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Comparison of reproductive performance in lactating dairy cows bred by natural service or timed artificial insemination

F. S. Lima^*, C. A. Risco^*,1, M. J. Thatcher^*, M. E. Benzaquen^*, L. F. Archbald^*, J. E. P. Santos and W. W. Thatcher

^* Department of Large Animal Clinical Sciences, College of Veterinary Medicine, and
Department of Animal Sciences, University of Florida, Gainesville 32611

¹ Corresponding author: riscoc{at}vetmed.ufl.edu

The objective of this study was to compare reproductive performance of lactating dairy cows bred by natural service (NS) or timed AI (TAI). One thousand fifty-five cows were blocked by parity and enrolled to receive either NS or TAI. Cows in both groups were presynchronized with 2 injections of PGF₂ given at 42 and 56 d postpartum. Fourteen days after the last PGF₂ injection, cows in the TAI group were enrolled in an Ovsynch protocol (d 0 GnRH; 7 d later, PGF₂; 56 h after PGF₂ injection, second dose of GnRH; and 16 h after second GnRH cows were TAI). Cows in the TAI group were resynchronized with an intravaginal insert containing progesterone inserted 18 d after TAI and removed 7 d later when GnRH was given. Cows were examined by ultrasonography on d 32 after TAI; nonpregnant cows received PGF₂ and GnRH 56 h later followed by TAI 16 h after the GnRH injection. Nonpregnant cows in TAI group were reinseminated up to 5 times using the same scheme. Cows in the NS group were exposed to bulls 14 d after the second PGF₂ injection, and ultrasonography was performed 42 d after exposure to bulls to determine pregnancy status. Nonpregnant cows in the NS group were reexamined by transrectal palpation combined with ultrasound every 28 d until diagnosed pregnant or 223 d postpartum, whichever occurred first. Cows diagnosed pregnant in TAI or NS were reconfirmed 28 d later to determine pregnancy loss. All bulls underwent an evaluation of breeding soundness and were rested for 14 d after 14 d of cow exposure. Health disorders were evaluated up to 70 d postpartum, and body condition score was evaluated at d 70 postpartum. Blood was collected on d 56 and 65 postpartum and analyzed for progesterone to determine cyclicity. The proportion of pregnant cows in the first 21 d of breeding did not differ between groups. The overall 21-d cycle pregnancy rate (PR), which included a total of 8 and 5 service opportunities for NS and TAI, respectively, was not different between groups (25.7 and 25.0% for NS and TAI, respectively). The daily rate of pregnancy was 15% greater for NS than TAI because cows in NS had a greater PR, which resulted in fewer median days open (111 vs. 116 d). Proportion of pregnant cows at 223 d postpartum was greater in the NS than TAI group (84.2 vs. 74.8%, respectively). Cyclicity did not affect reproductive performance. Cows with body condition score 2.75 had greater proportion of pregnant cows in the first 21 d of breeding and daily PR in the first 223 d postpartum Primiparous cows had greater proportion of pregnant cows and daily PR than multiparous cows at 223 d postpartum. In conclusion, the greater proportion of pregnant cows in the NS group was attributed to more opportunities for breeding than in the TAI group.

Key Words: timed artificial insemination • natural service • reproductive performance • dairy cow