PSXV-14 The impact of pre-mating nutrition during optimal and sub-optimal breeding seasons on reproductive outcomes in an accelerated lamb production system.

Low flock productivity and seasonal supply are two major constraints in the lamb industry. Accelerated lamb production systems have the potential to simultaneously overcome seasonal supply and productivity constraints. Nutritional management of these systems has not been studied however, so we examined the impact of premating nutrition on reproductive outcomes. Prolific, crossbred (Polypay x Dorset), multiparous ewes housed at 42.73°N and 84.5°W were randomly assigned to three nutritional treatments receiving 100% (control, C), 50% (underfed, UF), or 200% (overfed, OF) of predicted energy requirements for maintenance (MAINT) during the 21-day premating period following weaning. Treatments were replicated over the sub-optimal (April 24-May 27; n=99, 33 per treatment) and optimal (Dec 23-Jan 25; n=81, 27 per treatment) breeding seasons. During the ensuing 34-day mating period, ewes were fed a common diet (1.2 x MAINT) and randomly allocated to 3 mating groups (one ram per group, rams rotated daily). Pregnancy status was determined and litter size measured using transabdominal ultrasound scanning. The pre-mating diet resulted in consistent changes in body weight (sub-optimal and optimal P<0.001; Table 1) and subcutaneous back fat depth (sub-optimal and optimal P<0.001; Table 1) during both breeding seasons. Conception rate did not differ among treatments in either breeding season, whereas twinning rate was consistently elevated in OF ewes during both seasons (sub-optimal P<0.01 and optimal P<0.01; Table 1). Across season, fat depth at start of the pre-mating period/end of lactation was positively associated with fertility (P<0.05) and twinning rate (P<0.001). We conclude that regardless of season, a state of energy deficit prior to mating did not impact reproductive outcomes whereas a state of energy excess had a profound positive effect on prolificacy. These data also suggest that energy reserves during late lactation may also be an important determinant of reproductive outcomes in accelerated systems regardless of season. Key Words: