Comparison of old permanent grassland, Lolium perenne and Lolium multiflorum swards grown for silage 3. Effects of varying fertiliser nitrogen application rate

The aim was to identify modifications to the N-fertiliser regime that would optimise the annual productivity of three grass swards managed within inte nsive silage production systems. Replicate main plots within an old permane nt grassland (OPG) sward of diverse botanical composition were unaltered or were reseeded with Lolium perenne (LP) or Lolium multiflorum (LM). In the first and second years after reseeding, plots located within three main plo ts per sward were sub-divided into six sub-plots, and six annual rates of f ertiliser N (0, 230, 330, 430, 530 and 630 kg N / ha) were randomly allocat ed among the latter. There were four harvests each season of OPG and LP and five of LM. Neither sward botanical composition or tiller density (P > 0.0 5) were altered by the rate of fertiliser N applied. In the absence of fert iliser N input, mean annual dry matter (DM) yields were 7.3, 6.2 and 8.7 t/ ha, mean in vitro DM digestibilities were 721, 775 and 786 g/kg, mean crude protein concentrations were 153, 153 and 132 g/kg DM and mean water-solubl e carbohydrate (WSC) concentrations were 19, 29 and 33 g/l for the OPG, LP and LM swards, respectively. Increasing rates of fertiliser N application g enerally increased annual DM yield and crude protein concentration and redu ced WSC concentration. Whereas they did not affect the mean digestibility o f OPG or LP swards, they caused a reduction with the LM sward. Whereas appa rent recovery of applied N in herbage decreased in the OPG sward with incre asing fertiliser N input, this pattern did not occur with the ryegrass rese eds. It was concluded that under the harvesting-interval system used, avera ge annual inputs of approximately 360, 460 and 480 kg N/ha could be appropr iate for OPG, LP and LM swards. Furthermore, ensilability and digestibility characteristics should be better for ryegrass reseeds than the OPG sward.