Agistment is a common law concept; there is no relevant statute law. The law of agistment is entirely domestic (although a similar concept is known in EU law).
The law as stated in this article is accurate for England, Wales, Scotland and Northern Ireland.
Overview of Topic
1. Agistment is a contractual arrangement under which the owner of an animal (the "grazier") is allowed to keep it on land belonging to someone else (the "agister"), for the purposes of grazing. The owner of the animal will typically pay the agister a periodic amount under the contract.
2. The essence of agistment is that it confers on the grazier only a right to graze and not possession of the land in law (still less a tenancy or other interest in land). The arrangement has been analysed as a profit a prendre which by way of exception to the normal rule does not require to be created by deed. The arrangement is also similar to a contractual licence in respect of the land.
Nature of agistment
3. " The features of an agistment agreement are well known. The essentials are first that the animals to be agisted are the property of the person delivering them to the farmer, and they remain that person's property. The farmer's obligation is to feed the animals and deliver up those animals to the owner on demand, but the animals are not at his risk. He is only liable for injury arising from negligence or neglect of reasonable and proper care of the animals. All the farmer gets out of the agreement is the agreed remuneration for feeding the animals. "
Capon, Re  Ch. 442.
4. " Labelling an arrangement as an agistment will not make it one if its true substance is that of a sale subject to conditions or some other kind of arrangement "
Capon, Re  Ch. 442.
5. " Agistment arrangements do not give the agister a lien over the animals to enforce payment under the contract "
Southern Livestock Producers, Re  1 W.L.R. 24.
6. The agister is responsible, subject to limits discussed in the authorities, for the welfare and safe-keeping of the animals, with a view to returning them in good condition on demand.
7. " The law of England regarding agistment is the same. The English law is "if a man takes in a horse or other cattle to graze and depasture in his grounds, which the law calls agistment, he takes them upon an implied contract to return them on demand to the owner" (Blackstone, Commentaries, Book II. p.404). "
Hutchinson v MacInnes 1947 S.L.T. (Sh. Ct.) 51.
8. " ...at common law the agister takes none of those risks, but is only liable for negligence. "
Capon, Re  Ch. 442.
Agistment distinguished from other arrangements
9. " The sale or lease of the grass keep on pasture land for a definite period is not to be confounded with agistment or "tacking" as it is locally termed. The latter consists of the taking in by the farmer of another man's sheep or cattle (usually at so much per head per week) without binding himself to depasture them on any particular part of the farm, whereas the former consists of a grant for a definite period (usually in consideration of a lump sum) of the growing grass crop on a specified field or fields. "
Richards v Davies  1 Ch. 90.
Reasons for entering agistment
10. " Such arrangements can occur where an active farmer has some surplus land from which he wishes to derive an additional income. They commonly occur where an elderly farmer who no longer wishes to actively farm wants to earn an income from his land without losing control of his property. "
McCall v Revenue and Customs Commissioners  NICA 12.
11. The existence of agistment arrangements may be of importance in determining the nature of an agricultural establishment, and the availability of reliefs, for tax purposes: see: McCall v Revenue and Customs Commissioners  NICA 12 and journal articles written since the decision:
a. Business property relief: evolving case law and practice for landed estates P.C.B. 2010, 6, 328-335;
b. Taking stock: future approaches to grazing arrangements P.C.B. 2009, 6, 381-388;
c. McCall and Another (Personal Representatives of McClean Deceased) v Revenue and Customs Commissioners: no BPR on land subject to Northern Ireland grazing agreement (Case Comment) P.C.B. 2009;
d. Taxpayer loses McClean appeal - what are the implications for conacre and agistment lettings? Writ 2009, 199 (Sep), 22-25;
e. When will farming activities attract business property relief? (Case Comment) P.C.B. 2008, 5, 272-279.
12. The agister will acquire some responsibility for the animals in relation to third parties, but the responsibility falls well short of anything like strict liability for any damage caused.
13. " One error which plainly runs through the judge's observations is the repeated reference to malice. One thing that is clear in this case is that the filly had no malice whatever. But, apart from this, in my opinion the judge puts too high a standard of care upon the defendants when he required them to guard against the chance of a purely accidental and unpremeditated injury from young horses loose in a field. To impose such a duty, in my opinion, is quite unreasonable, and goes far beyond anything established by legal authority. It might well render agistment of animals impossible wherever a field abuts on an unfenced highway. "
Fitzgerald v Cooke (ED and AD) Bourne (Farms)  1 Q.B. 249.
14. England and Wales, Scotland and Northern Ireland: The substance of the law of agistment appears to be broadly the same in all three UK jurisdictions, despite some changes of terminology and local custom, and the courts appear to apply authorities between the three jurisdictions. See, for example: Hutchinson v MacInnes 1947 S.L.T. (Sh. Ct.) 51; McCall v Revenue and Customs Commissioners  NICA 12.
15. Agistment is a term found in EU legislation and decided cases, being used more or less in the same way as in UK common law. See, for example, Regulation 868/2008 on the farm return to be used for determining the incomes of agricultural holdings and analysing the business operation of such holdings.
Modern wintering contracts
16. " We have come to the view that Mr Kermack was right to say that a modern wintering contract has elements which can be seen as additions to the basic concept discussed by Bell. If there was a dispute about an admitted contract of agistment, evidence of current practice might well show how the implications of the contract had grown over the years. Such considerations would not address the question of whether any particular arrangement was agistment or not. Whatever might fall to be implied in the case of any specific contract in modern circumstances we accept that the contract of custody of sheep, known to Bell as depasturage, would have involved only a duty to provide adequate grass and take care for the safety of the stock. ... In short, we are satisfied that the fact that the applicant might never have considered whether the sheep needed extra care or treatment, or provided food for them when they could not get at the grass, would not by itself demonstrate that the arrangement was not one of agistment. The fact that the respondents might have provided extra feeding for the stock would not be critical if there was enough grass there in any event to keep the animals in reasonable health. There might be inferences to be drawn from an arrangement which left all responsibility for stock to the stock-owner but this would simply be a factor to weigh against other matters in considering whether the underlying arrangement was properly described as agistment or let. ... We determine that a contract of care and custody of sheep, of the type referred to as "agistment", does not necessarily involve the custodier in provision of feeding other than by reference to there being adequate pasture to maintain the agisted stock in a reasonable state of health. "
John Pollock and Sons Ltd v Robert Rennie (A Firm) 2014 G.W.D. 19-358.