Effect of milk performance and udder health on the zinc content of cow's milk

Milk is an important source of zinc (Zn), which performs various catalytic, structural, and regulatory functions in the human body. Milk yield, composition, and quality are considerably affected by the incidence of mastitis. The aim of this study was to determine the effect of milk performance and udder health on the Zn content of cow's milk. A total of 605 milk samples were collected from 32 Polish Holstein-Friesian (HO) cows (8 animals in lactations 1, 2, 3, and 4 each). Milk samples were taken once a week, between lactation days 7 and 90, and then once a month until the 10th month of lactation. The proximate composition of milk and the somatic cell count (SCC) were determined. The Zn content of milk was determined by atomic absorption spectrometry (AAS). In order to evaluate udder health, milk samples were classified as follows: <= 200 ths - healthy udder, 201 -400 ths - risk of mastitis, 401 -1 000 ths - subclinical mastitis, and > 1 000 ths somatic cells mL(-1) - severe subclinical mastitis. Two models of analysis of variance (ANOVA) were applied. The effect of lactation number (LN) and udder health on the Zn content of milk was evaluated in the first model. The effect of daily milk yield and the fat/ protein ratio on the Zn content of milk was evaluated in the second model. The average Zn content of milk was 4.6 mg dm-3. The concentration of Zn in milk was highest in the second week of lactation, and it decreased in subsequent weeks. Milk from the oldest cows (lactation 4) and the most productive cows had the lowest (P<0.05) Zn content. The fat/protein ratio, which is an indicator of energy balance in cows, was positively correlated with the Zn content of milk. The Zn content of milk increased (P<0.05) with increasing SCC. A high content of Zn in milk can be an additional indicator of mastitis in cows.