(plaintiffs entered into an oral lease agreement with defendant under which plaintiffs used defendant’s property to operate a cow-calf operation; the parties also entered into an oral agreement under which plaintiffs cared for defendant’s cattle, in exchange for one-half of the net calf sale proceeds ; when a dispute arose between the parties and defendant refused to allow the October sale of his cattle, plaintiffs served upon defendant an agister’s lien for continuing to care for the cattle; plaintiffs obtained a court order to sell the cattle in February, and the district court ruled that although plaintiffs entitled to ½ of the sales amount, the agister’s lien was “invalid as under the terms of [an] implied contract”; in reversing, the South Dakota Supreme Court ruled that the plaintiffs acquired a valid agister’s lien when defendant refused to sell his cattle in October, as he had always done; under SDCL 40-27-1, plaintiffs were entitled to impose a lien, retain possession of the cattle, and continue caring for them until the February sale; prejudgment interest was also mandatory, not discretionary).