122 Reproductive performance of beef cattle submitted to resynchronisation at 16 or 25 days after first service

Colour Doppler ultrasonography (CDU) of the corpus luteum at Day 22 after AI allows for early identification of nonpregnant cattle and prompt resynchronisation for timed AI (TAI). The objective was to compare the performance of beef cattle submitted to resynchronisation for second service at 16 or 25 days after first service. Nulliparous Angus-cross heifers (n = 214) and suckled cows (n = 379) were submitted to a 6-day CO-Synch for first service. At the time of the first AI, cattle were randomly assigned to begin resynchronisation for second service at Day 16 (Resynch-16) or Day 25 (Resynch-25) while managed as a single group. Cattle in the Resynch-16 group received an intravaginal progesterone (P4) device (1.38 g of P4, controlled internal drug release (CIDR), Zoetis) at Day 16 post-first TAI. On Day 22, CIDR were removed, and, pregnancy diagnosis was performed using CDU (Esaote MyLab Delta, PRF 960Hz). Cattle were considered nonpregnant if colour pixels covered < 10% of the periphery of the CL and were absent internally. Nonpregnant cattle were administered 1000 µg of cloprostenol (PGF, Estroplan, Parnell). Cattle in the Resynch-25 group received a CIDR and 100 µg of gonadorelin (gonadotrophin-releasing hormone, Gonabreed, Parnell) at Day 25 post-first TAI. On Day 31, CIDR were removed, pregnancy was diagnosed using B-mode ultrasonography, and nonpregnant cattle were administered 1000 µg of PGF. In both groups, nonpregnant cattle were submitted to second TAI 60 (heifers) or 72 h (cows) after CIDR removal, concurrently with administration of 100 µg of GnRH. Pregnant cattle at Day 22 (Resynch-16) were evaluated using B-mode ultrasonography at Day 31 to determine diagnostic accuracy of CDU. Pregnancy diagnosis was performed in all cattle using B-mode ultrasonography 31 days after second service. Data were evaluated using generalised linear mixed models and results are shown in Table 1. The pregnancy per AI (P/AI) at Day 22 for first service was 66.5% (133/200) and 62.5% (70/112) for Resynch-16 cows and heifers, respectively. First-service P/AI (Day 31) tended to be greater (P = 0.07) for Resynch-25 than Resynch-16 cows, while there was no difference (P = 0.52) between groups for heifers. The proportion of false positive pregnancy diagnosis at Day 22 was 19.5% and 21.4% for cows and heifers, respectively. Fertility to second service was greater (P = 0.01) for cows submitted to Resynch-25 compared with Resynch-16; however, there were no differences (P = 0.25) between groups for heifers. Cumulative P/AI was greater (P < 0.01) for Resynch-25 than for Resynch-16 cows and tended to be greater (P = 0.09) for Resynch-25 than for Resynch-16 heifers. In conclusion, resynchronisation at Day 25 in beef cattle results in improved reproductive performance compared to resynchronisation at Day 16.

Table 1.  First, second, and cumulative pregnancies per AI (P/AI) for cattle submitted to a resynchronisation program at 16 or 25 days after first service