Effects of delay in reapplication of nitrogen fertilizer following cutting silage from a ryegrass sward (research note)

Plots of a 2-year-old sward of Merlinda perennial ryegrass received a routine dressing of 100 kg N ha-1 as compound fertilizer in March 1991, followed by further dressings of 100 kg N ha-1 after cuts 1, 2 and 3 in a simulated four-cut silage system, either the same day as cutting or with a delay of 3, 7, 10 or 14 d. Partial irrigation ensured that fertilizer could be taken up immediately. Annual total dry matter yield, the dry matter yield for the experimental cuts 2-4 and their apparent response to N showed no effect of delays of 0-10 d in reapplication of fertilizer, but a significant (P < 0.01) reduction for 14 d delay. Significantly (P = 0.001) higher apparent recovery of N in the cut herbage was recorded where reapplication of fertilizer had been delayed by 7 or 10 d (93% and 91% respectively) in comparison with either smaller (0 delay, 78%; 3 d delay, 87%) or greater delay (14 d, 86%). It is suggested that lower recoveries at 0 and 3 d result from the sward's reduced NO3 uptake following defoliation and that, while in practical situations the conditions may not be as rigorous as those imposed in this experiment, further experiments to determine the fate of N not recovered in the herbage are warranted, so defining management circumstances in which immediate reapplication of N might not be advisable.