Productivity and seasonal constraints in lamb production can be overcome with accelerated systems. Accelerated lambing system 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: the last 28 days of lactation, then followed by the flushing period consisting of a 21-day pre-breeding and 34-day breeding period. During lactation, ewes were fed 100% (C), 70% (L), or 150% (H) of lactation energy requirements according to lamb rearing status (single or twin). During the flushing treatment, L or H ewes were then fed either 70% (L) or 160% (H) of energy requirements for maintenance (C ewes fed at 100% maintenance). This design was repeated over two seasons: optimal (Short, November-January, n=117) and suboptimal (Long , March-May n=108) breeding periods at 42.73°N and 84.5°W. Ewes were exposed to Dorset rams in 5 pens with a ram to ewe ratio of between 1:19 and 1:25; rams were rotated every 12 h. Conception rate, litter size, and incidence of fetal loss were examined by transabdominal ultrasonography. Linear mixed model analysis revealed that the dietary treatments produced changes in bodyweight and body condition score (BCS) that were largely consistent between seasons. Regardless of dietary treatment, litter size was higher by 0.28 lambs per ewe in Short (P < 0.05), while fetal loss tended to be higher in the Long (P = 0.06). Within season, nutritional treatments in either lactation or flushing had no effect on conception rate or litter size. Despite this, there was a significant regression relationship between BCS change during lactation on litter size across season (P < 0.05). After accounting for the seasonal effect on litter size, our analysis revealed that gestational length differed between seasons (145±0.2 days in Long vs. 147±0.18 days in Short; P < 0.05) as did lamb birth weight (5.10.09 kg for Short, 4.6 kg 0.05 kg for Long P < 0.01). These results indicate that high conception rates in the long day season are attainable with natural mating and that NRC feeding standards are not sufficient to both allow for the recovery of body condition and evoke a flushing response in the context of an accelerated lamb production system.
Read
