What slaughter laws exist in the US and Europe?
<p>I mean actual laws for industrial farms applicable to killing animals.</p>

<p><a href="https://www.law.cornell.edu/uscode/text/7/1902" rel="nofollow">7 USC 1902</a> allows one of two slaughter methods:</p>

<blockquote>
  <p>(a) in the case of cattle, calves, horses, mules, sheep, swine, and
  other livestock, all animals are rendered insensible to pain by a
  single blow or gunshot or an electrical, chemical or other means that
  is rapid and effective, before being shackled, hoisted, thrown, cast,
  or cut; or</p>
  
  <p>(b) by slaughtering in accordance with the ritual requirements of the
  Jewish faith or any other religious faith that prescribes a method of
  slaughter whereby the animal suffers loss of consciousness by anemia
  of the brain caused by the simultaneous and instantaneous severance of
  the carotid arteries with a sharp instrument and handling in
  connection with such slaughtering.</p>
</blockquote>

<p>Individual states typically have versions of this law (so that it's not necessary to make a federal case out of a violation), for example in Washington we have <a href="http://app.leg.wa.gov/RCW/default.aspx?cite=16.50&amp;full=true" rel="nofollow">RCW 61.50</a> which requires using a humane method, defined as:</p>

<blockquote>
  <p>(a) A method whereby the animal is rendered insensible to pain by
  mechanical, electrical, chemical, or other means that is rapid and
  effective, before being shackled, hoisted, thrown, cast, or cut; or </p>
  
  <p>(b) a method in accordance with the ritual requirements of any
  religious faith whereby the animal suffers loss of consciousness by
  anemia of the brain caused by the simultaneous and instantaneous
  severance of the carotid arteries with a sharp instrument.</p>
</blockquote>

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