Genetic selection for bovine chromosome 18 haplotypes associated with divergent somatic cell score affects postpartum reproductive and metabolic performance.

Infectious diseases, such as metritis and mastitis, have  serious economic consequences due to the use of antibiotic treatment, the withdrawal period thereafter, delayed onset of cyclicity after calving, extended intervals between calving, and poor fertility . Most high-yielding dairy cows struggling with such diseases also have metabolic disorders, and some results have shown a connection between immunological defense and metabolic regulatory mechanisms. One expects that future, it will be important to reconcile economic efficiency and ethical requirements by breeding cows that are high-yielding as well as healthy and long-lived. Genetic selection may offer a sustainable tool to improve dairy cattle farming via the selection of animals with reduced susceptibility toward diseases, which in turn should result in improved after-calving health. In this regard, linkage studies have shown promising results between bovine chromosome 18 (BTA18) and performance traits. Also, in udder health, the prediction of half-sib heifers with a high or low somatic cell score (SCS) is possible through marker-assisted selection, since a quantitative trait locus (QTL) for SCS exists in the region of BTA18.

To follow up on these findings, the aim of a study by Dr M M Meyerholz and colleagues was to compare the after-calving performance of two Holstein Friesian half-sib heifer groups genetically selected via single nucleotide polymorphism (SNP) genotyping for alternative paternal chromosome 18 haplotypes associated with favourable (Q) or unfavourable (q) effects on udder health, respectively. In this context, they examined whether this selection influenced the incidence of after-calving reproductive and metabolic diseases, in addition to the concentrations of selected blood parameters in the respective heifer groups. Their results were published in the Journal of Dairy Science, Volume 102 of 2019, page 9983 to 9994. The title of their paper is: Genetic selection for bovine chromosome 18 haplotypes associated with divergent somatic cell score affects postpartum reproductive and metabolic performance.

Thirty-six heifers (18 Q and 18 q) were monitored from 3 weeks before calving until necropsy on day 39 (± 4 days) after calving. Health status and rectal temperature were measured daily, and body condition score and body weight were assessed once per week. Blood samples were drawn twice weekly, and levels of insulin, non-esterified fatty acids, insulin-like growth factor-I, growth hormone, and β-hydroxybutyrate were measured, and the difference between groups Q and q statistically compared.

The results showed that Q-heifers had reduced incidence of metritis compared with q-heifers and were less likely to develop fever. Serum concentrations of β-hydroxybutyrate were lower and insulin-like growth factor-I plasma concentrations were higher in Q- compared with q-heifers. However, the body condition score and withers height were comparable between the haplotypes, but weight loss tended to be lower in Q-heifers compared with q-heifers. No differences between the groups were detected concerning retained foetal membranes, uterine involution, or onset of cyclicity.

In conclusion, selection of chromosome 18 haplotypes associated with a reduced SCS resulted in a decreased incidence of after-calving reproductive and metabolic diseases. The presented data clearly showed promise for selection against SCS and provided evidence that such selection strategy is unlikely to impair immune function. It also added to the existing knowledge aimed at avoiding negative consequences of genetic selection strategies in dairy cattle. However, the underlying causal mechanisms modulated by haplotypes in the targeted genomic region and immune competence necessitate further investigation.