Effect of nitrogen fertiliser rate and protein supplementation on the herbage intake and the nitrogen balance of grazing dairy cows

The objective of this study was to examine the effect of the level of reducing nitrogen (N) fertiliser rate on herbage intake and N balance of dairy cows grazing a pure perennial ryegrass pasture. The addition of a protein supplement to cows grazing the low N fertilised sward was also evaluated. Three treatments were compared over three periods of 2 weeks using 9 fistulated cows in a 3 x 3 Latin square design: HN ( 80 kg N.ha(-1).regrowth(-1)), LN (0-20 kg N.ha(-1).regrowth(-1)), LN+S (LN + 2 kg of soybean meal (SBM)). Daily herbage organic matter ( OM) intake was estimated by chromic oxide dilution in the faeces. Nitrogen and ADF contents in faecal OM were used to estimate the herbage digestibility. Herbage mass, grazing behaviour and rumen fermentation pattern were measured. Nitrogen intake was estimated by the chemical composition of the defoliated herbage and urinary N was calculated by subtracting milk Nand faecal N output from N intake. Digestibility (0.79), daily intake (16.4 kg OM), grazing time (512 min) and the proportion of volatile fatty acid were not affected by reducing the N fertiliser rate. These results may be explained by the moderate effect of N fertilisation on herbage mass which remained high in the LN swards (3.9 vs. 4.7 t OM.ha(-1) for HN) and the rather large herbage allowance which allowed the cows to graze a herbage with a crude protein content that still remained higher than 160 g.kg(-1) DM. On the LN sward, Nintake was significantly lower ( 80 g.d(-1)) (P < 0.01), faecal N and milk N output remained unchanged, whereas urine N output decreased (77 g.d(-1)) (P < 0.01). Protein supplementation did not depress HOMI or grazing time, supplemented cows consumed 2.4 kg OM more (P < 0.01) and this increased milk yield by 1.3 kg.kg(-1) SBM (P < 0.01). SBM supplementation largely increased N intake, and finally N excreted in the urine. It was concluded that N fertilisation, and N supplementation are efficient means to manipulate animal performances and N balance in grazing dairy cows.