Animal Feed Science and Technology 109 (2003) 19–33 Digestion and passage kinetics of fibre in mature dairy heifers maintained on poor quality hay as affected by the source and level of nitrogen supplementation P.S. Mlay a , A.E. Pereka a , M.R. Weisbjerg b , T. Hvelplund b,∗ , J. Madsen c a Department of Physiology, Biochemistry, Pharmacology and Toxicology, Sokoine University of Agriculture, Morogoro, Tanzania b Department of Animal Nutrition and Physiology, Danish Institute of Agricultural Sciences, Research Centre Foulum, 8830 Tjele, Denmark c Department of Animal Science and Animal Health, The Royal Veterinary and Agricultural University, Grønnegårdsvej 2, 1870 Frederiksberg, Denmark Received 18 September 2002; received in revised form 18 June 2003; accepted 18 June 2003 Abstract The effect of nitrogen (N) supplementation of poor quality hay on dry matter (DM) intake and digestibility, rumen fermentation and plant fibre kinetics was investigated using five ruminally fistulated mature heifers (1/2 Boran × 1/2 Friesian) with an average body weight of 210 kg. The experimental set-up was a 5 × 5 Latin square with five treatments and five periods of 28 days each. In each period, 14 days were for adaptation to treatments, 7 days for in vivo digestibility trial and rumen fluid sampling, and the last week was used for three rumen evacuations. The treatments were: poor quality hay only (HO) which was the control, hay with low urea (32 g per day) (HLU), hay with high urea (64 g per day) (HHU), hay with low soybean cake (210 g DM per day) (HLS) and hay with high (420g DM per day) soybean cake (HHS). At a given level of supplementation, urea and soybean cake were iso-nitrogenous, and hay was provided ad libitum. The intake varied from 5.7 to 6.8 kg DM between treatments. N supplementation gave significant increases in DM and organic matter (OM) intake and di- gestibility. Mean daily ruminal fluid ammonia (mg N/100 ml) was also significantly increased by Abbreviations: CHO, carbohydrates; CP, crude protein; DM, dry matter; DMI, dry matter intake; DNDF, digestible neutral detergent fibre; EE, ether extract; INDF, indigestible neutral detergent fibre; IVOMD, in vitro organic matter digestibility; MRT, mean retention time; N, nitrogen; NDF, neutral detergent fibre; NPN, non-protein nitrogen; OM, organic matter; VFA, volatile fatty acids; VFI, voluntary feed intake ∗ Corresponding author. Tel.: +45-8-999-1180; fax: +45-8-999-1378. E-mail address: torben.hvelplund@agrsci.dk (T. Hvelplund). 0377-8401/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0377-8401(03)00211-6