Be. Salfen et al., Effect of short-term calf removal at three stages of a follicular wave on fate of a dominant follicle in postpartum beef cows, J ANIM SCI, 79(10), 2001, pp. 2688-2697
The hypothesis that ovulation in response to short-term (48 h) calf removal
(CR) is dependent on the developmental stage of the dominant follicle was
tested in two studies. The objective of Exp. 1 was to characterize the fate
of a dominant follicle following 48-h CR on d 2, 4, or 8 of a postpartum f
ollicular wave. Ovaries of 61 beef cows were examined daily by transrectal
ultrasonography starting at d 20 to 21 postpartum. Treatments were no CR (n
= 14) and CR on d 2 (n = 12), 4 (n = 16), or 8 (n = 10) of first detected
follicular wave. Percentage of cows that ovulated a dominant follicle follo
wing treatment was not different among groups (P = 0.62). Maximum size of d
ominant follicles was larger in cows that ovulated (P = 0.002) than in cows
that did not ovulate. The objectives of Exp. 2 were 1) to determine whethe
r a follicular wave could be synchronized in anestrous cows following injec
tion of 1 mg of estradiol benzoate (EB) and 200 mg of progesterone (P-4; EB
+ P-4); 2) to characterize the fate of dominant follicles following 48-h C
R at three stages of a synchronized follicular wave; and 3) to determine wh
ether estrous cycles of normal length followed ovulation in cows pretreated
with EB + P-4. Ovaries of 50 anestrous beef cows were examined daily as in
Exp. 1. Treatments were sesame oil (SO) injected (i.m.) on d 25 postpartum
and no CR (n = 9); EB + P-4 and no CR (n = 9); EB + P-4 and CR on 6 (n = 1
2), 8 (n = 9), or 12 (n = 11) d after injection. The EB and P-4 injections
were given on d 25 postpartum. Variability in day of emergence of subsequen
t follicular waves was lower in cows receiving EB + P-4 than in SO-injected
cows (P < 0.05). The percentage of cows that ovulated was not different (P
= 0.16), but CR increased the percentage of cows that ovulated when groups
that received EB + P-4 were compared to the EB + P-4 group that did not ha
ve CR (53.1 vs 11.1%, respectively; P < 0.05). Maximum diameter of dominant
follicles was larger (P = 0.05) in ovulatory follicles. The luteal phase w
as longer (P < 0.03) in cows receiving EB + P-4 injection (10.6 +/- 1.2 d)
than in cows receiving SO (4.4 +/- 2.2 d). In summary, the maximum size of
ovulatory follicles was greater than that of nonovulatory follicles, the ov
ulatory response of postpartum anestrous cows was maintained through d 8 of
a follicular wave, synchronization of follicular waves was accomplished in
postpartum cows using EB + P-4, and the subsequent luteal phase length was
increased in animals that were administered EB + P-4.