Kidney function, but not nitrogen excretion, differs between Brown Swiss and Holstein dairy cows

Brown Swiss (BS) cows have greater urea concentrations in milk and blood compared with Holstein (HO) cows. We tested the hypothesis that BS and HO cows differ in kidney function and nitrogen excretion. Blood, saliva, urine, and feces were sampled in 31 multiparous BS and 46 HO cows kept under identical feeding and management conditions. Samples were collected at different lactational stages after the monthly DHIA control test-day. To test the glomerular filtration rate (GFR) and urea excretion, concentrations of creatinine and urea were measured in serum, urine, and saliva. As an additional marker to estimate GFR, we determined symmetric dimethylarginine (SDMA) in serum. Feces were analyzed for DM content and nitrogen concentration. Data on milk urea and protein concentrations and daily milk yield were obtained from the monthly DHIA test-day records. The effects of breed, time, and parity number on blood, saliva, urine, feces, and milk parameters were evaluated with the GLM procedure of SAS, with breed, time, and parity number as fixed effects. Differences between BS and HO were assessed by the Tukey-corrected t-test. Concentrations of urea, creatinine, and SDMA in serum, were greater in BS than in HO cows: 5.46 ± 0.19 versus 4.72 ± 0.13 mmol/L (urea), 105.96 ± 2.23 versus 93.07 ± 1.50 mmol/L (creatinine), and 16.78 ± 0.69 versus 13.39 ± 0.44 µg/dL (SDMA). We observed a greater urea concentration in BS cows (25.8 ± 0.7 vs. 21.8 ± 0.7 mg/dL) and protein content in milk (3.70% ± 0.08% vs. 3.45% ± 0.07%) than in HO cows. Urea and creatinine concentrations in urine and saliva did not differ between breeds. No differences between BS and HO were observed for milk yield, fecal DM, and fecal nitrogen content. Dry matter intake and BW were similar in BS and HO cows. Despite greater urea, creatinine, and SDMA concentrations in blood, as well as a higher milk urea content in BS compared with HO, respective concentrations in urine did not differ between breeds. In conclusion, our results demonstrate a lower renal GFR in BS compared with HO cows, thereby contributing to the greater plasma urea concentration in BS cows. However, estimation of nitrogen excretion via milk, urine, and feces does not entirely reflect nitrogen turnover within the animal.