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The health, well-being and reproductive status of dairy cows, apart from profitability in production, should be the most important considerations in animal breeding. Normally though, genetic evaluation focuses mostly on the genotypic background which influences economically important traits such as milk yield and composition. However, in many countries selection for enhanced milk performance has caused a decline in cow reproduction and consequently, this has led to increased interest associated with non-production traits, including reproductive efficiency, longevity and health. Assessment of the effects of genetic markers on reproductive performance may provide important clues to pursue a balance between production and functionality. To date though, considerable research has been conducted on the association of genetic markers with milk yield and composition traits but the information about the effects of these markers on reproductive traits is still limited. Therefore, further genetic research investigating the health and reproduction traits is needed to achieve sustainable dairy production systems. In this context, the objective of the study by Dr S. Ardicli and colleagues was to examine the impact of nine polymorphisms located in the CSN2, CSN3, CSN1S1, CSN1S2, OLR1, LALBA, STAT1, DGAT1 and LGB genes, together with environmental factors including calving year, season and parity on reproduction traits in Holstein cows. See the explanation and definitions of these markers below. The results of their study was published in the South African Journal of Animal Science, Volume 49 of 2019, page 379 to 394, the title being: Evaluation of candidate gene effects and environmental factors on reproductive performance of Holstein cows.
The analysis was conducted on 165 Holstein-Friesian cows of which the average milk yield was in excess of 8 500 litre per lactation. The data of reproductive traits of four lactations were included in the analysis. Genotypes were identified using PCR-RFLP and in the statistical analysis the least squares of the GLM procedures were used.
The results indicate that the CSN2, CSN3, OLR1, DGAT1 and LGB genotypes influence certain reproduction traits in Holstein cows. The A1A1 CSN2 genotype had positive effects on days before first insemination and first insemination to pregnancy interval. The BB variant of CSN3 was associated with shorter gestation length, and the heterozygote genotype of OLR1 was associated with lower age at first calving. The DGAT1 KA genotype and LGB B allele had positive effects on calving interval and days before first insemination, respectively. Therefore, these genotypes appear to be potential candidates for the selection of reproduction trait improvement. However, further studies performed with larger populations may be needed to confirm these associations. Reproductive performance was virtually unaffected by STAT1 genotypes. Calving year with days before first oestrus, days open, and first insemination to pregnancy interval; season with days open and first insemination to pregnancy interval and calving interval; parity with days before first oestrus and days open were determined as significant environmental effects.
The results suggest that a balance between production and functionality must be pursued, and proper economic weights must be applied to every trait because selection for favourable milk-related genes is expected to influence the reproductive performance of dairy cattle.
Explanations/definitions: More than 95% of cattle milk proteins are coded by six genes: the two main whey proteins, α-lactalbumin (α-LA) and β-lactoglobulin (β-LG) are coded by the LALBA and LGB genes and the four caseins β-CN, κ-CN, αS1-CN and αS2-CN by the CSN2, CSN3, CSN1S1 and CSN1S2 genes, respectively. The bovine LALBA gene, mapped to chromosome 5, and the LGB gene, mapped to chromosome 11, are candidate genes for milk production and composition. The CSN2, CSN3, CSN1S1 and CSN1S2 genes belong to the casein (CN) gene family. They are situated on chromosome 6, and are highly relevant in relation to milk yield and composition traits. The oxidised low-density lipoprotein receptor (OLR1) gene, which encodes surface receptors of vascular endothelial cells, is involved in fatty acid transport and plays an important role in the regulation of the oxidised form of low-density lipoproteins. The signal transducer and activator of the transcription 1 (STAT1) gene encodes the cytoplasmic transcription factor (STAT1) that acts as a regulator in cytokine signalling pathways and cellular functions involving proliferation, differentiation and apoptosis. STAT1 is located on chromosome 2 and is associated with improved milk yield and content. The diacylglycerol O-acyltransferase 1 (DGAT1) gene encodes the microsomal enzyme (DGAT1) in triglyceride synthesis, and it has recently become a highly interesting target for evaluating the fat-related traits including milk.