Michael Robinson • Link
"Hannah Glasse refers to both house lamb and grass lamb. House lambs were produced all the year round, being intensively reared under cover and sold at 8 to 10 weeks old. Grass lambs, however, being reared out of doors, were seasonal. It seems that, for the production of these, 'Dorset ewes' were favoured. They would be bought in October and bear their lambs about Christmas. The lambs, after being fed on turnips in the spring, would be sold in April and May (at 20s a head, according to one 18th century source).(Glasse, 1747)"
Glossary entry 'Lamb' at:-