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

Abstract

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 pas- ture. The addition of a protein supplement to cows grazing the low N fertilised sward was also evalu- ated. Three treatments were compared over three periods of 2 weeks using 9 fistulated cows in a 3 × 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 herb- age 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 N and faecal N output from N intake. Digestibility (0.79), daily in- take (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 fertilisa- tion 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 con- tent that still remained higher than 160 g·kg -1 DM. On the LN sward, N intake was significantly lower (- 80 g·d -1 )( P < 0.01), faecal N and milk N output remained unchanged, whereas urine N output de- creased (- 77 g·d -1 )( P < 0.01). Protein supplementation did not depress HOMI or grazing time, sup- plemented 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.