Feeding Tannins to Reduce Nitrogen Losses from Feedlot Cattle Fed High Protein Diets Containing Wheat Distillers Grains: Ruminal Fermentation, Digestibility, and Route of Nitrogen Excretion

Four ruminally cannulated and four ruminally and duodenally cannulated beef heifers (476 ± 25 kg, initial BW) were used in a replicated 4 × 4 Latin square to determine the effects of feeding condensed and hydrolysable tannin extracts with a high protein diet containing wheat distillers grains on ruminal fermentation, digestibility, and route of nitrogen (N) excretion. Periods were 4 wk and included a 1-wk washout, 2 wk for adaptation to the tannin extracts, and 1 wk for measurements. Dietary treatments were control (CON, basal diet), 2.5% Acacia mearnsii (ACA), 2.5% quebracho (QUE), and 2.5% chestnut (CHE; DM basis). The basal diet was composed of 71% barley grain concentrate, 20% wheat distillers grains and solubles, and 9% barley silage and contained 16.8% CP (DM basis). Diets were fed as total mixed rations once per day for ad libitum intake. Feed offered and refused were measured daily. Total collection of urine and feces were made for 6 d. Plasma was collected at 3 h and rumen fluid was collected at 0, 3, 6, 9, 12, 15, and 18 h after feeding at the end of the collection period. Data were analyzed using a mixed linear model with square, period, and treatment as fixed effects and animal within square as a random effect. Dry matter intake tended (P = 0.10) to be lower when heifers were fed the CHE compared to the CON and QUE, and was intermediate for the ACA. Total tract digestibility of DM was similar among heifers fed the tannin extracts (78.1, 77.4, and 77.6% for ACA, QUE, and CHE, respectively), however, all tannin extracts reduced DM digestibility compared to the CON (80.8%, SEM 0.98%; P < 0.05). Plasma urea-N and ruminal NH₃-N were also similar among heifers fed the tannin extracts and were reduced (P < 0.05) compared to the CON. There was no effect of the dietary treatments on total N output (P > 0.05), but fecal N output was increased (P < 0.05) and conversely urinary N output was decreased (P < 0.05) for heifers fed ACA and QUE compared to the CON. Fecal and urinary N outputs were intermediate for the CHE. Feeding tannins to beef cattle fed high protein diets containing wheat distillers grains reduced ruminal NH₃-N and plasma urea-N and shifted the route of N excretion from labile forms in urine to feces although there was a 4% decrease in DM digestibility.