|Van Vleck, Lloyd|
Submitted to: Journal of Animal Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: June 10, 1996
Publication Date: N/A
Technical Abstract: Estimates of cow and calf survival from a twinning experiment were used with a threshold level on an underlying normal scale to determine single or twin ovulations for 1, 3, and 6 estrous cycles prior to first breeding. Ovulation rate was considered perfectly correlated genetically with twinning rate. Thus, twinning rate would increase with selection for ovulation rate. This simulation compared response in ovulation rate in a herd with 376 cows and heifers mated and 300 calving. Initial herd was mated randomly to 12 bulls for 10 years to establish pedigrees. For next 20 years pedigree selection was practiced on bull calves (40 saved for a year) and sib/pedigree selection at one year of age (12 or 6 selected for mating to highest PBV cows). Selection was based on 1, 3, or 6 estrous cycles. With a threshold of .385 (initially .35 twin OR) and underlying heritability of .25, selection of 6 bulls on female measures on 1, 3 or 6 estrous cycles resulted in twinning rates of .79, .90, and .93 based on 25 replicates. With 12 bulls selected, comparable rates were .79, .87, and .90. Inbreeding accumulated to .06 with 12 and .11 with 6 bulls. Genetic means on normal scale showed larger response than rate on binomial scale. With heritability on normal scale of .125 corresponding twinning rates were .65, .73, and .79 with six and .60, .69, and .74 with 12 bulls. Responses to selection for a threshold of 1.282 (.10 twin OR) were less: with heritability = .25; (.54, .69, .73) and (.44, .66, .68); and with heritability = .125; (.27, .38, .46) and (.24, .34, .40) for 6 and 12 bulls selected. Inbreeding with selection was twice that for random mating to same number of bulls. These results suggest that increases in twinning in a herd selected for twinning should be expected.