Tandem administration of prostaglandin F2α and gonadotropin-releasing hormone in beef heifers and cows as a convergent presynchronization method in the 7 & 7 synch protocol
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An experiment was designed to evaluate modifications to the 7 & 7 Synch protocol with the hypothesis that tandem administration of prostaglandin F2? (PG) and gonadotropin-releasing hormone (GnRH) during the presynchronization portion of the protocol would generate a convergent ovarian response among anestrous and estrous cycling females. Nulliparous (n = 575), primiparous (n = 338), and multiparous (n = 1407) Angus-cross females across 13 locations were blocked based on parity and pre-treatment luteal status, as assessed via transrectal ovarian ultrasonography. Within block, females were randomly assigned to one of three treatments. Females assigned to the 7 & 7 Synch treatment (n = 765) received an intravaginal progesterone-releasing insert (CIDR®) and administration of PG on Day 0, administration of GnRH on Day 7, and administration of PG concurrent with CIDR removal on Day 14. Estrotect[trademark] estrus detection aids were applied on Day 14 at the time of CIDR removal and PG administration. Females assigned to the 7 & 7 + G treatment (n = 769) received the same treatment schedule as females assigned to the 7 & 7 Synch treatment, with the modification that GnRH was administered in tandem with PG on Day 0. Females assigned to the 7 & 7 + G + P treatment (n = 786) received the same treatment schedule as females assigned to the 7 & 7 Synch treatment, with the modification that GnRH was administered in tandem with PG on both Day 0 and Day 7 of the treatment schedule. Fixed-time artificial insemination (FTAI) was performed at 54 [plus or minus] 2 hours after CIDR removal for nulliparous females and at 66 [plus or minus] 2 hours for primiparous and multiparous females. Transrectal ovarian ultrasonography was performed on Day 7 and Day 14 to characterize the largest follicle diameter (LFD) and the presence and number of corpora lutea (CL). Expression of estrus prior to FTAI was characterized using estrus detection aids (Estrotect[trademark]). Approximately 60-100 days after FTAI, pregnancy status was determined via transrectal ultrasonography. Day 7 CL status was affected by treatment (P<0.0001), with a greater proportion of females presenting with CL on Day 7 when treated with the modified treatments 7 & 7 + G AND 7 & 7 + G + P (63 percent) as compared to females treated with 7 & 7 Synch (38 percent). Additionally, Day 7 CL status was affected by parity (P<0.0001), with a greater proportion of females presenting with CL on Day 7 if primiparous (71 percent) or multiparous (68 percent) as compared to nulliparous (38 percent). Day 7 LFD was affected by pre-treatment luteal status (P=0.0005) and parity (P<0.0001) and tended to be affected by treatment (P=0.06). There was a larger LFD on Day 7 among females that were luteal on Day 0 (12.2 [plus or minus] 0.17 mm) as compared to females that were non-luteal (11.2 [plus or minus] 0.21 mm), as well as a larger LFD on Day 7 among multiparous (12.6 [plus or minus] 0.23 mm) and primiparous (11.8 [plus or minus] 0.39 mm) females as compared to nulliparous females (10.6 [plus or minus] 0.26 mm). Females treated with 7 & 7 Synch tended to present with a larger LFD on Day 7 (12.1 [plus or minus] 0.16 mm) when compared to females on the modified treatments 7 & 7 + G AND 7 & 7 + G + P (11.6 [plus or minus] 0.22 mm). Additionally, Day 7 LFD was affected by the interaction of treatment and parity (P=0.01), with a larger Day 7 LFD among multiparous females treated with 7 & 7 Synch (13.1 [plus or minus] 0.29 mm) as compared to multiparous females treated with the modified treatments 7 & 7 + G OR 7 & 7 + G + P (12.3 [plus or minus] 0.20 mm; P=0.0009). Day 14 CL status tended to be affected by treatment (P=0.09) and pre-treatment luteal status (P=0.06). A greater proportion of females presented with CL on Day 14 when treated with 7 & 7 Synch (76 percent) as compared to females treated with 7 & 7 + G + P (73 percent), as well as a greater proportion of females presenting with CL on Day 14 if luteal on Day 0 (80 percent) as compared to females that were non-luteal (68 percent). In addition, Day 14 CL status was affected by parity (P<0.0001), with a greater proportion of females presenting with CL on Day 14 if multiparous (86 percent) or primiparous (71 percent) as compared to nulliparous (56 percent). Day 14 CL status tended to be affected by the interaction of treatment and parity (P=0.09), with a greater proportion of multiparous females presenting with CL on Day 14 if treated with 7 & 7 Synch or 7 & 7 + G (90 percent and 87 percent, respectively) as compared to 7 & 7 + G + P (82 percent; P<0.05). Day 14 LFD was affected by treatment (P=0.003), with a larger LFD on Day 14 among females treated with 7 & 7 + G + P (11.4 [plus or minus] 0.44 mm) as compared to females treated with 7 & 7 Synch (11.3 [plus or minus] 0.40 mm) or 7 & 7 + G (11.0 [plus or minus] 0.38 mm). Expression of estrus was affected by treatment (P<0.0001), pre-treatment luteal status (P<0.0001), parity (P=0.0013), and the interaction of treatment and parity (P=0.0024). A greater proportion of females expressed estrus prior to FTAI when treated with 7 & 7 + G + P (81 percent) as compared to females treated with 7 & 7 + G (75 percent) or 7 & 7 Synch (73 percent). A greater proportion of females classified as luteal on Day 0 expressed estrus prior to FTAI (84 percent) when compared to non-luteal females (64 percent). In addition, a greater proportion of females expressed estrus prior to FTAI if they were multiparous (82 percent) or primiparous (75 percent) as compared to nulliparous (65 percent). Lastly, a greater proportion of multiparous females expressed estrus prior to FTAI when treated with 7 & 7 + G + P (90 percent) as compared to multiparous females treated with 7 & 7 + G (79 percent; P=0.0018)) or 7 & 7 Synch (78 percent; P=0.0004)). Pregnancy rate to FTAI was affected by estrus expression (P<0.0001), Day 14 CL status (P=0.0038), and the interaction of estrus expression and Day 14 CL status (P=0.027). A greater proportion of females expressing estrus prior to FTAI became pregnant (59 percent) when compared to females failing to express estrus (35 percent). A greater proportion of females presenting with CL on Day 14 became pregnant as a result of FTAI (59 percent) when compared to females presenting with no CL (50 percent). Additionally, a greater proportion of females presenting with CL on Day 14 and expressing estrus prior to FTAI became pregnant to FTAI (62 percent) when compared to females presenting with CL and failing to express estrus prior to FTAI (48 percent; P=0.0055). In summary, tandem administration of GnRH and PG on Days 0 and 7 of the 7 & 7 Synch protocol effectively generated a convergent ovarian response among females with initially variable luteal status pretreatment.
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M.S.