Two studies (expt. 1, 185 cows in 1996/97; expt. 2, 168 cows in 1997/98) were conducted with Prim ‘Holstein dairy cows in the Mayenne region (France) to investigate suboestrus. Cows which had not been observed in oestrus since calving were allocated alternately to treatment groups between 60 and 90 days post partum. Expt. 1. Group 1: GnRH (Day 0, 100 μg im), PGF2α (Day 7, 25 mg im), GnRH (Day 9, 100 μg im) with artificial insemination (AI) Day 10. Group 2 : PGF2α (Day 0, 25 mg im), AI at oestrus or if oestrus not observed a second PGF2α injection was given (Day 13) with AI on Day 16 and Day 17. Expt. 2. Group 1 : GnRH (Day 0, 100 /μg im), PGF2α (Day 7, 25 mg im), GnRH (Day 9, 100 μg im) with AI at observed oestrus after Day 0 or at Day 10 if oestrus not observed. Group 2 : PGF2α (Day 0, 25 mg im), AI at oestrus, or if oestrus was not observed a second PGF2α injection was given (Day 13) and AI at observed oestrus. Progesterone was measured in serum at Day 0 and in milk at AI. Pregnancy diagnosis was performed by measuring bovine Pregnancy Specific Protein B (bPSPB) (Day 50 + 3) and confirmed by ultrasonography when the result was doubtful.
In expt. 1, farmers observed 47/101 (46.9 %) of group 1 cows in oestrus, 33/91 on Day 10 and 10 before Day 10. The progesterone concentrations were compatible with oestrus in 69/86 (80%) of cows on Day 10. In group 2, 36/83 (43.4 %) of cows were inseminated after the first PGF2α injection. After the second PGF2 α, only 29/43 (67 %) of cows had a low progesterone concentration at AI. The pregnancy rates were 36.1% and 32.5 % for groups 1 and 2, respectively. In expt. 2, oestrus was observed in 31/93 (33.7 %) of group 1 cows. In group 2, 51/75 (66 %) of cows were inseminated after the first injection of PGF2α, 13/75 (17.3 %) after the second and 11/75 (14.7 %) were not seen in oestrus. Pregnancy rates were 53.7% and 53.3% in groups 1 and 2, respectively. In conclusion, it is recommended that suboestrus be treated with PGF2α followed by AI at observed oestrus when oestrus detection is good while, the use of GnRH + PGF2α + GnRH is recommended when oestrus detection is poor.