Cows fed linseed reduce their methane emissions by 40%

The Torygraph has an article about reductions in cows’ methane emissions (from farting and burping) when they’re fed on linseed.

30% of farm greenhouse gas emissions comes from methane from sheep and cattle, so if feeding them linseed reduced that amount by 40%, that means it would reduce overall farm greenhouse gas emissions by 12%. Given that the current target is an 11% reduction in farm greenhouse gases by 2020,  based on the 1990 level, this sounds too good to be true?