A Single Prolonged Milking Interval

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

Is there a special mechanism behind the changes in somatic cell and polymorphonuclear leukocyte counts, and composition of milk after a single prolonged milking interval in cows?

<p>Abstract</p> <p>Background</p> <p>A single prolonged milking interval (PMI) e.g. after a technical stop in an automated milking system is of concern for the producer since it is associated with a short-lasting increase in milk somatic cell count (SCC), which is a major quality criterion used at the dairy plants. The content of polymorphonuclear leukocytes (PMN) and how the milk quality is influenced has not been much investigated. The SCC peak occurs without any obvious antigen challenge, possibly indicating a different leukocyte attraction mechanism after a PMI than we see during mastitis.</p> <p>Methods</p> <p>Composite cow milk samples were taken at the milkings twice daily during 7 days before and 5 days after a PMI of 24 h. Milk was analyzed for SCC, PMN, fat, protein and lactose, and at some occasions also casein and free fatty acids (FFA).</p> <p>Results</p> <p>During the PMI the proportion of milk PMN increased sharply in spite of marginally increased SCC. The peak SCC was not observed until the second milking after the PMI, in the afternoon day 1. However, the peak SCC value in <it>morning </it>milk did not occur until one day later, concomitantly with a <it>decrease </it>in the proportion of PMN. After declining, SCC still remained elevated while PMN proportion was decreased throughout the study as was also the milk yield, after the first accumulation of milk during the PMI. Milk composition was changed the day after the PMI, (increased fat and protein content; decreased lactose, whey protein and FFA content) but the changes in the following days were not consistent except for lactose that remained decreased the rest of the study.</p> <p>Conclusion</p> <p>The PMI resulted in increased SCC and proportion of PMN. Additionally, it gave rise to minor alterations in the milk composition in the following milkings but no adverse effect on milk quality was observed. The recruitment of PMN, which was further enhanced the first day <it>after </it>the PMI, appeared to be independent of milk volume or accumulation of milk per se. Hence, we suggest that there is a special immunophysiological/chemoattractant background to the increased migration of leukocytes into the milk compartment observed during and after the PMI.</p

Effects of a single prolonged milking interval in cows

A single prolonged milking interval (PMI), such as might be associated with technical failures in automatic milking systems, leads to a subsequent somatic cell count (SCC) peak in bulk tank milk. Increased SCC indicates mastitis in cows. It is generally correlated with reduced milk yield and quality, and is often used as a criterion for milk payment. Whether the transient SCC peak after a single PMI affects yield and quality is not known. The short duration of the inflammatory reaction after a PMI and its nonpathological history raise questions about the underlying immunological mechanisms and possible role of immunocompetent natural milk components. It is important to learn more about this kind of physiological inflammation to improve the interpretation of SCC and the general understanding of udder immunology. This thesis investigated the inflammatory reaction after a single PMI of 24 h at both the cow and the quarter level, and how it influences milk yield and quality. The PMI appeared to induce temporarily impaired epithelial integrity but no epithelial damage. Even so,milk yield per cow was reduced with 0.75 kg/day after the PMI, notably for up to the 10 days studied. Thus, based on the long-lasting reduction in yield and a possibly reduced milk payment due to the SCC peak, a PMI is of significant economic concern for the farmer although milk quality itself was shown not to be afflicted. The PMI caused a two-fold increase in the SCC, which then remained elevated for 2 days and was associated with an increased proportion of polymorphonuclear leukocytes (PMN) in milk. The most pronounced SCC reaction after the PMI was not seen until the udder had been emptied once. The initiation of inflammation occurred first during the PMI and elicited a systemic acute phase response of serum amyloid A (SAA), observed in blood prior to its appearance in milk. The milk showed consistently high chemotactic activity in vitro, although no increased content of the cytokines IL-1β and IL-8 was detected. The PMI induced significant alterations in the content of α-lactalbumin (ALA) and prolactin (PRL), in relation to the PMN reaction. ALA inhibited and PRL stimulated PMN migration, when tested in vitro. Based on the findings in this thesis, it is probable that SAA played a significant role in the inflammatory reaction after a PMI but it cannot be excluded that ALA and PRL might also have been contributing factors