|
The variability of milk production during a lactation can be seen directly in the daily yields. However, because high producing dairy cows develop an energy deficit especially in early lactation, which can have far-reaching consequences, it is sensible to breed for a low production yield in the first lactation weeks. Simultaneously, a good persistency can improve the overall yield to satisfy the farmers' and consumers' demands. From mammogenesis to actual milk synthesis in the mammary glands, many time points and mechanisms take part in the physiological time-dependent expression of milk. The largest complexes involved in milk production cover hormone and growth factors, feed intake and digestion, cell activity, nutrient transportation and synthesis. Despite these various starting points that impact upon milk production, only few genes have been identified. This could be due to many association studies using averaged yields and therefore not being able to detect genes that are only relevant during a specific time-point. We developed an approach to carry out association studies in regards to specific lactation periods. These periods were defined as the yield increase before the maximum, the persistency, and the overall level of production. With this approach we were able to show that the DGAT1 gene expresses its known genotype effects only after lactation day 40, and that several groups of markers are significant for the three different production periods. Further, gene enrichment studies showed that genes with different functions were accumulated around significant markers. Such gene groups were involved in the regulation of cell growth, gene expression and transcript regulation, the energy supply of cells, fat and protein synthesis, and immune response, which were specific for individual periods. Time-dependent association studies can provide new insights into the regulation of milk production. We could asses the processes of milk production that will be altered when breeding for significant markers. Genomic selection could incorporate time-dependencies and thus selectively change the production curve to improve the overall yield as well as animal health and fertility. |
|