Influence of the Ovary on Estradiol-Induced Luteinizing Hormone Release in Postpartum Milked and Suckled Holstein Cows

: Holstein cows were assigned at parturition to be milked (n = 10) or suckled by four calves each (n = 10). One-half of each group was ovariectomized bilaterally (ovex) at the end of wk 1 and the remaining one-half was ovariectomized unilaterally (intact) at the end of wk 2. Cows were challenged with 400 micrograms estradiol-17 beta (im) at the end of wk 1, 2, 3 and 4 (24 h after surgery) and blood was collected at 0, 12, 14, 16, 18, 20, 22, 24 and 26 h after estradiol to determine luteinizing hormone (LH) concentration. Blood also was collected every 3 d until 6 wk to estimate ovulation in intact cows by monitoring serum progesterone concentration. Calves were weaned from suckled cows after the third blood collection period (wk 3). Total ovarian follicles (16.5 +/- 4 vs 34.2 +/- 4) and follicular fluid weight (.7 +/- .2 vs 1.4 +/- .2 g) were less (P less than .05) in suckled than in milked cows during wk 1, respectively. But the reverse trend appeared to occur during wk 2, although total ovarian follicles (26.4 +/- 6 vs 16.4 +/- 6) and follicular fluid weight (.9 +/- .3 vs .8 +/- .3 g) were similar in suckled and milked cows, respectively. Fewer suckled than milked cows responded to estradiol (LH concentration 1 SD above the average pre-estradiol LH concentration) at the end of wk 1 and 2 (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)