Does the garlic cause methane reduction by the same mechanism as the seaweed?
The garlic study [2], states: "Garlic contains allicin, diallylsulide, dialyldisulfide and allyl mercaptan, which affect rumen methanogens, decreasing rumen CH4 and acetate production and increasing rumen propionate and butyrate production (Busquet et al., 2005)."
Allicin is not referenced in the sea-weed study. One would have to study the exact mechanism [3], to be conclusive.
[1]: https://journals.plos.org/plosone/article?id=10.1371/journal...
[2]: http://www.thecattlesite.com/articles/2858/effect-of-garlic-...