Impact of Tannins on Nitrogen Cycling in Urine and Feces in a Grazing System.

The fate of nutrients in animal excreta is becoming in issue of increasing concern – even in grazing systems.  Although plant growth can utilize some of the nitrogen deposited in a pasture, urine spots often contain the equivalent of 1,000 kg ha-1, which far exceeds any plant requirements. Gaseous emissions as ammonia and nitrous oxide, and losses due to leaching (nitrate) affect our air and water quality.  Tannins have the potential to improve nitrogen utilization and shift nitrogen from excretion in the urine to excretion in the feces.  Fecal nitrogen is released slowly and is more likely to be utilized by plant growth, thereby potentially reducing some environmental concerns. This study examines the effect of condensed tannins (CT) on nitrogen cycling in urine and feces of cattle. Grazing heifers were provided tannins – either in the forage, or added to the drinking water.  The following treatments were evaluated:  Tall Fescue/Birdsfoot Trefoil, Tall Fescue + Quebracho Tannin, and Tall Fescue (no tannin).  Heifers with similar weights, age, and body condition were outfitted with a urine and fecal collection system and placed in the appropriate paddock. Water intake was monitored, and fecal samples and urine samples were collected every 12 hours for 72 hours (3 days).  The feces were weighed and the volume of urine determined.  Feces and urine samples were frozen for further analysis. Fecal samples will be analyzed for total N and ammoniacal N.  Urine samples will be analyzed for total N, urea N, and ammoniacal N.