Without a male around then sorry no way , normally it is based on when the spring grass grows to produce milk so lets say the grass in your area starts early april as an ave then you can start lambing mid march , if you are willing to feed lots of hay and concs then you can lamb anytime . The weather is a factor if you wish to lamb outside the warmer /dryer the better but again if you have sheds /shelter then you can lamb earlier . Around your area a lot of lambs march /april . If you ask the seller of your hebs then you will know when they've lambed in previous years and this will provide you with a fair idea of when they are cycling , ewe lambs and primatives are normally late to cycle