EFFECT OF TWO LOW DOSES OF PROSTAGLANDIN F2α ON LUTEOLYSIS IN DAIRY COWS

In this preliminary study, we determined the effect of a modified method involving the administration of two low doses of prostaglandin F-2 alpha (PGF(2 alpha)) at an interval of 24 h on luteolysis in dairy cows, and compared it with the standard single-pdose method. Twenty-psix cows were assigned to three groups treated with two low doses (TLD group, n = 10), one standard dose (SD group, n = 10), and one low dose (OLD group, n = 6) on day 9 to 10 of the oestrous cycle (day 0 = the day of PGF(2 alpha) administration). Their serum progesterone (P4) levels and corpus luteum (CL) sizes were measured daily from day 0 to 4 to assess CL regression. The results indicated that the proportion of complete luteolysis, indicating a P4 value <= 1 ng/mL on day 3, was higher in the TLD group (100.0%) than in the SD (60.0%) and OLD (66.7%) groups. Ultrasonically detected changes in the CL area correlated with the shifts in the P4 values in both the TLD and the SD groups. The remaining CL area was significantly smaller in the TLD group (17.8% +/- 3.3%) than in the SD or OLD group on day 4. Thus, we concluded that the proportion of luteolysis in cows was increased with two low doses of PGF(2 alpha) as compared to a single PGF(2 alpha) dose, indicating the necessity of the second dose of PGF(2 alpha). However, further studies with larger sample sizes in the field are required.