|Gregory, Keith - RETIRED; 5438-01-30|
Submitted to: Journal of Animal Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: June 29, 1998
Publication Date: N/A
Technical Abstract: Selection of cattle using breeding values based on yearling ovulation rate and twinning rate of individuals and their relatives has increased twinning rate about 12% per generation. To determine associated responses in ovarian weight, follicular development, and corpus luteum (CL) number and weight, comparisons were made between ovaries of selected (S; n=24) and unselected (US; n=24) mature cows. Ovaries were collected immediately after slaughter at 0, 24, 48 or 72 h after a single injection of 30 mg prostaglandin F2alpha on d 17 (Estrus=d 0). Visible antral follicles were categorized by diameter as small (< 5 mm), medium (6 to 12 mm) or large (> 12 mm); CL (both active and regressing) were removed and weighed separately. Total CL weight decreased (P>.01) from 4.9 g at 0 h to 2.5 at 48 h and 1.8 at 72 h (SEM=.3). Ovaries of S cows were heavier (22.7 vs 16.1 g, SEM=1.2; P<.01) and contained more small (63.0 vs 43.6, SEM=5.2; P<.05), medium (7.9 vs 3.6, SEM=1.3; P<.05), and large (1.6 vs 1.1, SEM=.1; P<.01) follicles than ovaries of US cows, respectively. Number of CL (1.5 vs 1.1, SEM=.1; P<.01) and total CL weight (3.6 vs 3.0 g, SEM=.2; P<.05) were also increased for S cows compared to US cows. Number of large follicles increased (P>.01) from 1.0 at 0 h to 1.8 at 48 h (SEM=.1), primarily due to a greater number in S cows (population x time; P=.06). Results suggest that long-term selection for twin births in cattle has increased the size of the small follicle pool and the number of growing and ovulatory follicles.