Excessively large effect sizes are a warning sign in and of themselves. A good article on this is here: [0] -- simply if the effect is too huge, it becomes incumbent on the researcher to explain why no-one has noticed this before. Methane is a gas with a distinct and noticeable smell. Seaweed is plentiful in many places cows are kept. If the effect was really as dramatic as claimed you'd expect that it'd be well known that letting cows eat seaweed made them less smelly, and the authors of the original study make no attempt to explain why this might be the case.

0: http://daniellakens.blogspot.com/2017/07/impossibly-hungry-j...