Differential somatic cell count in milk as criteria for assessing cows’ udder health in relation with milk production and components

The somatic cell count in cow's milk is used to control the inflammatory infection process and to assess the likelihood of subclinical and clinical mastitis. In the article, within the framework of experimental design of observations in a dairy cattle herd, the possibility in the cows’ mammary gland monitoring status, based on the total somatic cell count determination and proportion for lymphocytes and polymorphonuclear neutrophils (PMN) in raw milk is shown for the first time in Russia. The obtained results confirm the relationship between somatic cell count (SCC) and daily milk yield for lactating animals. The aim of this work is to assess the relationship between the number of somatic cells in milk and their differentiation by species with milk production, milk component traits, and the risk of progressing subclinical and clinical mastitis in Holsteinized Black-and-White cows. The work was carried out from June 2020 to May 2021 (an experimental herd of Holsteinized Black-and-White cattle, PZ Ladozhsky - a branch of the Ernst Federal Research Center for Animal Husbandry, Krasnodar Territory). The total sample in a data set was 313 animals; the number of milk lactation records was 1931. The analysis of milk components was carried out using an automatic analyzer CombiFoss 7 DC (FOSS, Denmark) based on express methods of infrared spectroscopy and flow cytometry. The following milk traits were studied: daily milk yield, percentage of fat, protein, casein, lactose, dry matter, dry skimmed milk residue, traces of acetone and beta-hydroxybutyrate (BHB), freezing point and acidity, fatty acids (FA), SCC, DSCC (fraction of lymphocytes and PMN in the total amount of cells). In order to indirectly assess the mammary gland condition of cows, animals in the herd were conditionally divided into four groups: A - SCC ≤ 200 thousand cells per ml, DSCC ≤ 70 %; B - SCC ≤ 200 thousand cells per ml, DSCC > 70 %; C - SCC > 200 thousand cells per ml, DSCC > 70 %; D - SCC > 200 thousand cells per ml, DSCC ≤ 70 %. Also, the following animal gradation was applied regardless of the probability of mastitis: two groups with DSCC ≤ 70 and DSCC > 70 %; four subgroups with SCC ≤ 200, 201-500, 501-1000 and ≥ 1001 thousand cells per ml. We used logarithmic (normalized) SCC scores according to G.R. Wiggans et al. (1987) approach. The individual economic value of the daily milk yield of cows was determined. For assessing effects of environmental factors and their elimination on daily milk component traits, the equation of generalized linear models (GLM) was used. Estimates of phenotypic means for milk features were obtained by the GLM-equation using the least squares method. The pairwise comparison between means was performed using Tukey’s test. Principal component analysis (PCA) was used to study the variability of milk composition depending on their formation in the animal organism in order to determine the most significant parameters that determine the productivity of dairy cows. Healthy individuals and animals with suspected mastitis (predisposed to the onset of infection) (groups A and B) had desirable features of milk production, the daily milk yield was 25.7-27.7 kg, the average economic efficiency of milk production was 714-744 rubles per day per cow. Cows assigned to groups C (subclinical or clinical forms of active mastitis) and revealed as D (chronic mastitis) had the milk component traits superior to other groups with a relatively lower daily milk yield. Animals with high SCC values as well as with a chronic form of mastitis were most susceptible to metabolic disorders or ketosis, regardless of DSCC. An increase in the fat percentage in milk by 0.18-0.37 % (p ≤ 0.001) for animals with SCC ≥ 1001 thousand cells per ml led to rise the share of saturated FA by 1.1-1.4 percentage points (p.p.), palmitic FA - by 0.4-1.2 p.p., medium-chain FA - by 1.0-1.4 p.p. An increase in the normalized scores of SCC by one point (limits from 1 to 10) led to a decrease in the daily milk yield by 0.6 kg, lactose percentage - by 0.062 p.p. and an increase in fat and protein by 0.090 and 0.055 p.p., respectively. Analysis of the main components revealed clear clusters for the protein and fat milk fractions, urea and fatty acids, acetone and BHB, freezing point and pH values, SCC and DSCC. A separate group included the daily milk yield and lactose percentage (together with ketone bodies) traits not related to other milk composition traits, thereby indicating the independent nature of the variability of these features. Further study of the relationship between the milk components synthesis in mammary gland and animal physiological status will make it possible to clarify the direction of selection in dairy cattle and define the genetic determination of milk production traits.