Cows have the capacity to gestate twin calves, but not without decreased survival and body weight due to uterine crowding, researchers at USDA's Meat Animal Research Center (MARC) conclude.

The effects of increasing fetal numbers (twins and triplets) and their distribution between the left and right uterine horns were evaluated for calf survival, calf bodyweight at birth and weaning, gestation length, dystocia and calf sex ratio. Unilateral gestation refers to development in the same uterine horn; bilateral refers to separate uterine horns.

From 1994 to 2004, 1,587 single-born calves, 2,440 twins, and 147 triplets born to first-calf heifers and cows in the Twinner population at MARC were studied. Cattle were equally distributed between the spring and fall breeding seasons.

Fetal number and distribution in utero were determined by real-time ultrasonography at 40-70 days post breeding. Results found:

• From 1994 to 2004, the number of calves per calving increased from 1.34 to 1.56.
  
• Gestation length was 6.8 days shorter for twins compared with singles (277.5 ± 0.2 vs. 284.3 ± 0.2 days), and 12.7 days shorter for triplets (271.6 0.8 days).
  
• As fetal numbers increased, survival and birth weight of individual calves decreased, but total calf bodyweight produced per cow increased.
  
• Twins resulting from bilateral twin ovulations had increased survival and bodyweight at birth, longer gestation length and less dystocia than twins resulting from unilateral twin ovulations.
  
• Calf survivability and bodyweight: Singles: 97.2% ± 0.3% and 105.2 ± 0.2 lbs. Bilateral twins: 92.0 ± 0.4% and 85.9 ± 0.4 lbs. Unilateral twins: 83.2 ± 0.4% and 80.9 ± 0.4 lbs. Bilateral triplets: 73.8 ± 1.4% and 67.5 ± 1.5 lbs. Unilateral triplets: 51.9 ± 3.2% and 69.9 ± 3.5 lbs.
  
• Birth weight of single calves increased by 1.12 lbs./day for each additional day of gestation vs. 0.84 lbs./day for individual twins.
  
• Calf bodyweight increased with age of dam from 2 to 4 years old.
  
• Twins and triplets had a greater incidence of dystocia than single births.
  
• Ratio of male:female calves (0.52:0.48) was not affected by type of birth. Postnatal calf survival was similar for all three types of birth.
  
• Total progeny bodyweight at weaning by type of birth was: single (479.9 ± 5.5 lbs.), twin (723.7 ± 7.1 lbs.) and triplet (834.2 ± 33.1 lbs.).

Researchers conclude that the production of twin births has the propensity to increase reproductive efficiency in beef cattle by 20-30%. However, a portion of the potential gain from twinning in cattle is compromised by reduced calf survival at birth, lighter bodyweight of twin progeny at birth and weaning, and an increased incidence of dystocia associated with abnormal presentation of twin fetuses within the birth canal. Continued selection for the increased frequency of twin ovulations and births has increased the frequency of triplet ovulations and births, but triplet births provide little additional production benefits compared with twin births.

(S.E. Echternkamp, et al, 2007. Journal of Animal Science, 85:3239.)