Maintenance of Pregnancy in Beef Cattle after a Luteolytic Dose of ProstaglandinF2α

The increased use of assisted reproductive technologies (ARTs), increase the risks of erroneous prostaglandin F2α (PGF2α) administration. The objective of this study was to determine if pregnancies could be maintained if P4 was administered after PGF2α administration. A total of seven pregnant heifers between 30-40 days of gestation and 19 pregnant cows between 30 to 40 (n=12) or 80 to 90 (n=7) days of gestation were used. All females were administered a maintenance dose of P4 or progestin at either 2, 6, 8, 10, 12 or 18 hours post-PGF2α for 7 days. A total of 18 of 25 (72%) pregnancies were maintained for a minimum of 7 days. The success rate for cows and heifers receiving P4 or progestin 2, 6, 10, 12 or 18 hours after PGF2α was 4/6 (75%), 6/6 (100%), 2/3 (67%), 2/4 (50%), 1/2 (50%) and 3/4 (75%), respectively. Maintenance P4 or progestin was continued on selected females (12 of the 18) and they were examined via ultrasonography 2 to 3 times weekly until a follicle ≥10 mm developed on the ovary ipsilateral to the gravid uterine horn, at which time hCG (2,500 IU) was administered. Luteal tissue was successfully induced in 10 of the 12 (83%) and 8 of these calved (80%). These results indicate that it is possible to maintain a bovine pregnancy by administering maintenance dose of P4 from 2 to 18 hours post-PGF2α.


Introduction
Administration of prostaglandin F 2α (PGF 2α ) can induce luteolysis in cycling cattle and abortion at various times during pregnancy [1,2,3] and the most effective time for PGF 2α to induce abortion is from day 10 to 150 of gestation [4]. Although, pregnancy may be maintained on progesterone (P 4 ) following PGF 2α induced luteolysis, there have been no reports of reviving or rescuing the original corpus luteum (CL) of pregnancy.
In early studies exploring the effect of CL ablation on bovine pregnancy, maintenance P 4 was administered prior to CL ablation [5,6,7] or simultaneously with CL ablation [8,9,10,11,12]. In the previous cases, pregnancy was maintained until near the end of gestation; however, it is unknown if pregnancy can be maintained when maintenance P 4 is administered after PGF 2α administration. As this will be the often be the situation when PGF 2α is erroneously administered to pregnant cows as a result of on farm estrous cycle synchronization for use in artificial insemination (AI) or embryo transfer (ET).
In addition to rescuing and maintaining a pregnancy in PGF 2α -treated gestating cows an additional obstacle would involve maintaining the pregnancy to term and it resulting in normal parturition. Cows that undergo parturition without a CL have a higher incidence of premature calving, dystocia, increased incidence of retained placental tissues, increased cow and calf mortality, and insufficient milk production post-calving [8,10,11,13,14]. Additionally, there are reported cases of a lack of maternal instinct and increased incidence of calf abandonment [11]. An increase in dystocia occurring in the absence of a CL has also been described in swine [15,16].
There have been reports of induction of luteal tissue during early pregnancy in beef cattle, however the induced luteal tissue needed to be ipsilateral to the gravid uterine horn or it was only transient [12,17,18]. Later, it was reported that induced luteal tissue was capable of continuing a pregnancy to term regardless to which ovary produced the luteal tissue [19].
These reports provide insight into a possible mechanism to prevent problems that commonly occur in CL ablated pregnant cows. Therefore, the objectives of these studies were to: (1) determine if pregnancies could be maintained in the absence of a CL with P 4 or altrenogest administration post-PGF 2α treatment and (2) determine if induced luteal tissue could successfully support the maintained pregnancy to term and result in normal parturition, producing a viable offspring.

Animals
In this experiment, seven pregnant heifers from 30 to 40 days of gestation and 20 pregnant cows between 30 to 40 days (n=13) or 80 to 90 days (n=7) of gestation were randomly assigned to a treatment time. All females (cross-bred Angus and Simmental

Induction of luteal tissue
From females that successfully maintained pregnancy following PGF 2α -treatment; three cows from 30 to 40 days of gestation group, two cows from 80 to 90 days of gestation group and seven heifers from the 30 to 40 days of gestation group were selected for testing of the second experimental objective. These females were ultrasound three times per week to determine the development of a follicle ≥10 mm on the ovary ipsilateral to the gravid uterine horn. Once a follicle of this size and position occurred, the female received 2,500 IU of hCG (i.m., Chorulon, Intervet Inc., Millsboro, DE) and was continually monitored for luteal development via ultrasonography and plasma P 4 concentration. Once induced luteal tissue was detected via ultrasonography in cows receiving maintenance P 4 the dose of P 4 was decreased to 50 mg per day for 7 days then to 25 mg per day for 3 days and finally P 4 treatment was terminated. At this time blood samples were collected to verify induction of luteal tissue. Both norgestomet implants were removed from females once induced luteal tissue persisted for 10 days.

