(246) Accessory Corpora Lutea in Ewes: Influence on Fertility and CL Maintenance

Muhammad Saad¹; Caleb Rykaczewski¹; Alexus Edwards¹; Eduardo Ponte¹; Florentino P.J. da Silva Junior¹; Rafael Domingues¹; Alvaro García-Guerra¹

1. Department of Animal Sciences, The Ohio State University, Columbus, OH, USA

Abstract Text:

Progesterone (P4) is essential for the establishment of pregnancy in ruminants. The induction of accessory corpora lutea (ACL) increases circulating P4 and can improve fertility in dairy cattle, although its effect appears to be dependent on ACL location. Moreover, ACL maintenance in cattle is location-dependent, with ipsilateral ACL being sustained, whereas 53% to 66% of contralateral ACL regress, primarily between 30 and 60 days of gestation. The objective of the present study was to evaluate 1) the effect of ACL presence and location on fertility in ewes; and 2) the effect of relative ACL location on its maintenance during pregnancy. Dorset and Dorset-cross ewes (n=152), with mean (±SD) age of 3.8±2.4 years, body weight of 64.4±7.9 kg, and a body condition score of 3.4±0.7 (scale 1-5), were estrous synchronized for out-of-season breeding using a short-term P4 protocol. Ewes received an intravaginal P4 device (CIDR) for 6 days, with 125 µg of cloprostenol sodium administered at CIDR removal. Following CIDR removal, ewes were exposed to fertile rams fitted with marking harnesses for 4 days and monitored twice daily for estrus expression. A total of 129 ewes (84.8%) exhibited estrus and underwent ultrasonography on days 5 and 6 post-estrus to determine number and location of corpora lutea (CL). At least one CL was detected in nearly all ewes (99.6%; 126/129), with 19.8%, 69.8%, and 10.3% of ewes having 1 CL, 2 CL, or ≥3 CL, respectively. Among ewes with 2 CL, 51.1% (45/88) had both CL in the same ovary (unilateral), while 48.9% had one CL in each ovary (bilateral).  Ewes with 1 CL (n=25) or 2 unilateral CL (n=45) were administered 100 µg of gonadorelin acetate on day 6 to induce ACL formation. Ewes with ≥3 CL or 2 bilateral CL were excluded from further CL monitoring. Ultrasonography was performed every 5 days from day 10 to 60 post-estrus to determine number and location of CL and for pregnancy diagnosis. Based on ACL presence and location, ewes were categorized into three groups: Ipsilateral-ACL, Contralateral-ACL, and No-ACL. Ewes with ACL on both ovaries were included in the Contralateral-ACL group. Data were analyzed using generalized mixed models. The ovulatory response to GnRH was 51.4% (36/70) and did not differ (P=0.29) between ewes with 1 CL or 2 unilateral CL. The proportion of ewes with 1 ACL (61.1%; 22/36) or ≥2 ACL (38.9%; 14/36) was similar (P=0.37) between groups. Relative ACL location was 58.3% (21/36) ipsilateral and 41.7% (15/36) contralateral, and did not differ (P=0.14) between ewes with 1-CL or 2-unilateral-CL. Pregnancy rate at days 25 and 60 was greater (P< 0.04) in ewes with 2 unilateral CL (73.3%; 33/45) than in those with 1 CL (48.0%; 12/25), but was not influenced (P=0.11) by ACL presence. Additionally, pregnancy rates did not differ (P=0.28) among Ipsilateral-ACL (66.7%; 14/21), Contralateral-ACL (80.0%; 12/15), and No-ACL (55.9%; 19/34) groups.  Pregnancy loss between day 25 and 60 was not observed in any group. Regression of ACL was not observed in any ewe, regardless of whether ACL were located ipsilateral or contralateral. In conclusion, presence and location of ACL did not affect fertility of estrous synchronized naturally mated ewes. Furthermore, ACL were maintained regardless of their location until at least day 60 of pregnancy, suggesting that CL maintenance mechanisms after maternal recognition of pregnancy may differ between cattle and sheep.