Effect of melengestrol acetate on cyclicity induction in Bos taurus heifers

The aim was to compare the effect of progesterone on the estrus pre-synchronization protocol using intravaginal progesterone device (CIDR®, Zoetis, Brazil) or oral melengestrol acetate (MGA®Premix, Zoetis) on induction of estrous cyclicity and vaginal reaction in Bos taurus heifers. Prepubertal heifers (n = 245), Red and Aberdeen Angus, 18 months old and a body condition score of 2.74 ± 0.04 (1-5) were distributed randomly in three presynchronization groups: MGA (n = 84), uCIDR (n = 83) and CONTROL (n = 78). The MGA group was given 2500 mg daily of MGA mixed with mineral salt per heifer for 12 days (from D-26 to D-14). On D-24 a CIDR (previously used for 8 days) was inserted in the uCIDR group, and kept for 12 days (from D-24 to D-12). On D-12 0.6 mg of estradiol cypionate (ECP®, Zoetis) was injected im in MGA and uCIDR groups. CONTROL group was not administered any form of progesterone. On D0 uterus and ovaries were examined by transrectal ultrasound to observe the cyclicity, i.e. presence of corpus luteum. After ultrasound evaluation all heifers were injected with 2 mg of estradiol benzoate im (Gonadiol®, Zoetis) and a CIDR (previously used for 8 days) was inserted. On D7, 12.5mg of dinoprost tromethamine im (Lutalyse®, Zoetis) was given. The CIDR was removed on D9, and 0.6 mg of ECP and 200 IU of equine chorionic gonadotropin (eCG, Novormon®, Zoetis) were injected. The TAI was performed on D11 (48hr CIDR removal), and pregnancy diagnosis was performed on D41 (30 days after TAI). Data was analyzed using Proc Means, Proc Freq and Proc GLIMMIX on the statistical program SAS 9.3. The ciclicity rate after presynchronization was higher for MGA (79.7%, 67/84) and uCIDR (74.7%, 62/83) than CONTROL (7.7%, 6/78) (P < 0.0001). Estrous behavioral after pre-synchronization was higher for MGA (89.2%, 75/84) than uCIDR (78.3%, 65/83) and CONTROL (3.8%, 3/78) (P < 0.0001). Post-synchronization (D9) the number of heifers in heat was higher in MGA (91.6%, 77/84) than uCIDR (74.7%, 62/83) and CONTROL (57.7%, 45/78) (P = 0.0007). At the end of synchronization treatment (D9) more heifers with vaginitis were observed in the uCIDR group (16.9%, 14/83) than the other two groups (MGA = 4.76%, 4/84; CONTROL = 5.1%, 4/78) (P = 0.003). The pregnancy/AI was higher in heifers treated with MGA (69.04%, 58/84; P = 0.002) than uCIDR (50.6%, 42/83) and CONTROL (34.6%, 27/78). Also, the pregnancy rate in the end of the breeding season was higher in heifers treated with MGA (81%, 68/84, P < 0.0001) than uCIDR (67.5%, 56/83) and CONTROL (55.0%, 43/78). In conclusion, the presynchronization treatment with MGA was able to improve the estrous cyclicity and increase pregnancy rate after TAI and in the end of the breeding season in heifers. In addition, treatment with MGA may result in lower vaginitis in heifers than used progesterone intravaginal device.