 0.43, P<0.01). Gas production correlated (P<0.01–<0.001) negatively and positively, respectively to CP and WSC contents at 3 h of incubation. Results demonstrate that application of N increased DM yield and CP content of forage rape. There was no yield advantage due to the Early or Late sowing, but late sowing increased CP and decreased nitrate N content. Harvesting at H1 and H2 yielded similarly, but nitrate N decreased with the progress of harvest time.