Interpretive Summary: Optimal dose of PG600 when given to progestogen-synchronized ewes during anestrus as affected by day of the year and temperature
By: Jackie Walling
The goal of a producer is to maximize profitability while optimizing performance by increasing the number of lambs born within a year’s time. A recent article published in Translational Animal Science investigates the effects of varied levels of PG-600 and its impact on pregnancy rates when given during anestrus.
Most commonly used to induce estrous in gilts, PG-600 has also been used off label in sheep. By varying PG600 dose levels, researchers aimed to evaluate if variation in the day of the year (DOY) or temperature affected the dose-response.
From 1998 to 2016, 1402 mixed bred ewes between the ages of 1.5 to four years were split into weekly treatment groups of three to 12 animals. Ewes were synchronized with one of two progestogen devices (synthetic hormones used to prime estrus cycle): Syncro-Mate-B (SMB) or a controlled internal drug-releasing (CIDR) device. Devices were removed after synchronization and ewes were injected with a randomly assigned PG600 dose. Two rams were used for breeding and ewes were checked for pregnancy between days 85 and 99 with final confirmation on day 120 or at lambing. Average DOY was 199 and the average temperature was 11.31⁰C. Two types of statistical models were generated to analyze interactions of DOY, temperature, and dose with pregnancy rates and types (open, single, twins, etc).
Results calculated the highest pregnancy rate for CIDR (76%) and SMB (77%) between PG600 dose levels 2.0mL and 2.49mL with the most prevalent pregnancy type as twins (33%). Rates were highest during the months of June and July. Pregnancy rates reduced at dose levels below 2.0mL and above 3.0mL. Quadruplets and quintuplets were seen a few times in dose levels 2.5mL to 4.0mL.
Statistical Model (a) assumed treatment variables affected pregnancy outcomes linearly and separately. The only significant interaction from this model indicated SMB Device Reuse reduced pregnancy outcomes by 27%. Statistical Model (b) allowed nonlinear interaction between variables and produced several significant interactions.
For modeling purposes, temperature (11.31⁰C) and DOY (199) were held constant. Pregnancy probability was highest at double peaks of 2.0mL and 3.5mL differing from the first range of 2.0mL to 2.49mL. This suggests temperature and DOY influence pregnancy responses. When minimum daily temperatures were graphed, all pregnancy types varied at extreme temperature ends, but the open rate was smallest at 8.9⁰C suggesting cooler temperatures favored estrus induction. Resulting pregnancy type varied with PG600 dose levels suggesting dose level adjustment could select for the number of fetuses produced per ewe. Pregnancy probability, temperature, and dose level were graphed together revealing higher pregnancy rates occurred when the temperature was slightly under the average.
Overall, when temperature and DOY are considered, the optimal PG600 dose during anestrus should be 3.5mL. No interaction existed between dose and temperature, but temperature and dose each impacted pregnancy rate and type. Lower temperatures favored estrus induction. Dose levels above 3.5mL increased number of luteal structures available, but this came with large of follicles and an uninhabitable environment resulting in decreased fertility. With higher dose levels leading to reduced fertility, future research is set to focus on dose levels below 3.5mL to optimize PG600 usage.
For the full article, visit Translational Animal Science.