Epsilon Archive for Student Projects

Abstract

The aim of the present thesis is to describe the background and specifically investigate the effect of a single prolonged milking interval on cell traffic in the bovine mammary gland and on milk composition. The literature study of the thesis is slightly broader than this and
includes also other facts about the bovine mammary gland of interest and relevancy to the narrower subject of the research project. The reason behind performing the research project and writing the thesis is originating in questions raised in relation to practical observations in dairy herds. It has been noticed that after a technical stop in an automated milking system resulting in a prolonged milking interval (PMI), many cows show a short lasting increase in milk somatic cell count (SCC). This can influence the herd milk SCC and may result in reduced payment of the milk due to rules based on that increased milk SCC is associated with lowered milk quality. It is, thus, of economical importance for the producer. By
studying the cell traffic in the udder when the SCC is increased but under non-mastitic conditions it was expected that also new information could be gained about the cell traffic in the normal udder which has been poorly studied. The mechanisms behind these SCC peaks and whether they influence milk quality are not clarified.

In the research project 29 dairy cows milked twice daily were included. The cows were exposed to a single PMI of 24 hours by omitting one afternoon milking. Milk samples were
taken regularly during 1 week before and 5 days after the PMI and analyzed for SCC, percentage of PMNs, fat, protein, lactose, casein and FFA. The main effects of the PMI
were increased SCC and PMN proportion, most pronounced in the milkings during the first day. Interestingly, the proportion of PMNs was of similar size in both milkings day 1 although the SCC in morning milk was much lower than in the afternoon. Usually, the proportion of PMN is known to follow the different SCC in morning and afternoon milking,
respectively, well, with lower SCC and proportion of PMN in the morning. The output of number of cells per hour, a measure that is not influenced by a possible dilution effect of the large accumulated milk volume in the first morning after the PMI, showed that the highest recruitment rate of total cells and PMN occurred, between the first and second milking after the PMI even if the increase started already during the PMI. After the initial peak, cell counts declined but SCC remained higher while the proportion of PMNs declined to values lower than the baseline value during the rest of the study. Lactose content
decreased but in contrast to previous studies, fat and casein increased. The individual cow’s lactation stage prior to the PMI had a significant effect on the changes in milk composition. The alterations in milk composition were, however, numerically slight and did not impair
the milk quality. The first afternoon milk yield was reduced and, interestingly, remained lower than the baseline value throughout the study.

To conclude: The increased recruitment of PMN shows that there was an enhanced chemotactic activity in the milk already during the PMI without any obvious antigenic
challenge. Blood or damaged cells as sources of cytokines is not likely considering the decreased concentrations of serum proteins we observed in the milk and results from
previous studies indicating that a single PMI does not cause any cell damage. The results from this study indicate that the PMN infiltration after a single PMI is due to PMN
chemotactic factors that are different from the PMN chemotactic factors present in mastitic milk.