Methane mitigation in ruminants by dietary means: The role of their methane emission from manure

Methane emission from the excreta of ruminants is an important source of this greenhouse gas in addition to the methane produced in the rumen and the hindgut. This review compares the few existing studies where diet effects on methane emission from the animal and from the manure were measured within the same experiment. The article also describes a new technical solution to combine the respiration chamber equipment with manure storage units. The dietary measures for methane mitigation in the animal can elicit three types of response: (i) it is efficient through limiting fiber fermentation in the rumen and the hindgut, (ii) it limits methanogenesis at unchanged fiber digestibility, or (iii) it is effective in the animal and in the manure as well. Also combinations of these three responses are possible. Experimental evidence for type (i) responses exists for increasing the proportions of concentrate in the diet and for the use of certain fatty acids, by which decreases in enteric methane formation are partially or completely compensated by an enhanced manure-derived methanogenesis. By contrast, feeding concentrate ingredients rich in lignified fiber shows additive declines of enteric and manure-derived methane (response iii). This review discusses also other dietary means for methane mitigation, which are currently regarded as promising, with respect to their potential effects on emission from manure. Finally, the importance of the type and duration of manure storage on the expression of diet effects on manure-derived methane is described. The survey clearly illustrates the need to include measurements of manure-derived methane more often.