182 Effect of P.G. 600 dose on expression of estrus, fertility, and prolificacy in ewes synchronized out-of-season using a short-term progesterone-based protocol

Out-of-season breeding in ewes requires oestrous induction, which is characterised by reduced ovulation rate. Equine chorionic gonadotropin (eCG) is often utilised to increase ovulation rate and prolificacy. In the United States, P.G. 600 (Merck Animal Health, Madison, NJ), containing both eCG and human chorionic gonadotropin (hCG), is used to induce oestrus during out-of-season breeding in ewes. However, information on the effect of dose of P.G. 600 on reproductive performance for ewes bred out-of-season is limited. The present study aimed to evaluate the effect of P.G. 600 dose on the expression of oestrus, fertility, and prolificacy in ewes synchronized out-of-season (April–May) using a short-term progesterone (P₄)-based protocol. A total of 421 Dorset and Dorset-cross ewes, with a mean (±s.d.) age of 3.1 ± 2.2 years and body condition score of 3.1 ± 0.8 (scale 1 to 5) were synchronized using a short-term P₄ protocol at two locations in Ohio. On Day −6, ewes were administered a controlled internal drug release device (CIDR, 0.3 g of P₄, Zoetis, Kalamazoo, MI), which remained in place for 6 days. On Day 0, the CIDR was removed, 125 µg of cloprostenol sodium (Parnell, Overland Park, KS) was administered, and ewes were randomly assigned to one of two treatment groups as follows: Low dose (2.5 mL of P.G. 600 containing 200 IU of eCG and 100 IU of hCG) or High dose (5.0 mL of P.G. 600 containing 400 IU of eCG and 200 IU of hCG). Ewes were exposed to fertile rams fitted with marking harnesses from Day 0 to Day 4, and ewes were monitored twice daily for expression of oestrus. Pregnancy diagnosis was performed 30 ± 2 days after breeding, and lambing data were obtained from farm records. Data were analysed using generalized linear mixed models (SAS 9.4) and results are reported in Table 1. The percentage of ewes that exhibited oestrus within 96 h after CIDR removal was not different (P = 0.54) between treatment groups. The mean interval between CIDR removal and the onset of oestrus tended (P = 0.07) to be shorter in the Low dose (44.9 ± 1.0 h) than the High dose ewes (47.2 ± 1.1 h). However, there were no differences (P > 0.3) in pregnancy, lambing, or pregnancy loss percentage between ewes treated with a Low or High dose of P.G. 600. The percentage of ewes with multiple births and the mean number of lambs born per ewe were not different (P > 0.6) between treatment groups. In conclusion, similar outcomes were obtained with both P.G. 600 doses and the administration of a larger dose is not necessary in ewes synchronized out-of-season using a short-term P₄-based protocol.