Scholarship & Research

The objective of this study was to determine the temporal relationship of ovarian follicles and corpora lutea (CL) on the time to onset (TO) and duration (D) of behavioral estrus in PGF 2 alpha- treated ewes. Forty normally cycling Western White-Faced ewes were randomly assigned to one of eight treatments to be either sham-operated (SO) or ovariectomized (OVX) at 36, 44, 52 or 60 h after a PGF 2 alpha injection given on day 12 of the estrous cycle. At the time of injection (Oh) and every 4 h afterwards for 96 h, each ewe was observed for estrus using a teaser ram. At surgery, ovaries of each ewe were macroscopically examined for number and size (mm) of antral follicles and CL. Ovaries of OVX ewes were prepared for histological evaluation. Actual size and number of follicles and CL were determined by taking photographic slides of stained tissue sections and projecting them onto a paper screen. Follicles were classified as atretic (A) or non atretic (NA), and CL as normal or regressing. Proportion of ewes in estrus was lower (P<.05) for OVX ewes (11 of 20; 55%) than for SO (20 of 20; 100%) and there was a time by treatment interaction (P<.05). Mean TO of estrus was 48.2 + or - 3.6 h for OVX ewes which showed estrus and 52.2 + or - 2.4 h for SO ewes (P>.05); and the overall mean was 50.7 + or - 1.9 h. Duration of estrus was 25.6 + or - .8 h for OVX ewes which showed estrus and 32.6 + or - 3.2 h for SO ewes (P>.05); and the overall mean was 29.8 + or - 2.5 h. There were no treatment effects for either TO or D of estrus (P>.05). At 44, 52 and 60 h fhe number of actual large NA (LNA) follicles was negatively associated with TO of estrus (r= -.97, -.73, -.87, respectively) and positively associated with D of estrus (r-.76, .70, .65). Actual CL volume was positively associated with D of estrus (r=.84, .58, .68) and negatively associated with TO of estrus (r= -.93, -.36, -.39). Total number of actual antral follicles did not change over time (P>.05), nor did number of follicles in the small (1-3.9mm), medium (4.0-5.9mm) and large (>6.0mm) size groups (P>.05). There was a shift in class of follicles such that number of medium NA follicles decreased and number of LNA follicles increased from 36 to 52 h (PC.05). CL size was smaller at 60 h (P<.05), while CL volume remained constant (P>.05). These results indicated that there is a temporal relationship of the ovary for the TO and D of estrus in that the ovary is required for at least 44 h, probably longer, for estrus to occur. However, once estrus has been initiated the presence of the ovary is no longer required for the normal expression of estrus. Individual variation in the TO and D of estrus may be related to differences in the number of LNA follicles and (or) CL volume.

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