Increasing Stocking Rate Affects Fecal Deposition and N Return On Signal Grass Pastures.

Nutrient return in pastures occurs either via litter deposition or animal excreta and grazing management may affect these pathways. Monitoring this return is necessary to model nutrient flow in grazed ecosystems. Nutrient losses are expected to increase when more nutrients return via excreta; also, nutrient is more evenly distributed on pasture surface when returned via litter. This experiment aimed to evaluate the effect of three stocking rates [2, 4, and 6 Animal Units (AU)/ha; 1 AU = 450 kg live weight] on fecal deposition, fecal N concentration, and N return via feces on rotationally stocked signal grass (Brachiaria decumbens Stapf.) pastures. Grazing period was three days. Resting period was 32 d and 72 d, on rainy and dry season, respectively. Fecal deposition and chemical composition were determined in eight evaluations. Two cross bred Holstein/zebu cows were used and daily fecal output per animal was estimated by giving a feed marker (purified and enriched lignin - LIPE^® ; 500 mg/d). Feces were collected (~200 g/an.) at the second and third day of grazing. Composite samples of the two cows from the same treatment were analyzed for modified lignin and N. Fecal output was affected by the interaction between stocking rate and evaluation period, with average values of 8.3 g DM of feces/kg of animal live weight; average fecal output per animal was 4.3 kg DM of feces. Fecal N concentration was also affected by a stocking rate vs. evaluation interaction, with average of 20 g N/kg of feces (DM basis). In general, these interactions occurred because fecal output or chemical composition variation caused by stocking rate was not consistent across evaluation periods. Increasing stocking rate increased linearly (P<0.0001) N daily return via feces (172, 335, and 533 g N/ha for 2, 4, and 6 AU/ha, respectively). In general, a more pronounced effect of stocking rate and evaluation period was observed for fecal deposition and N return per area than per animal. This likely occurred because herbage mass was not limiting animal intake even at the highest stocking rate.