Nitrogen transformations and greenhouse gas emissions during composting of manure from cattle fed diets containing corn dried distillers grains with solubles and condensed tannins

Condensed tannins (CT) in ruminant diets reduce ruminal N degradation, but there is little research on how these phenolic compounds alter N metabolism during manure composting. This study investigated effects of CT additives in cattle diets on N content and greenhouse gas (GHG) emissions from manure composting. This open windrow composting experiment consisted of two replications and three treatments being: (1) CK: manure from cattle fed a diet containing 860g/kg barley (Hordeum vulgare L.) grain, 90g/kg barley silage and 50g/kg supplement on a dry matter (DM) basis, (2) DDGS: diet similar to (1) with 400g/kg corn (Zea mays L.) dried distillers grain with solubles (DDGS) replacing barley grain, and (3) DDGS+CT: diet as described in (2) but with the addition of 25g/kg DM Acacia mearnsii CT. After 56 days of composting (Phase 1), windrows in each treatment were thoroughly mixed and divided into four portions. Two portions received additional mature compost (100kg/tonne DM) as a source of nitrite-oxidizing bacteria (NOB) and the other two received none. Portions were moved into individual bins for 57 days (Phase 2) of composting, followed by 104 days (Phase 3) of composting. Adding NOB had no effect on final compost properties or GHG emissions. Total C, total N and NH₄ ⁺ in final compost were higher (P<0.001) in DDGS+CT than in the DDGS and CK treatments, increasing the value of the compost as a fertilizer. Including CT in the diet did not affect CO₂, CH₄ or N₂O emissions during composting. Results demonstrate that substituting DDGS for barley at 400g/kg and adding 25g/kg CT to cattle diets increases the agronomic value of the manure and compost as fertilizer without increasing GHG emissions. This paper is part of the special issue entitled: Greenhouse Gases in Animal Agriculture – Finding a Balance between Food and Emissions, Guest Edited by T.A. McAllister, Section Guest Editors: K.A. Beauchemin, X. Hao, S. McGinn and Editor for Animal Feed Science and Technology, P.H. Robinson.