New research evaluated presynchronization and resynchronization protocols on grazing dairy cows at first and second service. The study compared the two reproductive methods and their effect on pregnancy rate through a 100-day breeding season.

1,262 cows were randomly assigned to one of four treatments with two presynchronization treatments and two resynchronization treatments. Estrous cycles of the cows were presynchronized with one of the following methods:



PGF program (PreSynch) consisting of two treatments of PGF administered 14 days apart, followed by artificial insemination (A.I.) 11 days later.




PGF-GnRH-based presynchronization program (G6G) where cows received a dosage of PGF and a GnRH treatment three days later, followed six days later by A.I.

All cows were bred on the same day (day 0) using the same A.I. protocol which included GnRH on day -8, PGF on day -3 and -2 and GnRH plus timed A.I. on day 0. Also, on day -8, blood samples were analyzed for progesterone. On day 12 cows either remained as a control or received a CIDR® insert containing progesterone for 7 days.

Estrus was observed daily starting on day 19 and, when observed in heat, cows were bred. On day 35, bulls were placed with the cows for an additional 65 days. Results published in the October 2011 issue of the Journal of Dairy Science concluded: 

Compared to Jerseys and Crossbreds, Holstein cows were less likely to have progesterone >1 ng/mL on day -8, expressed estrus less and had a lower conception rate. This resulted in a slower rate of pregnancy and smaller proportion of pregnancies at the end of the breeding season.

Body condition, days in milk and progesterone levels at the first GnRH dosage of the timed A.I. protocol had marked effects on the reproductive performance of lactating grazing cows.

More G6G cows had progesterone >1 ng/mL at the first GnRH injection of the time A.I. protocol compared with PreSynch cows (82 vs. 74.3 percent).

Researchers concluded the presynchronization treatment did not influence conception rate, but cows in G6G had increased risk of pregnancy loss between 30 and 65 days after the first A.I. (12.9 vs. 8.1 percent).

Source: Dairy Cattle Reproduction Council