Sample collection and progesterone assay
Blood samples were collected at 0, 2, 6, 12, 24 and 30 hours post-PGF 2α for cows and 0, 6, 12, 18 and 30 hours for heifers following PGF 2α and again following hCG administration. The rational for the change from P 4 to altrenogest to maintain pregnancy the first 24 hour time period was to allow for more accurate plasma P 4 assays as altrenogest does not and did not cross-react in radioimmunoassay (RIA), nor did the norgestomet.
Blood samples were collected via jugular venipuncture using sodium heparin tubes, and plasma samples were stored at -4˚C until the assay was performed. Plasma P 4 concentrations were determined using a commercial P 4 assay kit (Diagnostics Systems Laboratory, Webster, TX) and the intra-and interassay CV and assay sensitivity were 5%, 10% and 0.05 ng/ml for progesterone.

Induction of luteal tissue and continuance of pregnancy
Induction of luteal tissue was successful in 2 of 3 (67%) cows from the 30 to 40 days group; however, only one of the two cows continued gestation to parturition with induced luteal tissue. The cow that successfully maintained pregnancy to parturition on induced luteal tissue received hCG at day 10 and 15 post-PGF 2α . The other cow received one administration at day 15 and aborted 37 days following hCG, which was 27 days following removal from P 4 . The third cow aborted 3 days doi: 10.7243/2054-3425-2-1

Days of gestation at time of PGF 2α treatment
No. of maintained pregnancies (%) 1,2 No. of cows with induced luteal tissue (%) 3,4 No. of cows calving (%) 5 30 to 40 days (mature cows) 8  post-hCG at day 10 post-PGF 2α . Two of the three cows (from the 80 to 90 days group) maintaining pregnancy were attempted to developed induced luteal tissue after receiving hCG 7 days post-PGF 2α . One receiving a second hCG administration 7 days later. In these two cows, plasma P 4 levels were 0.24 and 1.19 ng/ml at 24 hours post-PGF 2α and were 2.51 ng/ml and 7.32 ng/ml 7 days post-hCG.
In the heifer 30 to 40 days group, induction of luteal tissue was successful in 6/7 (86%) and 5/6 (83%) of those who calved. The mean±SE hCG administrations to induce luteal tissue was 2.0±0.4 per heifer (range of 1 to 4). The mean±SE time to induce luteal tissue was 38±5.5 days (post-PGF 2α ) (range 7 to 49 days). The mean±SE P 4 concentration (72 hours post-PGF 2α ) for heifers maintaining pregnancy on induced luteal tissue was 0.75±0.29 ng/ml. The mean±SE P 4 concentration at 49 days post-PGF 2α with induced luteal tissue was 6.39±1.60 ng/ml. Of the five heifers maintaining pregnancy on induced luteal tissue, gestation was ~285 days, and 4/5 (80%) were normal with the exception of one heifer that died of dystocia. The number of successful luteal tissue inductions and calving is summarized in (Table 1).

Discussion
In this experiment, a total of 24 of 26 (92%) cows and heifers (between 30 and 90 days of gestation) responded to the PGF 2α administration with a decline in plasma P 4 concentrations below 2 ng/ml occurring by 30 hours of PGF 2α which is consistent with other reports [3,4,20]. The decline in plasma P 4 concentrations below 2 ng/ml occurred between 12 to 24 hours post-PGF 2α . The decline pattern may contribute to the 86% success in maintaining a pregnancy if P 4 is admin-istered prior to 12 hours compared to a 60% success rate when P 4 is administered 12 hours or later post-PGF 2α . This outcome indicates that the sooner P 4 supplementation occurs following PGF 2α the more likely the pregnancy can be maintained. Of the six pregnancies successfully maintained with P 4 supplemented at 12 hours or later, five were among heifers from 30 to 40 days of gestation. This may be the result of heifers experiencing significantly higher P 4 concentrations following P 4 supplementation compared with cows [21].
The second objective of this experiment was to prevent dystocia and loss of calves at parturition. This problem has been well documented in cattle (beef and dairy) calving without a CL [10,11,13,14], and although it has been reported that induced luteal tissue was capable of supporting pregnancy in luteoctomized cattle in the absence of maintenance progestin [12,17,18] the success rate was low when the induced luteal tissue was located on the contralateral ovary to the gravid uterine horn [12]. Only one study reports induced luteal tissue doi: 10.7243/2054-3425-2-1 continuing a bovine pregnancy to term [19].
The two pregnancies lost during the process of induction of luteal tissue may have been the result of loss due to rectal manipulation during ultrasonography two to three times per week [22,23]. There seemed to be no effect of follicle size and subsequent luteal tissue formation as the majority of the follicles were ~11 mm at the time of hCG administration (range 10 mm-15 mm) and several calves were born from cows receiving only one administration of hCG. All pregnancies maintained to term were the result of induced luteal tissue. This was evident as all females were also removed from maintenance P 4 or progestin prior to 120 days of gestation which is the minimum stage of gestation for a cow to continue pregnancy in the absence of luteal tissue [10].
The length of gestation in cows with induced luteal tissue ranges from 271 and 287 days with the majority calving~285. Among the eight females calving only one developed dystocia with the other seven demonstrating normal parturition, lactation and maternal behavior. Calves were normal with one calf having a low birth weight.
In conclusion, these data illustrate that valuable pregnancies can be maintained following an accidental administration of PGF 2α without the need for extended exogenous P 4 or progestin supplementation. In addition, pregnancies maintained on induced luteal tissue can result in normal parturition, maternal behavior and lactation.