SOME EFFECTS OF APPLYING UNDILUTED SILAGE EFFLUENT TO GRASSLAND

Two small-plot experiments were carried out to assess the influence on herbage dry matter (DM) production, chemical composition and soil fer tility status of applying undiluted silage effluent at a range of appl ication rates and intervals after a silage, cut. In the first experime nt, in 1990, silage effluent was applied at 25, 50, 100, 150 and 200 m (3) ha(-1) 1, 8, 15 and 22 d after a silage cut in August. In the seco nd experiment, in 1991, silage effluent was applied at 7, 14, 21, 28, 35, 42, 49, 75, 100, 125 and 150 m(3) ha(-1) 1, 4, 8, 15, 22 and 29 d after silage cuts were taken from different sites in May, July and Aug ust. An untreated control and an inorganic fertilizer treatment were i ncorporated in both experiments. The immediate effects of the treatmen ts on herbage yield, chemical composition and soil nutrient status wer e assessed 6-8 weeks after the initial application; residual effects o n herbage and soil fertility were measured at a subsequent harvest. Co mpared with the untreated control, herbage yield increases were obtain ed with increasing rates of effluent application. Although there was e vidence that higher yields could be obtained from earlier applications , up to 50 m(3) ha(-1) of effluent could be applied up to 15 d after t aking a silage cut with little damage to the su ard. Delaying the timi ng of application, and increasing the application rate, increased the proportion of the sward which was damaged; this reached a maximum of 0 .84 when the highest application rates were applied 29 d after a silag e cut. The increase in the proportion of dead herbage in the sward, as sociated with increasing rate of effluent application, reduced the qua lity of the herbage harvested in Experiment 1. In Experiment 2 the N, P and, in particular, the K content of the herbage increased with incr easing rate of effluent application, whereas the effect on Mg content was variable with contents generally being less than 2.0 g kg(-1) DM. Apparent recovery of nutrients applied in the effluent was both low an d variable ranging from 0.58 to -0.03 for N, 0.10 to -0.005 for P, 0.3 4 to -0.02 for K and 0.21 to -0.002 for Mg over both experiments. Effl uent had little effect on soil pH, whereas P and, in particular, K con tents increased with increasing rate of effluent application. There wa s evidence that effluent had a beneficial effect on both herbage yield and chemical composition at the residual cut, the extent depending up on rate and time of effluent application.