1. A method for supplying bioavailable methionine to a cow which comprises administering to the cow an ester of methionine or methionine amide and/or an ester of the hydroxy analogue of methionine or a salt thereof. 2. A method as claimed in claim 1 in which the ester is introduced as a supplement to the feed. 3. A method as claimed in claim 1 or claim 2 in which the ester is an alkyl ester having 1 to 12 carbon atoms. 4. A method as claimed in claim 3 in which the ester is an alkyl ester having 1 to 10 carbon atoms. 5. A method as claimed in any one of the preceding claims in which the alkyl ester is methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tertriary-butyl, n-pentyl, isopentyl, n-hexyl or isohexyl. 6. A method as claimed in claim 4 in which the ester is an alkyl ester having 1 to 4 carbon atoms. 7. A method as claimed in claim 6 in which the alkyl ester is branched. 8. A method as claimed in claim 7 in which the ester is the isopropyl ester. 9. A method as claimed in claim 8 in which the ester is the isopropyl ester of the hydroxy analogue of methionine 10. A method as claimed in claim 7 in which the ester is the tertiary-butyl ester. 11. A method as claimed in claim 10 in which the ester is the tertiary-butyl ester of methionine. 12. A method of supplying at least 50% bioavailable methionine to a cow which comprises administering to the cow the tertiary butyl ester of methionine or the isopropyl ester of the hydroxy analogue of methionine. 13. A method of improving milk from obtained from a dairy cow which comprises supplying to the cow an ester of methionine or methionine amide and/or an ester of the hydroxy analogue of methionineor a salt thereof as claimed in any one of the preceding claims. 14. A method as claimed in claim 13 the improvement in which comprises increased protein content in the milk. 15. A method as claimed in claim 13 the improvement in which comprises increased fat content in the milk. 16. A ration comprising a grain portion, a concentrate portion and a supplement, said supplement comprising an ester of methionine or methionine amide and/or an ester of the hydroxy analogue of methionine or a salt thereof. 17. A ration as claimed in claim 16 in which the supplement comprises an amount of ester calculated as methionine equivalent of up to 75g. 18. A ration as claimed in claim 17 comprising an amount of ester calculated as methionine equivalent of 10 to 30g. 19. A ration as claimed in any one of claims 16 to 18 in which the ester is the isopropyl ester of the hydroxy analogue of methionine 20. A ration as claimed in claim 19 wherein the isopropyl ester is present in an amount of from 7 to 65g per cow per day. 21. A ration as claimed in any one of claims 16 to 18 in which the ester is the tertbutyl ester of methionine. 22. A ration as claimed in claim 21 in which the ester is present in an amount of from 7 to 65.g per cow per day. 23. A unit dosage form comprising an amount of ester as claimed in any one of claims 3 to 11 suitable for dosage for one cow for one day. 24. A method of improving the condition of a cow which comprises supplying to the cow an ester of methionine or methionine amide and/or an ester of the hydroxy analogue of methionine or a salt thereof. 25. A method as claimed in claim 24 in which the ester is an ester according to any one of claims 3 to 11. 26. A method as claimed in claim 24 or claim 25 in which the improvement comprises improved fertility. 27. A method as claimed in claim 24 in which the improvement comprises improved liver function. 28. A method as claimed in claim 24 in which the improvement comprises an increase in energy.