You miss the point; cows are not the problem. In fact; there is no problem. The level of methane, even according to the UN’s own studies remains constant despite a rise or fall in cattle numbers. Again, it is NOT the cows.
But to humor your point: It is bacteria in the animals gut that aids in digestion. There are pro-biotic products that attempt to restore the bacterial balance to the gut. If the ph of the gut is upset, these bacteria die off quickly rapidly followed by the animal. The bacteria or microbial activity in the gut does an amazing thing; it allows us to eat sunshine! The cows is a vehicle to transport bacteria that breaks down cellulose, which we cannot, and provides us a consumable protein. Essentially, ALL meat is grass, all grass is sunshine.
Regardless, even if you could genetically engineer a steer that could consume a grass that produced no methane waste, should one consider methane a waste (I don’t, even the poop that comes out of the animal is not a waste, it has a purpose and is used), think of the outrage. People freak out over genetically modified corn, there is almost no way you could do an animal or grass. And who knows what domino effect will be set in motion by not feeding the soils via grass. In fact, the alleged carbon problem would bloom out of control. Healthy soils, via quality grazing practices will sequester more carbon quicker (Some say 5 years) than if we stopped all fossil fuel usage (some say 100 years)
Furthermore, if you could do a grass, there still remains the rest of the world is covered in your non-modified grass that still is eaten and decays and produces methane.
The point remains; cows are not the problem, nor is the alleged environmental imbalance, nor is environment the concern.