Reduction in size of the ovulatory follicle reduces subsequent luteal sizeand pregnancy rate

We hypothesized that reducing the size of the ovulatory follicle using aspi ration and GnRH would reduce the size of the resulting CL, reduce circulati ng progesterone concentrations, and alter conception rates. Lactating dairy cows (n=52) had synchronized ovulation and AI by treating with GnRH and PG F(2 alpha) as follows: Day -9, GnRH (100 mug); Day -2, PGF(2 alpha) (25 mg) ; Day 0, GnRH (100 mug); Day 1, AI. Treated cows (aspirated group; n=29) ha d all follicles > 4 mm in diameter aspirated on Days -5 or -6 in order to s tart a new follicular wave. Control cows (nonaspirated group; n=23) had no follicle aspiration. The size of follicles and CL were monitored by ultraso nography. The synchronized ovulation rate (ovulation rate to second GnRH in jection; 42/52=80.8%) and double ovulation rate of synchronized cows (6/42= 14.3%) did not differ (P > 0.05) between groups. Aspiration reduced the siz e of the ovulatory follicle (P < 0.0001; 11.5 +/- 0.2 vs 14.5 +/- 0.4 mm), and serum estradiol concentrations at second GnRH treatment (P < 0.0002; 2. 5 +/- 0.4 vs 5.7 +/- 0.6 pg/mL). The volume of CL was less (P < 0.05) for a spirated than nonaspirated cows on Day 7 (2,862 +/- 228 vs 5,363 +/- 342 mm (3)) or Day 14 (4,652 +/- 283 vs 6,526 +/- 373 mm(3)). Similarly, serum pro gesterone concentrations were less on Day 7 (P < 0.05) and Day 14 (P < 0.10 ) for aspirated cows. Pregnancy rate per AI for synchronized cows was lower (P < 0.05) for aspirated (3/21=14.3%) than nonaspirated (10/21=47.6%) cows . In conclusion, ovulation of smaller follicles produced lowered fertility possibly because development of smaller CL decreased circulating progestero ne concentrations. (C) 2001 by Elsevier Science Inc.