Assessment of the Loss in Milk Yield due to Subclinical Mastitis tested with Electrical Conductivity

Abstract

The effects of subclinical mastitis on milk yield were studied by analyzing data on quarter-milk yields and data on mastitis tests.Six cows were milked for the experiment twice a month by using a separate quarter-milker for 4-6 months.The quarter difference of electrical conductivity (QdEC) or of the concentration of chloride (QdCI) was used as a criterion for subclinical mastitis, in place of the conventional cell count. The QdEC value of each quarter milk is the difference in conductivity between the lowest among 4 quarter-milks from an udder and that of the other quarters in an udder.The degree of subclinical mastitis of a quarter was expressed by the mean value of the QdECs. A decrease in milk yield in a quarter was expressed by the quarter-milk yield ratio, (R, %).This is the ratio of a sum of actual milk yield from an affected quarter to a presumed sum of milk yield which would be produced from this quarter if the quarter were healthy.The ratio R was calculated according to the following procedure on the basis of assumptions as described below.Each milk yield obtained by a series of milkings from a quarter was expressed as a percentage of the yield at the first milking time for that quarter.These milk yields in percent are denoted as the relative quarter-milk yield, (relative yield, yi).When the quarters in an udder are healthy, the relative yield of each quarter in an udder should change coincidentally from one milking to the next, notwithstanding that milk yields in weight differ or that yields diminish gradually, and the sum of the relative yields (%) for each quarter (Σyi) is assumed to be equal in an udder.Therefore, when a cow has at least one healthy quarter in an udder, its sum of the relative yield should represent the sum of all of the other quarters when they were healthy.Then, the quarter-milk yield ratio, R (%), which assesses the loss due to subclinical mastitis over a few months'period for a quarter, is calculated as a percentage of the sum of relative yield for a quarter (Σyi) to the sum for the healthy quarter in anudder (Σyn).The healthy quarter was chosen by comparing the mean of the QdEC for 4 quarters in an udder and the quarter with the lowest mean was selected as healthy.We found a negative, highly significant correlation between the mean of QdEC and the ratio R for 24 quarters in 6 cows;the correlation coefficient, r=-0.72, (P<0.001).The relationship expressed by a linear regression equation was;
y=-(2.556±0.531) x+99.724, where x is the mean of the QdEC (10^-4S), and y is R (%) for a quarter.
From the equation it was assessed that a 2.6% reduction in milk yield occurs for every 1×10^-4S increase in the mean of QdEC in the affected quarters.A similar relationship was found between the quarter difference of chloride (mM) and the ratio R (%).The correlation coefficient was;r=-0.75.In this paper, we discussed an example in an assessment on the milk loss in a herd, where frequency distribution of the mean of the QdEC for all quarters of the cows in a herd was known.