Determining Effects of Multiple Tannin Manure Applications on Dairy Forages and Soil.

Dietary choices for dairy cows have direct implications to nutrient availability from land-applied manure because of alterations to manure chemistry. Tannin additions to a dairy cow’s diet protect feed protein through rumen fermentation and digestion, resulting in reduced concentrations of urea-N in urine and higher N concentrations in feces. The objectives of this study were to determine the impact of repeated tannin manure application on forage production, quality and available N in soil. Tannin manures were obtained from Holstein cows during an experimental, 90-d feeding trial.  Cows were fed diets containing 0, 0.4, or 1.8% tannin of total dry matter intake. Manure was applied to St. Charles silt loam soil contained in greenhouse pots.  Manure from all tannin diets were studied, each at two N application rates (240 kg ha^-1 and 360 kg ha^-1). Manures treatments were applied at three frequencies, which corresponded to three crop cycles over a six month period: first cycle to winter wheat, followed by a second cycle of sorghum, followed by a third cycle of sorghum ratoon. Following the first manure application, there were no significant differences in treatment effects on wheat yields. Nitrogen concentrations in wheat shoots was higher (P= 0.04) in 0.4% tannin, low manure N treatment than the control (no manure) treatment.  After the second manure application, sorghum aboveground yield were higher in the manure treatments than the non-manure (control) treatments. After the third manure application, aboveground sorghum ratoon yield was higher (P= 0.002) in pots that received the high manure N rate than in pots that received the low manure N rate. Sorghum ratoon yield and N concentrations in the 0.4% tannin treatments were higher (P= 0.08 and P=0.06, respectively) than the 0% tannin treatment.