Fm. Rhodes et al., CHANGES IN OVARIAN-FUNCTION AND GONADOTROPIN-SECRETION PRECEDING THE ONSET OF NUTRITIONALLY INDUCED ANESTRUS IN BOS-INDICUS HEIFERS, Biology of reproduction, 55(6), 1996, pp. 1437-1443
The aim of this study was to monitor endocrine and ovarian changes imm
ediately preceding the onset of nutritionally induced anestrus. Daily
blood samples were obtained from 14 postpubertal heifers for one estro
us cycle (initial estrous cycle). Subsequently, heifers designated ''r
estricted'' were given a limited diet (n = 9), and daily blood samples
were obtained for approximately 21 days preceding onset of anestrus (
anovulatory cycle). Controls were allowed ad libitum dietary intake (n
= 5), and daily blood samples were collected for a complete estrous c
ycle during a time period that coincided with that preceding onset of
anestrus in restricted heifers. Plasma samples were assayed for LH, FS
H, progesterone, and estradiol-17 beta. The ovaries of all heifers wer
e examined daily using transrectal ultrasonography from the initial un
til the final or anovulatory estrous cycles to determine changes in gr
owth of follicles and corpora lutea. Anestrus was defined as failure o
f ovulation of the dominant follicle following luteolysis. When anovul
atory and initial estrous cycles in restricted heifers were compared,
mean concentrations of LH were lower (p < 0.01), and diameters of domi
nant follicles were smaller (p < 0.01); mean concentrations of estradi
ol-17 beta were also lower in the three days following luteolysis (P =
0.06), but concentrations of FSH appeared to be higher (p = 0.003); m
aximum diameters of corpora lutea were smaller (p < 0.001), but durati
on of luteal phases and concentrations of progesterone preceding luteo
lysis were similar (p > 0.10). In controls, no differences were found
between estrous cycles for any of these variables. It is concluded tha
t failure of ovulation, following reduced dietary intake, resulted fro
m insufficient circulating LH to stimulate maturation of the ovulatory
follicle.