RELATIONSHIP BETWEEN GENETIC MERIT AND BIOLOGICAL RESPONSE IN LACTATING COWS

The research reported in this thesis refers to two studies, that were conducted in commercial farms. Part 1 deals with the association between estimated breeding values (EBVs) and hematological, milk and urine markers of metabolism and well being of dairy cows after the peak of lactation. The study 1 used 153 lactating cows, selected from 3 Simmental (IS) and 2 Holstein (IH) commercial farms. Cows were ranked according to the EBVp from minus to positive and selected every 5 EBVp values from minus to positive. Milk samples were individually collected and analysed for protein, fat, lactose,(BOHB), cortisol contents and somatic cell count (SCC). Blood was sampled after the morning milking and before the morning meal, and urines were sampled after stimulation of micturition. Blood samples were analysed for total protein, albumin, globulin, urea, glucose, creatinine, aspartate aminotransferase (AST), haemoglobin (Hb), non-esterified fatty acids, BOHB, glutathione peroxidase (GPx), total antioxidant status (TAS), cortisol and Zn, and urine for total N, urea and purine derivatives (PD) N. Significantly higher milk (P < 0.001) and fat (P < 0.01) yields and significantly lower body condition score (BCS), protein percentage, SCC, urea and BOHB (P < 0.001) were observed for IH cows in comparison to IS. Significant differences were observed for plasma creatinine (P < 0.001), higher in IS, and for AST (P < 0.05), Zn, TAS and GPx (P < 0.001), higher in IH cows. The concentrations of N (P < 0.01), creatinine N and the creatinine N to N ratio (P < 0.001) in urine were significantly higher in IS than IH cows. Instead, the PD N to total N (P < 0.05) and PD N to creatinine N ratio (P < 0.001) were lower in IS than in IH cows. A positive linear effect of EBVp was observed for milk yield, milk protein and fat yields (P < 0.05) for IS and IH cows. The EBVp was negatively related to BCS and glucose in IS cows (P < 0.05) and to plasma BOHB in both breeds (P < 0.05). EBVp was negatively related (P < 0.05) to urinary urea to total N and PD N to total N ratios in IS cows and to PD N to creatinine N ratio for IH cows. Biomarkers of metabolism measured in blood, urine and milk, often used to assess the energy and nitrogen balance during peripartum, can also be used to understand differences related to the breed and to the genetic merit. Study 2 was conducted on 10 commercial farms, 6 of IS and 4 of IH, for a total number of 1200 lactating cows. For these animals, EBV for milk fat (EBVf) and protein (EBVp) yields and for somatic cell count (EBVc) were associated with markers of metabolism and well being in blood and with cortisol in milk and hair. The results confirmed the negative association of BOHB with EBVp (P < 0.01) for both breeds and also a positive linear association with EBVf (P < 0.01). Differences between breeds were also confirmed for plasma creatinine, that was significantly higher (P < 0.01) for IS than IH. However, cortisol in hair and milk was not related to EBVs. In part 2, cows were grouped on the base of SSC (class1 4000000). The aim was to investigate the relationship between cortisol concentrations, in milk, blood or hair, and the SCC, considering also the effect of farm, milk yield and days in milking (DIM). For study 1, a total of 135 cows were sampled from 2 commercial farms of Italian Simmental (IS) cows and 2 commercial farms of Italian Holstein (IH), whilst in study 2, a total of 1041 cows were sampled from 6 commercial farms of IS and 4 commercial farms of IH. In study 1, the values of cortisol content in milk were higher in IH than IS cows. Significant effect (P < 0.01) was shown between farms for milk and plasma cortisol concentrations. Cortisol content in milk was not correlated to plasma content in study 1 and the mean milk to plasma cortisol ratio was about 1:30. In study 2, significantly higher values of milk cortisol for IH cows in comparison to IS cows were reported (P < 0.01). A significant effect of class of SCC was observed, cows belonging to class 3 showed significant higher milk cortisol content. The mean values of milk cortisol were 6.15 for class 1, 6.14 for class 2 and 6.27 for class 3. Acute phase proteins in plasma significantly differ for class of SCC. Haptoglobin and ceruloplsmin in plasma were significantly higher (P < 0.05) for class 3 in comparison to class 2 3 and to class 1. Paroxonase and albumin were signicantly lower (P < 0.05) in class 3 than in the other classes of SCC. The farm effect was significant also in the study 2, confirming the results obtained in the first study. Biomarkers of metabolism and oxidative stress were also in line with the EBVs and in particular BOHB is a promising compound to measure also in mid lactation, when subclinical ketosis is not more a potential problems for the lactating cows, since contains genetic information other than nutrition relationship. Instead, biomarkers of animal health, as acute phase proteins in plasma and cortisol in plasma, milk or hair, are more related to environment and, probably, the complex regulatory mechanisms of these compounds does not allow to link them directly to genetic background. These biomarkers, and milk cortisol in particular, are promising compound to measure to complement the assessment of animal welfare protocols