No , I'm not suggesting fitting Underwear to cows... as such!
(although it is an amusing image!)

For one thing, a cow has no self control when it comes to distinguishing between a release of gas, or something more substantial (!)

Cows also can not pull down underwear, or seek a discreet place for relief.

This system sees a kind of coat for the cow with semi inflated pockets, that are used to create a pumping action as the cow walks.  Attached to this is a solar panel and an empty gas bottle on the cow's back with a hose, leading to a small funel or 'hood' positioned just above the posterior of the cow (Only rising emmisions are to be caught!)

The cow walks, pumping the coat to create a vacuum in the hose and funel, and when the cow expels gas it is sucked into the hose, where the solar powered compressor squeezes it into the bottle, cutting off when the bottle is full.

The bottle is only a few litres, but with compression can store a little more.
...and when the solar panel is not working the compressor, it charges a small  battery (so you have a solar powered cow too!)

Upon return to the cow shed, both bottle and battery can be removed and collected, and shipped for commercial use, giving the farmer a new source of income, and making his cow more valuable into the bargain.

But it's not all work for the  beloved cow, as the coat will help keep him warm in winter months and cool in summer, because when the sytem is not working to scoop up gas, the battery can stop charging when a thermostat indicates temperature outside the optimal comfort for her, and either heats the coat like an electric blanket, or pumps the heat out llike a fridge.

Which will keep Buttercup quite happy.

As cow flatulence is said to be responsible for a sizable proportion of emmisions attributable to indirect human activity, removing even some of the stink can only be good.