A:   The U. S. Meat Animal Research Center has been studying twinning for a number of years in what they call their twinning population. The base herd was established some 25 to 30 years ago with 96 cows of various breeds. All base cows had two or more twin calvings. Since then, female replacements have been selected over four to five generations for production of twin births. From 1994 to 2004, ovulation rate increased from 1.46 to 1.89, being higher in fall than spring breeding. There were more twins from bilateral (egg ovulated from toe ovaries) ovulations than unilateral (eggs ovualted from the same ovary). Pregnancy rates were lower after cows produced twins or triplets. Number of calves per parturition increased from 1.34 to 1.56, and there were more triplets than early in the study. There were 38% single births, 58% twins, and 4% triplets. Gestation length was 6.8 days shorter for twins and 12.7 days shorter for triplets, so birth weights were lower accordingly. Calving difficulty was higher with multiple births. Calf survival at birth was 97% for single, 89% for twin, and 70% for triplet, but beyond that there was no difference in survival to weaning. Total weaning weight per cow calving was 479 lb for singles, 722 lb for twins, and 832 lb for triplets. So, increased calf production from multiple births was accompanied by higher fetal mortality. There might be some opportunity for increased production with multiple births, but only under very high levels of management.

So there appears to be females within some breeds that have a greater ability for multiple ovulations. This may be a function of some hormones that are produced that stimulate more than one ovulation. At the MARC they selected not only from the female side, but also used males as sires that produced females that had a greater ability to ovualte more than one egg during estrus or heat. Since 1981, cattle at MARC were selected for fraternal twin births using estimated breeding values for twinning calculated from repeated measurements of ovulation rate in all heifer progeny starting at about 12 months of age, from twinning rate in the selected females, and from progeny testing and subsequent assortive mating of progeny proven sires. Twinning rate increased from 4% in 1984 to 52% in 2001, an increase of 3% per year, as a result of an increase in the frequency of twin and triplet ovulations. Although a twin/multiple ovulation is the first and limiting prerequisite for multiple births in cattle, fetal mortality was increased with twin and, especially, triplet fetuses.