It has been observed that many ewes in Utah are somewhat under size when eighteen months old, the usual age for first breeding. It is a common experience for these ewes to produce much smaller lamb and wool crops than the older ewes.
Esplin, Alma C.; Madsen, Milton A.; and Phillips, Ralph W., "Bulletin No. 292 - Effects of Feeding Ewe Lambs During Their First Winter" (1940). UAES Bulletins. Paper 254.