166 The effect of late lactation and pre-breeding nutrition on reproductive outcomes in an accelerated lambing system

Abstract Productivity and seasonal constraints in lamb production can be overcome with accelerated systems. Accelerated adoption, however, has been limited by seasonal constraints in reproduction. Preliminary data suggest that periconceptional nutrition affects reproduction in the sub-optimal, long day breeding season. To evaluate this, we randomly assigned prolific, multiparous, Polypay x Dorset ewes at mid-lactation to nutritional treatments (5) over 2 periods: last 30 days of lactation (LACT) followed by the flushing period (FLUSH) consisting of the 21-day pre-breeding and the 34-day breeding period (total 55 days). During LACT, ewes were fed 100% (C), 70% (Low), or 150% (High) of late lactation energy requirements according to lamb rearing status (single or multiple). At FLUSH, Low or High ewes were fed either 70% (Low) or 150% (High) of energy requirements for maintenance (C ewes fed at 100% maintenance during FLUSH). This design was repeated over two seasons: optimal (SHORT day, n = 117) and suboptimal (LONG day, n = 108) breeding periods at 42.73°N and 84.5°W. Ewes were exposed to Dorset rams in 5 pens at 4–5% rotated every 12h. Reproductive outcomes were examined by ultrasound (transabdominal for litter size and conception rate; transrectal for early pregnancy loss). GLM analysis revealed changes in bodyweight and body condition score (BCS) consistent between seasons (Table 1). Regardless of dietary treatment, litter size was higher in SHORT (P < 0.05), while embryonic loss tended to be higher in LONG season (P = 0.08). Within season, treatment had no effect on reproductive outcomes, however prolonged undernutrition (Low-Low) reduced conception rate in LONG vs. SHORT season (100 vs.78%; P < 0.05). These results indicate that 1) high conception rates in LONG are attainable with natural mating and 2) chronic undernutrition hampers conception during long day periods. Finally, traditional flushing protocols are inadequate to invoke an optimal ovulation response regardless of season in accelerated systems.