Developing a system of year-round lamb production can address issues of continuous supply and product uniformity. In this project, Barbados Blackbelly (BB) and St. Croix (STX) Landrace hair sheep were used to produce cohorts of lambs in 4 mo intervals. Lambs were born on pasture, weaned at approximately 9 weeks, and returned to pasture after a period of post-weaning transition in pens. Ram lambs were rendered short-scrotum at weaning and grazed with ewe lambs as a single group in a rotational grazing system using cool season perennial grasses, warm and cool season annual forages, hay, and soy hull supplementation based on season and forage availability. Lambs were removed as they reached 35–40 kg or 12 mo of age. Data from two lamb cohorts born in April (n = 45) and August (n = 39), respectively, were analyzed for the effect of breed and sex on BW and ADG at different production stages. Birth weights of the April and August cohorts were similar (3.36 kg), but pre-weaning ADG and weaning weights were higher (P < 0.001) in the April (400 g/d and 15.6 kg) than August cohort (304 g/d and 12.1 kg). In contrast, pasture ADG (to 9 mo of age) was higher (P < 0.001) in the August (114 g/d) than April (73 g/d) cohort. Pasture ADG was higher (P < 0.05) in STX (100 g/d) than BB (88 g/d), and higher males (106 g/d) than females (81 g/d). During joint grazing of the two cohorts (December through March) ADG was initially higher (P < 0.001) in the older April (76 g/d) than August (16 g/d) lambs, but similar as they transitioned to annual forage (310 g/d). Pasture ADG fluctuated greatly in relation to forage quality, and to a lesser degree with breed and sex, and 79% of April and 84% of August cohort lambs reached target weights as yearlings.
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