Effect of exogenous progesterone and oestradiol on plasma progesterone concentrations and follicle wave dynamics in anovulatory anoestrous post-partum dairy cattle

The effect of exogenous progesterone (P4) and of oestradiol benzoate (ODB) on plasma progesterone concentration and follicle dynamics was studied in anovulatory anoestrus (AA) post-partum pasture-fed dairy cattle. Cows (n=32) were defined AA based on not detecting a corpus luteum upon transrectal ultrasonography of the ovaries. Cows were randomly assigned to treatment with an intravaginal P4-releasing device containing 1.56 g of P4 (1Q; Cuemate, Pfizer Animal Health, Auckland, NZ; n=11) or with modified devices with double (2Q; n=11) or triple (3Q; n=10) the normal P4 dose for 8 days. Half of each group received 2 mg ODB at device insertion (Day 0) while the other half did not receive ODB at this time. All cows were treated with 1 mg ODB 1 day after intravaginal device removal (Day 9). Ultrasonography occurred daily until either ovulation or Day 15 whichever occurred sooner. Blood samples were drawn on Days 0, 1, 3, 5, 7, 8, 9, 15 and 22 for plasma P4 determination. Increasing P4 dose was associated with an increase in plasma P4 concentration during the time of device insertion (P<0.05). The highest P4 dose was associated with a delay in emergence of, but a shorter interval from emergence to maximum diameter and ovulation of, the subsequent dominant follicle (DF2) compared to the lowest P4 dose. Treatment with ODB resulted in a delay in emergence of DF2 (4.2 (0.4) versus 2.0 (0.4) days (S.E.M.) to emergence for ODB versus no-ODB; P=0.01), a smaller maximum diameter of DF2 (15.2 (0.5) versus 17.9 (0.6) mm (S.E.M.) for ODB versus no-ODB; P<0.01), a shorter interval to maximum DF2 diameter (5.0 (0.3) versus 6.8 (0.3) days (S.E.M.) for ODB versus no-ODB; P=0.03), a shorter interval from DF2 emergence to ovulation (6.3 (0.4) versus 8.5 (0.4) days (S.E.M.) for ODB versus no-ODB; P=0.02) and a tendency for a lower average plasma P4 concentration post-ovulation (i.e. average of Days 15 and 22; 2.5 (0.4) versus 3.4 (0.4) ng/ml plasma P4 for ODB versus no-ODB, respectively; P=0.08). The DF present at device insertion, was still present at device removal in three (9%) cows of which two were treated with 1Q + no-ODB and one with 3Q + ODB. It is concluded that increasing P4 dose and ODB treatment are associated with a delay in subsequent follicle wave emergence and more rapid follicle growth. Oestradiol benzoate treatment also tends to reduce the plasma P4 concentration in the subsequent luteal phase in post-partum, anoestrous dairy cattle.