The effects of synchronizing the rate of dietary energy and nitrogen supply to the rumen on the production and metabolism of sheep: food characterization and growth and metabolism of ewe lambs given food ad libitum

Abstract

Abstract The effects of diets formulated to have two rates of organic matter (OM) release and to be either synchronous or asynchronous with respect to their hourly release of nitrogen (N) and OM in the rumen on the growth, intake and metabolism of ewe lambs was studied. In experiment 1, the in situ degradation characteristics of N and OM for 16 food ingredients was determined. The foods varied considerably in their chemical composition and degradability coefficients of N and OM. Based on this information, four diets were formulated to differ in their rate of N and OM release in the rumen but to have a similar content of metabolizable energy (10·4 MJ/kg dry matter (DM)), crude protein (140 g/kg DM), daily ratio of N: OM released (34 g N per kg OM) and digestible undegradable protein (32 g/kg DM) but to be synchronous or asynchronous with respect to hourly N: OM ratio. The four diets were slow energy, synchronous (SS), slow energy asynchronous (SA), fast energy, synchronous (FS) and fast energy asynchronous (FA). In experiment 2 the four diets were offered ad libitum to 24 ewe lambs of an initial live weight of 25 kg in a 2 × 2 factorial design. Lambs were slaughtered at 40 kg live weight. Animals offered diets FS and FA had a higher growth rate than those offered diets SA and SS (266 v. 225 g/day respectively; P < 0·05). There was a significant interaction between rate ofOM and N release on DM intake (DMI) with lambs offered diet FS consuming less than those offered FA (1·47 v. 1·67 kg DM per day; P < 0·05). Rate of energy release and synchrony had an effect on food conversion efficiency (FCE; kg gain per kg DMI) with lambs offered diets FS and FA having a greater FCE than those offered SS and SA (0·170 v. 0·146 respectively; P < 0·001) and those offered diets SS and FS had a greater FCE than those offered diets SA and FA (0·164 v. 0·152 respectively; P < 0·05). Diet had little effect on carcass composition. The proportion of propionate in rumen fluid was greater in lambs offered diets FS and FA than in those offered diets SS and SA (222 v. 168 mmol/mol respectively; P < 0·01). Plasma urea concentrations exhibited a cyclical trend throughout the day with highest concentrations occurring 2 h after fresh food was offered. Lambs offered diet FS had lower plasma urea concentrations at 6, 10 and 14 h after feeding. Plasma concentrations of ß-hydroxybutyrate were lower throughout the day in lambs offered diet FS. The current findings are consistent with the view that a rapid release of OM and synchronizing hourly N: OM release in the rumen can improve the efficiency of growth through improvements in protein and/or energy metabolism.