Abstract
The onset of lactation results in a sudden irreversible loss of Ca for colostrum and milk synthesis. Some cows are unable to quickly adapt to this demand and succumb to clinical hypocalcemia, whereas a larger proportion of cows develop subclinical hypocalcemia that predisposes them to other peripartum diseases. The objective of this study was to perform a comprehensive genomic analysis of blood total Ca concentration in periparturient Holstein cows. We first performed a genomic scan and a subsequent gene-set analysis to identify candidate genes, biological pathways, and molecular mechanisms affecting postpartum Ca concentration. Then, we assessed the prediction of postpartum Ca concentration using genomic information. Data consisted of 7,691 records of plasma or serum concentrations of Ca measured in the first, second, and third day after parturition of 959 primiparous and 1,615 multiparous cows that calved between December 2015 and June 2020 in 2 dairy herds. All cows were genotyped with 80k SNPs. The statistical model included lactation (1 to 5+), calf category (male, females, twins), and day as fixed effects, and season-treatment-experiment, animal, and permanent environmental as random effects. Model predictive ability was evaluated using 10-fold cross-validation. Heritability and repeatability estimates were 0.083 (standard error = 0.017) and 0.444 (standard error = 0.028). The association mapping identified 2 major regions located on Bos taurus autosome (BTA)6 and BTA16 that explained 1.2% and 0.7% of additive genetic variance of Ca concentration, respectively. Interestingly, the region on BTA6 harbors the GC gene, which encodes the vitamin D binding protein, and the region on BTA16 harbors LRRC38, which is actively involved in K transport. Other sizable peaks were identified on BTA5, BTA2, BTA7, BTA14, and BTA9. These regions harbor genes associated with Ca channels (CACNA1S, CRACR2A), K channels (KCNK9), bone remodeling (LRP6), and milk production (SOCS2). The gene-set analysis revealed terms related to vitamin transport, calcium ion transport, calcium ion binding, and calcium signaling. Genomic predictions of phenotypic and genomic estimated breeding values of Ca concentration yielded predictive correlations up to 0.50 and 0.15, respectively. Overall, the present study contributes to a better understanding of the genetic basis of postpartum blood Ca concentration in Holstein cows. In addition, the findings may contribute to the development of novel selection and management strategies for reducing periparturient hypocalcemia in dairy cattle.
Highlights
The period around parturition is critical for dairy cows because of the metabolic adaptations required to adjust to parturition and lactation when cows transition from the pregnant and nonlactating state to the nonpregnant and lactating state
Most studies have evaluated hypocalcemia as a binary trait using records based on clinical symptoms of severe hypocalcemia
The incidence of milk fever is low, and this binary trait does not capture cows with subclinical hypocalcemia. In this context, blood Ca concentration measured after parturition is a better indicator trait to evaluate periparturient hypocalcemia in dairy cows
Summary
The period around parturition is critical for dairy cows because of the metabolic adaptations required to adjust to parturition and lactation when cows transition from the pregnant and nonlactating state to the nonpregnant and lactating state. Most metabolic disorders in dairy cattle occur in the first weeks of lactation, such as milk fever, ketosis, and displaced abomasum (Bell, 1995; Goff and Horst, 1997). Periparturient hypocalcemia is a complex metabolic disorder that occurs at the onset of lactation because of a sudden irreversible loss of Ca incorporated into colostrum and milk (Horst et al, 1997). The economic losses for the dairy industry due Cavani et al.: GENOMIC ANALYSIS OF POSTPARTUM BLOOD CALCIUM to periparturient hypocalcemia are substantial, with a total cost estimate of $246 per case, because of reduced milk production, extended days open, and early culling, among others (Liang et al, 2017)
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