Dietary Tannin and Crude Protein Impact Dairy N Excretion and Ammonia Emissions From Barns Floors and Manure-Amended Soils.

Feeding tannin extract and less crude protein (CP) to dairy cows may have synergistic impacts on reducing urinary N excretion and NH₃ emissions from dairy barns and land applied manure. Holstein dairy cows were fed four levels (g kg^-1) of dietary tannin extract (mixture from red quebracho and chestnut trees): 0 (0T), 4.5 (LT), 9.0 (MT) and 18.0 (HT); each fed at two levels (g kg^-1) of dietary CP: 155 (LCP) and 168 (HCP). The addition of tannin extracts to the diets did not significantly impact animal performance but increased feed N use efficiency and decreased N excretion in urine. Reductions in NH₃ emission from barn floors due to tannin feeding were greatest when tannin was fed at LCP: the LCP-LT and LCP-HT treatments emitted 30.6% less NH₃ than LCP-0T; and the HCP-LT and HCP-HT treatments emitted 16.3% less NH₃ than HCP-0T.  Feeding tannin extract decreased urease activity in feces resulting in 11.5% reduction in NH₃ loss. The application of tannin directly to simulated barn floor also reduced NH₃ emissions by 19.0%.  Tannin did not impact NH₃ emissions from soils. But emissions from the HCP slurry were 1.53 to 2.57 times greater than from the LCP slurry.  At trial’s end concentrations of soil inorganic N were greater in HCP slurry-amended soils than in LCP slurry-amended soils.  Emissions from the sandy loam soil were 1.07 to 1.15 times greater than from silt loam soil, a result which decreased soil inorganic N in the sandy loam compared to the silt loam soil. Larger scale and longer term field trials are needed to ascertain the effectiveness of feeding tannin extracts to dairy cows in abating NH₃ loss from barns and land-applied slurry, and the impact of tannin-containing slurry on soil N cycles.