1) The meaning could differ regionally and chronologically, but generally it meant a bullock or heifer up to two years old.
1292 In equis, bovettis et stircis cxvijs vjd, Bolton Priory
1346 lego Elen? de Lathum iiijs et unum stirk, Sutton on Derwent
1446-58 iij twynters j sterke j vacca, Fountains Abbey
1524 certain styrkes delivered to the Cellerar of Fontance ... and seyn the burning of the same, Kirkby Malzeard
1607 with ten stirkes coming out of the calfe close, Airton. It was not unusual for the sex and age of the animal to be made clear: 1548 to the saide Henrie Waddisworthe an oxe stirke, Halifax
1551 6 stirk milk kye prised to Ł6, Marske
1607 7 yearing stirkes, Brandsby. It remained active in the farmers’ vocabulary: 1816 Jany 7th Stirk bulled at John Roberts, South Crosland. Stirk is a well-established Yorkshire surname and the by-name provides an early example of the word: 1275 Ricardus Styrke, Sandal. See GRDict.