I have worked in a state-of-the-art slaughterhouse (one of the largest in the world), working to the standards set by the US, UK and New Zealand simultaneously. That is, hygiene was awesomely thorough, but there was no attempt to meet kosher or halal standards. I have also watched a cow being slaughtered in the traditional way at a village festival in north-east Turkey, with nothing more high-tech than a rope and a sharp knife.

In the slaughterhouse we got through about half a million sheep while I was there. Each one of them was driven up a ramp with prods into the killing area at the end of the factory, which was maybe fifty feet wide, twenty feet high, thirty feet long, and by lunchtime was covered inches thick in congealed blood almost to the ceiling. Killing was done in an elevated pen, at the start of the conveyor chain, about 12 feet above floor level. The last sight each sheep saw was six lines of twitching corpses hanging on hooks through their rear ankles moving away towards the processing areas. (They were skinned first, then the arsehole was removed, then they started the rest of the gutting. My main job was emptying the arsehole buckets and picking up stray guts that fell off the conveyor). At that point, even if your brain is the size of a golfball you probably have a pretty good idea what's about to happen to you. Which was an electric shock to the head, enough to disorient and incapacitate the sheep (the same as ECT or an epileptic seizure) and then a series of knife cuts that slashed through the arteries of the neck and sometimes the spinal cord. The slaughtermen were highly skilled, efficient, and in no way cruel, but with that volume of animals to process (about one every ten seconds) there were regular accidents, like a half-electrocuted sheep falling off the killing pen to smash into the floor and wallow around in the pool of blood before they could get it back.

The calf in Turkey knew exactly nothing until the blood supply to its brain failed. Consciousness is lost within 20 seconds when that occurs. Kosher and halal requirements (they are the same apart from the prayers) prescribe that the animal should not know what's going to happen.