Effect of undegradable intake protein supplementation on urea kinetics and microbial use of recycled urea in steers consuming low-quality forage

We evaluated the effect of undegradable intake protein (UIP) on urea kinetics and microbial incorporation of urea-N in ruminally and duodenally fistulated steers (n4; 319 kg) providedad libitumaccess to grass hay in a 4 × 4 Latin square. Casein was continuously infused abomasally in amounts of 0, 62, 124 and 186 mg N/kg body weight per d to simulate provision of UIP. Periods were 13 d long with 7 d for adaptation and 6 d for collection. Jugular infusion of [15N15N]urea followed by determination of urinary enrichment of [15N15N]urea and [14N15N]urea was used to measure urea kinetics. Forage and N intake increased (quadratic,P < 0·02) with increasing UIP. Urea synthesis was 27·1, 49·9, 82·2 and 85·8 g urea-N/d for 0, 62, 124 and 186 diets, respectively (linear,P < 0·01). The proportion of urea synthesis that entered the gastrointestinal tract was 0·96 for steers receiving no UIP and decreased linearly (P = 0·05) to a low of 0·89 for steers receiving 186. The amount of urea entering the gastrointestinal tract was least for 0 (26·3) and increased (linear,P < 0·01) to 48·7, 77·2 and 76·6 g urea-N/d for 62, 124 and 186 diets, respectively. Microbial incorporation of recycled urea-N increased quadratically (P = 0·04) from 13·9 for 0 to 47·7 g N/d for 124. The proportion of microbial N derived from recycled urea increased (quadratic,P = 0·05) from 0·31 to 0·58 between 0 and 124 and dropped to 0·44 for 186 mg N/kg body weight per d. UIP increased intake of hay and provided a N source for ruminal microbes via urea recycling.