Pulling all of this evidence together, the big picture is that there are no magic technology bullets that can drastically alter the reality that ruminants emit methane for a good reason. This methane is the outcome of evolutionary processes that produce animals that are fit for the grassland environment in which they live naturally.Farmers will no doubt continue chasing this mirage (and forcing us to pay for it) for as long as they can, because the alternative is to accept that they have to change what they are doing. The rest of us shouldn't waste any more time on it. Because we already know how to reduce agricultural emissions, with positive side-benefits for water quality: cut the herd. This does not mean a giant cow cull, or cowpocalypse: it can be done painlessly, within the normal agricultural business cycle, just by not letting farmers replace as many animals as they send them to the works.
Want to cut agricultural methane by 10%? Cap cow numbers at a level 10% lower than at present. Want to cut them by 50%? Set a lower target. Ideally, we should set a long-term, downward path to cap the herd at a sustainable level. And if farmers find some technological magic bullet after all which significantly reduces emissions per animal, then we can revise it upwards. But it is important to set the default as decline, rather than business-as-usual. And we already have the policy tools required to do this, in the form of the National Animal Identification and Tracing programme.
Farmers have had more than fair warning of the need to reduce emissions. Their "plan" - basicly hope - has failed. Its time we made them do what is required, rather than letting them delay action further.