Conception rate for postpartum dairy cows treated with different gonadorelin (GnRH) products for first or resynchronized timed AI

Abstract Text:

Different GnRH products are used for timed AI (TAI) in postpartum dairy cows. The objective was to compare conception rate (CR) for cows treated either Factrel (Zoetis Animal Health, Florham Park, NJ) or Cystorelin (Merial, Duluth, GA) GnRH before first TAI or resynchronized (resynch; second service or greater) TAI. Holstein cows (n=3938) in a confinement-style dairy in northeast Missouri were assigned to weekly cohorts (n=22) based on calving date. Cows were treated with “Presynch Ovsynch” (PGF_2α, 14d, PGF_2α, 14d, GnRH, 7d, PGF_2α, 56h, GnRH, 16h, TAI) so that first TAI was 68 to 74 d postpartum. PGF_2α was Lutalyse (5mL; 25mg; Zoetis). The GnRH product was either Factrel (2mL; 100μg; n=1945) or Cystorelin (2mL; 100μg; n=1993) and alternated weekly for cows assigned to cohorts. There were first TAI (n=1790) and resynch TAI (n=2148) cows within each cohort. The resynch began 32d after TAI (GnRH, 6d, ultrasound pregnancy diagnosis, 1d, and then for nonpregnant cows, PGF_2α, 56h, GnRH, 16 h, TAI).  The trial was conducted in two seasons [winter (Jan to Feb; n=1203) and summer/fall (July to Oct; n=2735)] during 2012.  Cows were fed a TMR, milked thrice daily, and milk tested monthly for volume, somatic cell count (SCC), fat%, protein% (prot%) and milk urea nitrogen (MUN). Data were analyzed by using logistic regression. There was no effect of GnRH treatment (P>0.10) on TAI CR [36% vs. 37%; Factrel vs. Cystorelin, respectively) and no treatment interaction with any of the tested effects (service number, parity, season, etc.; P>0.10). TAI CR was greater (P<0.05) at first TAI (38%) than resynch TAI (35%) and was greater (P<0.001) in winter (44%) than summer/fall (33%). Milk production (305ME), fat%, or MUN did not affect TAI CR but both SCC and protein% affected TAI CR. The TAI CR decreased (P<0.05) with from lowest (<100,000; 38%; n=2493) to highest (>500,000; 32%; n=440) SCC quartile. The TAI CR increased from lowest (2.5 or less; 25%; n=185) to highest (3.2 or greater; 39%; n=560) octile for milk prot%. In conclusion, GnRH product did not affect TAI CR for first or resynch TAI in an Ovsynch-based program. Other factors affected TAI CR including service number (lesser for second service or greater), season (lesser in summer), SCC (lesser for cows with greater SCC), and milk prot% (greater for cows with greater prot%).

Keywords: GnRH, dairy cow, Ovsynch