Nutrient Leaching Under Grazing Systems with Grass Monocultures and Grass-Legume Mixtures.

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. Losses due to nitrate leaching can affect our water quality.  Grass monocultures require the addition of nitrogen fertilizer.  Grass-legume mixtures can eliminate the need for nitrogen fertilizer.  Tannins present in some legumes 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 examined the effect of legumes with and without condensed tannins (CT) on nitrogen cycling in a grazing system. This research compared the following treatments:  1) tall fescue-alfalfa (TF+ALF, Festuca arundinacea Schreb, Medicago sativa L.); 2) tall fescue-birdsfoot trefoil (TF+BFT, Lotus corniculatus L.) mixtures; 3) tall fescue with nitrogen fertilizer (TF+N); and, 4) tall fescue without nitrogen fertilizer (TF-N). Plots were grazed using a rotational grazing system, with livestock being moved each week.  Leachate was collected bi-weekly throughout the grazing season and analyzed for nitrate concentration.  Soil samples were collected at the beginning and end of each grazing season to a depth of 150 cm and analyzed for inorganic nitrogen content.  Results will be presented.