drug is protected from loss due to volatility and from the effect of moisture and other materials in animal feeds during processing of the treated feed and during subsequent storage of the feed and is released at the desired rate when the treated feed is administered to an animal it has been found that by appropriate selection of the particular copolymer s and the amount thereof the rate at which the drug is released into the gastrointestinal tract of an animal to be treated can be preselected a variety of styrene maleic anhydride copolymers are suitable each of them providing a different predictable release rate this enables the preparation of compositions having drug release rates over a wide range of values with the particular release rate being preselected by selection of the appropriate copolymer by using a mixture of particular copolymers or by using a mixture of different compositions of this invention in which each of the compositions employs a different copolymer the organophosphorus drugs contemplated in this invention are organophosphates and their sulfur analogs represented by the formula str1 wherein each r which may be the same or different represents alkyl of from 1 to 10 carbon atoms x is oxygen or sulfur y is hydrogen or middle halogen i e cl or br and 34 hal 34 is middle halogen preferred subclasses of these organophosphates are described by the formulae str2 wherein each r 1 is alkyl of from.