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ONE VERSUS TWO PROTAGLANDIN ADMINISTRATIONS ON CORPUS LUTEUM REGRESSION AND FERTILITY

Modifications to the Ovsynch protocol to increase conception are of primary interest. One of the issues under investigation is the viability to use a second prostaglandin F2αtreatment and a second one before synchronisation with gonadotropin-releasing hormone before initiation of the Ovsynch procedure. These were the topics of investigation of the Wiltbank group, which were published in the Journal of Dairy Science, Volume 98 of 2015, pages 8644 to 8654 and 8741 to 8752, with the respective titles: Effect of a second treatment with prostaglandin F2α during the Ovsynch protocol on luteolysis and pregnancy in dairy cows and Modifications to Ovsynch improve fertility during resynchronization: Evaluation of presynchronization with gonadotropin-releasing hormone 6 d before initiation of Ovsynch and addition of a second prostaglandin F2α treatment.

The main objective of the first study was to evaluate the effect of a second treatment of prostaglandin F2α (PGF) during Ovsynch on regression of the corpus luteum and on fertility during timed artificial insemination. Two experiments were performed. In both, cows were randomized to receive no additional treatments with PGF (1PGF), or a second PGF (2PGF) treatment at 24 hours after the first PGF treatment. In the first experiment with 344 synchronized lactating dairy cows that received artificial insemination at about 80 days in milk, the Double-Ovsynch protocol for synchronizing ovulation was used. Blood samples were collected at the PGF and final gonadotropin-releasing hormone (GnRH) treatments (respectively 72 and 16 hours before timed artificial insemination), during the breeding Ovsynch protocol, to determine corpus luteum regression in response to the protocol.

Treatment with 2PGF increased corpus luteum regression from 83% with 1PGF to 97% with 2PGF. The effect of 2PGF on corpus luteum regression was observed in both first-calf heifers and in cows with more than one lactation (multi-lactation). Cows with lower compared to greater blood levels of progesterone at the time of PGF administration had a lower percentage of complete corpus luteum regression after 1PGF (66.7 vs. 88.1%) but not after 2PGF (95.1 vs. 97.6%).

In Experiment 2 of the first study, 2148 lactating dairy cows on 11 dairy farms in four different regions of the United States were used. The cows were synchronized with Ovsynch and received timed artificial insemination at about 60 days in milk. Cows that received 2PGF had a tendency for increased pregnancies per artificial insemination (AI) compared with cows with 1PGF. This tendency for improvement in pregnancy per AI was observed in multi-lactation cows but not in first-calf heifers. If the data from the 2 experiments are combined, a 9.45% increase in pregnancy per AI for cows receiving two compared with one PGF was observed, with the increase in pregnancy per AI observed in multi-lactation cows rather than in first-calf heifers. Thus, a second PGF treatment in Ovsynch-type protocols can increase pregnancy success by about 10%, primarily due to enhanced fertility in multi-lactation cows.

In the second study, 897 lactating Holstein cows were randomly assigned to a 2 × 2 factorial arrangement of treatments to compare the main effects of pre-synchronization with GnRH six days before beginning an Ovsynch protocol and a second PGF treatment 24 hours after the first (1 vs. 2PGF) on pregnancies per AI. This resulted in the following four treatments: (1) an Ovsynch protocol (Control); (2) pre-synchronization with GnRH followed by an Ovsynch protocol; (3) an Ovsynch protocol with a second PGF treatment; and (4) pre-synchronization with GnRH followed by an Ovsynch protocol with a second PGF treatment. All cows were submitted for first timed artificial insemination (AI) using a Presynch Ovsynch protocol, and cows detected in oestrus after the second PGF treatment of the Presynch portion of the protocol were inseminated and removed from the experiment. Non-pregnant cows were resynchronized using an Ovsynch protocol initiated about 32 days after artificial insemination. Blood samples were collected at the first GnRH treatment, at the PGF treatment, and at the last GnRH treatment of the Ovsynch protocol and were assayed for progesterone concentrations.

Overall, pregnancy per AI tended to be greater for cows receiving a second PGF treatment compared with cows not receiving the second PGF treatment [40 and 37% for treatments (4) and (3) respectively, versus 33 and 32% for treatments (2) and (1), respectively]. Fewer cows pre-synchronized with GnRH had low progesterone concentrations at the first GnRH treatment than cows not pre-synchronized (13 versus 25%), and progesterone concentrations were greater at PGF administration for cows pre-synchronized with GnRH. Cows receiving two PGF treatments had lower progesterone concentrations at the last GnRH treatment than cows receiving one PGF treatment. Differences in progesterone concentrations at PGF administration were detected only for re-synchronized cows and not for cows submitted for first timed AI. It was concluded that pre-synchronization with GnRH six days before beginning an Ovsynch protocol will tend to increase progesterone concentrations at the first GnRH treatment of an Ovsynch protocol, and that a second PGF treatment 24 hours after the first will decrease progesterone concentrations at first timed AI, thereby possibly increasing pregnancy per AI in cows re-synchronized for the second and greater timed AI.