Effect of high rates of nitrogen fertiliser on perennial ryegrass growth and morphology in grazed dairy pasture in northern New Zealand

Nitrogen (N) fertiliser was applied to dairy pastures at rates of 0,200, or 400 kg N/ha per year on farmlets stocked at 3.24 (LS) or 4.53 (HS) Friesian cows/ha from June 1993 to June 1995. Annual net herbage accumulation increased 22 and 23% on farmlets LS200 and HS200 respectively, and 37 and 24% on farmlets LS400 and HS400. Most of the extra herbage was perennial ryegrass. Ryegrass contents (% total dry matter (DM)) were 71 and 76% on farmlets LS200 and LS400 respectively compared with 61% on farmlet LS0. Ryegrass tiller densities were also higher on farmlets LS200 and LS400 (6295 and 6673 tillers/ m2 respectively) than on farmlet LS0 (4072 tillers/ m2). On all farmlets there was a post‐flowering (January‐February) increase in tiller density although this increase was enhanced by N. N application also increased tiller number/plant (3.37, 4.10, and 4.26 tillers/plant on farmlets LS0, LS200, and LS400 respectively). The greater leaf number and leaf dry weight (DW)/plant on farmlets LS200 and LS400 was connected to the increased tiller number/plant, although DW/leaf was also significantly (P < 0.05) greater under N. There was no N effect on leaf sheath or basal stem, but DW/ tiller was significantly (P < 0.05) greater. Ryegrass plants formed two types of internodal stolon: I(v) associated with vegetative growth throughout the year; and I(r) associated with reproductive growth and only formed during November—January. N had no effect on I(v) formation but 35 and 34% of plants formed I(r) on farmlets LS200 and LS400 respectively compared with 22% on farmlet LS0. It is suggested the positive effects of N on tiller number and DW/plant together with the post‐flowering increase in tillering may improve persistence of perennial ryegrass in intensively grazed dairy pastures.