Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/21/2007
Publication Date: 12/1/2007
Citation: Echternkamp, S.E., Cushman, R.A., Allan, M.F., Thallman, R.M., Gregory, K.E. 2007. Effects of ovulation rate and fetal number on fertility in twin-producing cattle. Journal of Animal Science. 85(12):3228-3238. Interpretive Summary: The objective of this study was to evaluate the effects of ovulation rate and of fetal number and distribution within the uterus on pregnancy rate and on fetal survival in heifers (2.5 years of age) and mature cows. Cattle are primarily monovulatory and, thus, their reproductive rate is low compared with swine and poultry. Experimental results indicate that the production of twin births has the potential to increase reproductive efficiency in beef cattle by 20 to 30%. Twin ovulations are the first prerequisite for fraternal twin births, and continued long-term selection for twin ovulations in the MARC Twinner population has increased the frequency of twin ovulations as well as the frequency of triplet and quadruplet ovulations and of triplet births. Calves/parturition increased from 1.34 in 1994 to 1.56 in 2004, and the combined frequency of fraternal twin and triplet births is currently 62%. Because death of a twin or triplet bovine fetus within a shared placenta results in termination of the pregnancy, the correlated increase in twinning rate with increasing ovulation rate indicates that most of the females in this cattle population have the uterine capacity to gestate twin fetuses to term. In mature cows, in utero survival of twin fetuses was the same whether twins were gestated in separate uterine horns (bilateral twins) or both within one uterine horn (unilateral twins). However, twin calves gestated in separate uterine horns did have greater vigor and heavier body weight (especially male calves) at birth than twins gestated in only one uterine horn. Thus, the additional uterine space for bilateral twin fetuses appeared to improve fetal development, providing evidence for some maternal uterine or placental restrictions or both on twin fetal growth in mature cows. In contrast, pregnancy losses in heifers were increased when both twin fetuses were gestated within one uterine horn compared with separate horns, and most of this reduction had already occurred by day 75 to 135 of gestation. Also, total calf birth weight for twins increased with age of dam from 2 to 4 or 5 years of age. In contrast, pre- and post-natal mortality were increased for triplets regardless of age of the dam, but their death did occur later in gestation in mature cows. Thus, although most cattle can gestate twins to term, differences in maternal uterine capacity or placental development for twins and triplets appear to exist among cattle, especially for first-calf heifers, and the selection for twin births may have increased uterine support for the twin fetuses. Also, the additional uterine space for twin or triplet calves distributed between both uterine horns appears to enhance fetal development. Because of the increased fetal mortality and pregnancy losses associated with triplet pregnancies, it appears that little additional beneficial gains will be achieved from further increases in ovulation rate. Fertility losses of 20 to 30% also occur in single-ovulating females but their losses result primarily from conception failure or embryonic mortality (< 15 days postbreeding). Likewise, 20 to 30% of the individual ovulations in twin- and triplet-ovulation failed to produce a conceptus, reconfirming that anomalies in ovulation, oocyte quality, and embryos are major contributors to reproductive inefficiencies in cattle, regardless of ovulation rate.
Technical Abstract: Effects of ovulation rate and of fetal number and distribution within the uterus on pregnancy rate and fetal survival were evaluated in nulliparous (n = 1,331) and parous (n = 3,517) cattle selected for twinning. Cattle were divided into a spring (70 d) and fall (60 d) breeding season and bred by a combination of AI and natural service. Ovulation rate, pregnancy status, and fetal number and distribution were determined by transrectal, real-time ultrasonography of the uterus and both ovaries at the end of the breeding season. Pregnancy was reconfirmed by rectal palpation at 75 to 135 d of gestation. For heifers and cows combined, ovulation rate increased (P < 0.01) from 1.46 ± 0.4 in 1994 to 1.89 ± 0.4 in 2004; number of calves/parturition increased (P < 0.01) from 1.34 ± 0.3 to 1.56 ± 0.3, respectively, which included an increase in triplet and quadruplet ovulations and triplet births. Bilateral twin ovulations yielded proportionately more (P < 0.01) twin births than unilateral twin ovulations. Ovulation rate was greater (P < 0.01) in the fall than spring breeding season. Pregnancy rate at ultrasound diagnosis did not differ among females with one, two, or three ovulations (89.1 ± 0.7, 91.2 ± 0.7, or 91.5 ± 2.8%, respectively), but rates at calving decreased (P < 0.01) with increasing ovulation rate (85.1 ± 0.6, 82.7 ± 0.6, or 64.2 ± 2.7%, respectively). Pregnancy rate was lower (P < 0.01) after a twin or triplet birth than a single birth. For dams birthing twins or triplets, pregnancy rate was lower in the fall versus spring, but rates were similar between seasons for dams of a single (type of birth x season; P < 0.05). Cows </= 60 d postpartum had a decreased (P < 0.01) pregnancy rate compared with cows > 60 d regardless of type of birth. Maintenance of pregnancy to term differed (P < 0.01) among females diagnosed with one, two, or three fetuses (95.7 ± 0.6, 87.8 ± 0.8, and 54.9 ± 2.3%, respectively). Reduced survival of twin and triplet fetuses in heifers had occurred (P < 0.01) by d 75 to 135 of gestation and losses were greater (P < 0.01) for unilateral than for bilateral twins or triplets. Fetal survival in cows was not affected by uterine location and loss of twins or triplets occurred later in gestation. Thus, increased calf production from selecting for increased ovulation rate in beef cattle is tempered by increased fetal mortality partially associated with crowding of two or three fetuses within one uterine horn, especially in heifers.