Dietary Crude Protein and Tannin Impact Dairy Manure Chemistry and Ammonia Emissions from Incubated Soils

Excess crude protein (CP) in dairy cow diets is excreted mostly as urea nitrogen (N), which increases ammonia (NH 3 ) emissions from dairy farms and heightens human health and environmental concerns. Feeding less CP and more tannin to dairy cows may enhance feed N use and milk production, abate NH 3 emissions, and conserve the fertilizer N value of manure. Lab‐scale ventilated chambers were used to evaluate the impacts of CP and tannin feeding on slurry chemistry, NH 3 emissions, and soil inorganic N levels after slurry application to a sandy loam soil and a silt loam soil. Slurry from lactating Holstein dairy cows ( Bos taurus ) fed two levels of dietary CP (low CP [LCP], 155 g kg −1 ; high CP [HCP], 168 g kg −1 ) each fed at four levels of dietary tannin extract, a mixture from red quebracho ( Schinopsis lorentzii ) and chestnut ( Castanea sativa ) trees (0 tannin [0T]; low tannin [LT], 4.5 g kg −1 ; medium tannin [MT], 9.0 g kg −1 ; and high tannin [HT], 18.0 g kg −1 ) were applied to soil‐containing lab‐scale chambers, and NH 3 emissions were measured 1, 3, 6, 12, 24, 36, and 48 h after slurry application. Emissions from the HCP slurry were 1.53 to 2.57 times greater ( P < 0.05) than from the LCP slurry. At trial's end (48 h), concentrations of inorganic N in soils were greater ( P < 0.05) in HCP slurry–amended soils than in LCP slurry–amended soils. Emissions from HT slurry were 28 to 49% lower ( P < 0.05) than emissions from 0T slurry, yet these differences did not affect soil inorganic N levels. Emissions from the sandy loam soil were 1.07 to 1.15 times greater ( P < 0.05) than from silt loam soil, a result that decreased soil inorganic N in the sandy loam compared with the silt loam soil. Larger‐scale and longer‐term field trails are needed to ascertain the effectiveness of feeding tannin extracts to dairy cows in abating NH 3 loss from land‐applied slurry and the impact of tannin‐containing slurry on soil N cycles.