Milk lactose has recently gained interest in the scientific community thanks to its association with udder health and the need of novel indicators for cattle breeding. Moreover, there is economic interest behind solid lactose worldwide for manufacturing purposes, especially food and pharmaceutical industry. This is fully demonstrated by the increased number of papers dealing with lactose in the last decade. Most publications report lactose percentage (LP) to be negatively correlated with milk somatic cell count (SCC), used as indicator of mastitis together with its score (SCS). On the other hand, lactose yield (LY) directly determines milk volume.
The overall aims of the present thesis were: i) to identify sources of variation of LP and LY; ii) to assess genetic parameters of LP and LY; iii) to estimate phenotypic and genetic associations of LP and LY with traditional traits (milk yield, fat percentage, and protein percentage), milk coagulation properties, freezing point, and minerals content; iv) to estimate genetic correlations of LP and LY with mastitis and other common health disorders; and v) to identify genomic regions coding for LP and LY, and search for overlaps with regions coding for SCS and mastitis.
Chapter 1 provides an overall picture of phenotypic and genetic variation of LP and LY in Italian Holstein breed, and of their associations with traditional milk-related traits, including SCS. In Chapter 2, the relationships of LP and LY with milk freezing point, minerals and coagulation properties predicted from milk mid-infrared spectra are presented and discussed. Chapter 3 deals with estimation of genetic correlations of LP and LY with major health disorders of dairy cows diagnosed by veterinarians, namely mastitis, ketosis, milk fever, retained placenta, and ovarian cysts. Finally, results of the genome-wide association study carried out for LP and LY are presented in Chapter 4.
Based on the results of the present thesis, LP is a potential indicator of udder health and mastitis resistance in dairy cattle. However, due to limited variability of LP at population level, some derived traits, e.g. based on lifetime LP trend in cow, should be further investigated and exploited in order to maximise response to selection in the long term.