Propionate is one of the major products formed during rumen fermentation (Lindsay 1978). Propionate formation in the rumen can occur via two pathways, the non-randomising (acrylate) pathway associated with no ATP generation or the randomising (succinate) pathway associated with ATP generation (Thauer et al. 1977, Russell & Wallace 1997, Ungerfeld & Kohn 2006). Acrylate, as acryl-CoA, is an intermediate in the non-randomising pathway, and is reduced to propionate. The randomising pathway is the major propionate synthesis pathway from glucose (Baldwin et al. 1963). Fumarate and malate are intermediates in this pathway, which are further reduced to succinate, which is decarboxylated to propionate. In several in vitro and in vivo ruminal studies, addition of fumarate, malate or acrylate resulted in reduced methane and increased propionate formation (Martin & Streeter 1995, Asanuma et al. 1999, Carro et al. 1999, Iwamoto et al. 1999, Lopez et al. 1999, Bayaru et al. 2001).
