Abstract
The objective was to determine the incidence of LH surges and ovulatory responses in lactating dairy cows enrolled in a timed artificial insemination (TAI) program. Cows were assigned randomly to 2 presynchronization treatments: (1) Pre10 (n=37): 2 injections of PGF2α (PG; PG-1 and PG-2) 14 d apart (Presynch); or (2) PG-3-G (n=33): one 25-mg injection of PG (Pre-PG) administered 3 d before a 100-µg GnRH injection (Pre-GnRH). Ten days after PG-2 or Pre-PG, all cows were enrolled in a 7-d Ovsynch TAI program [injection of GnRH (GnRH-1) 7 d before PG (PG-3) and GnRH (GnRH-2) administered at either 56 or 72 h after PG-3; TAI at 72 h]. Blood was collected to determine LH at (1) Pre-GnRH: 48 to 80 h after PG-2 and hourly from 72 to 78 h (Pre-GnRH at 72 h); (2) GnRH-1: 0 to 6 h after GnRH-1; and (3) GnRH-2: 48 to 80 h after PG-3 and hourly from 56 to 62 h or 72 to 78 h for cows injected with GnRH-2 at 56 or 72 h after PG-3, respectively. Ovaries were scanned and pregnancy per TAI (P/AI) was diagnosed 31 and 61 d post-TAI by transrectal ultrasonography. The Pre-GnRH injection increased the incidences of LH surges (100 vs. 43%) and ovulation (91 vs. 60%) and subsequent concentrations of progesterone in PG-3-G cows compared with Pre10 cows, respectively. Seven days later, incidence of ovulation (48 to 62%) and occurrence of LH surges (100%) did not differ between treatments after GnRH-1. In contrast, LH concentrations and area under the LH curve of Pre10 cows were greater than that of PG-3-G cows because progesterone was greater in PG-3-G than in Pre10 cows (4.6±0.4 vs. 2.8±0.4 ng/mL), respectively. Concentrations of LH did not differ after GnRH-2 at either 56 or 72 h; however, 1 cow receiving GnRH-2 at 56 h and 3 cows at 72 h had early spontaneous LH surges before GnRH-2. Ovulation was suppressed overall in 210 blood collection windows in cows with elevated progesterone concentrations. When progesterone was <1 ng/mL after either PG-2 or PG-3 injections, GnRH-induced LH surges occurred in more than 90% of cows, and incidence of ovulation exceeded 80%. Pregnancy per AI tended to differ for PG-3-G (56.7%) compared with Pre10 (37.8%) and for 56 h (54.5%) compared with 72 h (38.2%), with the Pre10–72 h treatment combination producing less than half (22.2%) the pregnancies compared with all other treatment combinations. Furthermore, in these same cows, post-TAI luteal tissue volume tended to be compromised. We conclude that incidences of GnRH-induced LH surges and ovulation are suppressed in cows with elevated progesterone, possibly contributing to some loss in P/AI in TAI programs.
Highlights
Synchronization of ovulation to allow for timed artificial insemination (TAI) has become one of the most adopted reproductive technologies by dairy producers (Caraviello et al, 2006; Moeller et al, 2010)
Improved per AI (P/AI) resulting from presynchronization programs before Ovsynch has been associated with synchronizing the majority of estrous cycles to d 5 through 12, which improved ovulation incidence to the first GnRH injection of Ovsynch and resulting P/AI compared with cows treated at random stages of the estrous cycle (Vasconcelos et al, 1999)
In our previous study of these same presynchronization treatments, we reported that PG-3-G tended to increase the proportion of cows with progesterone concentrations ≥1 ng/mL and increased the number of Corpora lutea (CL) per cow (Stevenson et al, 2012)
Summary
Synchronization of ovulation to allow for timed artificial insemination (TAI) has become one of the most adopted reproductive technologies by dairy producers (Caraviello et al, 2006; Moeller et al, 2010). Presynchronizing the estrous cycles of cows improves pregnancy per AI (P/AI) compared with cows starting Ovsynch at random stages of the estrous cycle (Moreira et al, 2001; El-Zarkouny et al, 2004; Navanukraw et al, 2004). Improved P/AI resulting from presynchronization programs before Ovsynch has been associated with synchronizing the majority of estrous cycles to d 5 through 12, which improved ovulation incidence to the first GnRH injection of Ovsynch and resulting P/AI compared with cows treated at random stages of the estrous cycle (Vasconcelos et al, 1999). Other intervals between the second PG Presynch injection and the onset of Ovsynch, 14 d (14 × 14; Navanukraw et al, 2004), 11 d (14 × 11; Galvão et al, 2007), and 10 d (14 × 10; Stevenson et al, 2012), have been investigated. The 14 × 11 Presynch program increased P/AI compared with Presynch 14 × 14 (Galvão et al, 2007)
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