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

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 (420 g 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 digestibility. Mean daily ruminal fluid ammonia (mg N/100 ml) was also significantly increased by urea supplementation. Mean daily ruminal fluid pH was not significantly affected by treatments. Ruminal fluid total volatile fatty acids (mM) increased with N supplementation and were significantly ( P<0.05) higher in HHS compared to the control. Both urea and soybean cake supplementation significantly increased the rate ( P=0.05) and extent ( P=0.01) of digestible NDF (DNDF) degradation compared to the control. In conclusion, N supplementation of poor quality hay greatly improved the intake and digestibility of DM, OM and NDF. The effect of soybean cake or urea on plant fibre digestibility was equivalent, indicating no effect of intact protein compared to NPN on ruminal NDF digestion.