109 Pre-synchronization and reutilization of progesterone devices during a 6-day CO-Synch protocol for fixed-time artificial insemination in beef heifers
Authors: Canadas E. R. , Duran B. J. , Machado G. , Nall A. , Battista S. E. , Mussard M. L. , Baruselli P. S. , Garcia-Guerra A. 
Published: January 2021

Ovulatory response to the initial gonadotrophin-releasing hormone (GnRH) of the CO-Synch protocol is affected by circulating progesterone (P4) and follicle size. In addition, heifers that ovulate to the initial GnRH treatment have greater fertility after AI. Thus, this study determined the effect of (1) presynchronization (Presynch) before a 6-day CO-Synch protocol and (2) circulating [RCE1] (P4) on ovulatory response, oestrus expression, and pregnancies per AI (P/AI) in beef heifers. Yearling beef heifers (n=233) at three locations were randomly assigned in a 2×2 factorial design to the following treatments: (1) Presynch+6-day CO-Synch with a new P4 device; (2) Presynch+6-day CO-Synch with a once-used P4 device; (3) 6-day CO-Synch with a new P4 device; and (4) 6-day CO-Synch with a once-used P4 device. Presynch consisted of insertion of a new P4 intravaginal device (1.38g of P4) on Day −17 and removal of P4 device on Day −11 concurrently with 500µg of cloprostenol sodium (PGF). On Day −9, all heifers received either a new (New) or once-used (Used) CIDR and 100µg of gonadorelin acetate (GnRH). Six days later (Day −3), CIDRs were removed, 1000µg of PGF was administered and an oestrous detection patch applied (Estrotect). At 72h after CIDR removal, AI was performed concurrently with administration of 100µg of GnRH. Pregnancy was determined by transrectal ultrasonography 31 days after AI. A subset of heifers (n=155) was examined on Day −9 and Day −3 by ultrasonography to determine ovulation to Day −9 GnRH. Data were analysed using generalized linear mixed models (SAS 9.4; SAS Institute Inc.). Presynch heifers had larger follicle diameter on Day −9 (12.7±0.3 vs. 10.1±0.3 mm; P&lt;0.001), greater ovulatory response to Day −9 GnRH (82.5%; 66/80 vs. 56%; 42/75; P&lt;0.001), greater expression of oestrus (90.6%; 106/117 vs. 78.4%; 91/116; P&lt;0.02), and earlier oestrus (49.8±1 vs. 53.1±1 h; P&lt;0.01) compared with controls. There was a treatment×CIDR interaction on oestrous expression, such that a lesser (P&lt;0.05) percentage of control heifers with new CIDR expressed oestrus compared with all other groups (Table 1). Heifers with a used CIDR during the 6-day CO-Synch tended (P=0.08) to have greater P/AI (52.1%; 61/117) than those with a new CIDR (40.5%; 47/116). In conclusion, presynchronization before initiation of a 6-day CO-Synch increased follicle diameter, enhanced ovulatory response and oestrous expression, but did not affect fertility. The earlier onset of oestrus in presynchronized heifers suggests that the timing of AI may need to be modified.

Table 1.
Oestrous expression and pregnancy per AI (P/AI) in beef heifers with or without presynchronization and treated with a new or used CIDR during a 6-day CO-Synch

Treatment
CIDR
Oestrus (%; n/n)
Time of oestrus (h)
P/AI (%; n/n)

Control
New
67.8a (40/59)
53.7±1.5a
33.9 (20/59)

Used
89.5b (51/57)
52.7±1.6a
50.9 (29/57)

Presynch
New
94.7b (54/57)
50.9±1.4b
47.4 (27/57)

Used
86.7b (52/60)
48.7±1.3b
53.3 (32/60)

P-value
Treatment
0.03
0.01
0.21

CIDR
0.62
0.19
0.08

Interaction
0.003
0.75
0.38

a,bValues with different superscripts differ (P&lt;0.05).