The effect of nitrogen fertilization level and protein supplementation on herbage intake, feeding behaviour and digestion in grazing dairy cows

An experiment is described to study the influence of nitrogen (N) fertilization and protein supplementation on the nutrition of grazing dairy cows. Two levels of N fertilization (0 and 60 kg N per ha per regrowth) and two levels of soyabean meal supplementation (0 and 2 kg day−1) were factorially combined and compared in a 4 × 4 Latin square design using periods of 11 days. Eight fistulated Holstein cows were strip grazed on perennial ryegrass pastures at a constant daily herbage allowance — measured above 5 cm from ground level — of 20 kg OM cow−1. The individual herbage OM intake was calculated using chromic oxide and faecal N and ADF contents. Grazing behaviour, duodenal digesta flow and ruminal fermentation patterns were also measured. eraction was found between fertilization and supplementation levels. Herbage OM intake was greatly depressed in unfertilized swards (−2.0 kg OM day−1). This could be attributed to a decrease of the rate of intake due to the lower green leaf mass (1.6 vs. 2.3 t OM ha−1). Herbage OM digestibility, non-ammoniacal N (NAN) flow into the duodenum and ruminal fermentations were reduced on unfertilized swards. However, the sites of OM digestion and NAN flow expressed g per kg digestible OM intake were similar at both fertilization levels. Cows receiving protein supplementation showed similar herbage intake, but higher total OM intake and NAN flow than unsupplemented cows. The low level of concentrate fed, and perhaps also its composition, may explain the additivity between herbage and concentrate. Treatments had no effect on total grazing time nor on mean rate of biting.