Frequency of leukochimerism in Holstein and Jersey twinsets

Increased heterozygosity in low-pass sequencing data allows identification of blood chimeras in cattle

In about 90% of multiple pregnancies in cattle, shared blood circulation between fetuses leads to genetic chimerism in peripheral blood and can reduce reproductive performance in heterosexual co-twins. However, the early detection of heterosexual chimeras requires specialized tests. Here, we used low-pass sequencing data with a median coverage of 0.64× generated from blood samples of 322 F1 crosses between beef and dairy cattle and identified 20 putative blood chimeras through increased levels of genome-wide heterozygosity. In contrast, for 77 samples with routine SNP microarray data generated from hair bulbs of the same F1s, we found no evidence of chimerism, simultaneously observing high levels of genotype discordance with sequencing data. Fifteen out of 18 reported twins showed signs of blood chimerism, in line with previous reports, whereas the presence of five alleged singletons with strong signs of chimerism suggests that the in-utero death rate of co-twins is at the upper limit of former estimates. Together, our results show that low-pass sequencing data allow reliable screening for blood chimeras. They further affirm that blood is not recommended as a source of DNA for the detection of germline variants.

Degree of leukochimerism with calf age in Angus-crossbred twin sets

Twinning in cattle is infrequent and usually undesired. It can result in an increased occurrence of abortion and dystocia, reduced calf survival and a high likelihood of freemartinism in mixed-sex twins. Twin gestations are also commonly associated with the formation of placental vascular anastomoses (PVA) between twins. Through PVA they share blood, hormones (leading to freemartinism in mixed sex twins) and hematopoietic stem cells, which are the progenitors of white blood cells. The sharing of stem cells between twins can result in leukochimeric twin sets. These are twins that have white blood cells derived from both self and the co-twin owing to the fetal migration of hematopoietic stem cells from the extraembryonic mesoderm of the yolk sac to final sites like bone marrow and thymus. This study examined the degree to which this leukochimerism changes with age. DNA was extracted from hair bulbs containing mesenchymal dermal papilla to determine the individual's true genotype and blood samples were obtained at six time points from 1 week to 8 months of age to assess leukochimerism. Samples were genotyped using a medium density SNP chip, and quantitative estimates of allele frequency were determined using SNPs for which members of a twin set had alternative homozygous genotypes. The results indicate statistically significant changes in the proportion of self and co-twin with age and suggest that by 2-4 months of age the genotypic mix in white blood cells represents the hematopoetic stem cell population resident in the individual (i.e. permanently found in thymus and bone marrow).

Effect of Genetic and Environmental Factors on Twin Pregnancy in Primiparous Dairy Cows

Twin pregnancies are highly undesirable in dairy cattle; they compromise the health and wellbeing of a cow and dramatically impair the farm economy. Recently, a genomic prediction for twin pregnancies has been developed. The objective of this study was to assess environmental and management risk factors affecting the incidence of twin pregnancies in high-producing dairy cows in their first lactation, with a special emphasis placed on the genomic prediction values of twin pregnancy. Our study population of primiparous cows proved valuable in identifying factors other than genomic predictive values that influence twin pregnancy rates. The odds ratio for twin pregnancies was 0.85 (p < 0.0001) for each unit of a prediction value increase, 3.5 (p = 0.023) for cows becoming pregnant during the negative photoperiod, and 0.33 (p = 0.016) for cows producing ≥42 kg of milk at AI, compared with the remaining cows who produced <42 kg of milk. As a general conclusion, the practical implication of our findings is that genomic prediction values can identify the risk of twin pregnancy at a herd level. Given the cumulative effect of genomic selection, selecting animals with a reduced genetic risk of twin pregnancies can contribute to reducing the incidence of twin pregnancies in dairy herds.

Clinical management of pregnancy-related problems between days 28 and 60 in the dairy cow

The biological and economic impacts of pregnancy loss in dairy herds are well recognized. This review examines clinical aspects of late embryonic/early fetal loss of non-infectious cause in the dairy cow. The period of interest spans from shortly after observation of at least one embryo with a heartbeat upon pregnancy diagnosis, at around Day 28 of pregnancy (late embryonic period), until approximately Day 60 of pregnancy (early fetal period). This last time point is when pregnancy is firmly established and beyond which the risk of pregnancy loss is greatly reduced. We particularly focus on the role of the clinician in managing a pregnancy and discuss findings to predict the viability of a pregnancy, available therapies for expected pregnancy problems and possible implications of the newer technologies.

Short communication: Heritability of twinning rate in Holstein cattle

Multiple births or twinning in cattle is a naturally occurring reproductive phenomenon. For dairy cattle, twinning is considered a detrimental trait as it can be harmful to cow and calf as well as costly to the producer. The objective of this study was to examine recent US calving records for the Holstein breed to determine a current estimate of heritability for twinning rate along with effects of season and parity. Two models were used in this study: a linear sire model and a binary threshold-logit sire model. Both were mixed models considering fixed effects and random effects. Analyses were conducted using a restricted maximum likelihood method. Heritability estimates were 0.0192 ± 0.0009 and 0.1420 ± 0.0069 for the linear and threshold models, respectively. Repeatabilities from the linear and threshold-logit models were 0.0443 ± 0.0012 and 0.2310 ± 0.0072, respectively. The nonzero estimates of heritability indicate the potential to select against this trait for genetic improvement of Holstein cattle.