121 Breed and Mating Season Effects on Gestation Length in Landrace Hair Sheep Ewes

Gestation length in sheep varies with breed, but little information is available on hair sheep and any effects of time of year of mating. Data were collected between 1997 and 2021 on straight-bred Barbados Blackbelly (BB) and St. Croix (SC) ewes managed under pasture-based, accelerated mating. Rams were fitted with marking harnesses and ewe gestation length was determined as the difference between lambing and marking date. Gestation length (n = 1291) was analyzed with fixed class effects of ewe breed (BB or SC), age class at mating (4 levels), mating month (March, July, or November), and litter size (1, 2, or ≥ 3) and the linear and quadratic effects of total litter birth weight (LBW). Two-way interactions were initially fit but removed as none were significant (P > 0.15). Random effects included ewe and mating month – year (n = 350 and 32, respectively). Gestation length was 0.8 d longer for SC than BB ewes and 0.5 to 0.7 d longer (P ≤ 0.03) for > 5.5 yr-old ewes than all younger age classes. However, mating month did not impact gestation length (P = 0.11). Ewes that gestated a single lamb had longer gestation length (149.8 ± 0.20 d; P < 0.001) than twin- (147.4 ± 0.14 d) and triplet-bearing ewes (146.7 ± 0.28 d), which were also different from each other (P = 0.01). Finally, both quadratic and linear effects of LBW were significant (P < 0.001) and indicated gestation length increased with LBW until reaching an inflection point at approximately 9.1 kg. Gestation length in landrace hair sheep ewes was in line with the 147 d commonly used in gestation calculators. Slight adjustments can be made for ewe age, breed, and litter size, but are not needed for mating month.