A SOLUTION FOR USE IN ASSESSING THE CELL COUNT OF COW'S MILK

A study of methods for estimating the cell content of bulk milk

Four methods of estimating the cell content of bulk milk were studied, correlated statistically and their degree of reproducibility estimated. The relative merits of the 4 methods are discussed, and the need for defining the object of testing before selecting a test is stressed. If designed for use in a programme for mastitis control, or the control of milk quality based on cell content, each country must choose a screening test or a precise cell-counting technique, depending on the availability of laboratory resources, extension advisory services and other economic factors.

Screening tests studied were the California Mastitis Test (CMT) and the Brabant Mastitis Test (BMR), the former showing a correlation coefficient of 0·817 against the Electronic Cell Count (ECC) method in 320 samples. The more objective BMR correlated well with the ECC method, showing a correlation coefficient of 0·987 for 406 samples. Direct microscopic counts (DMC) were correlated with a centrifugal method and with a chemical method of preparing samples for electronic cell counting. For 430 samples in each case the correlation coefficients obtained were 0·930 and 0·966 respectively. The 2 ECC techniques were compared using 430 samples, and a very close relationship was found (r=0·988).

For regular examination of large numbers of milk samples using the electronic cell counter we consider the chemical method of sample preparation more suitable than the centrifugal method. Differences in the reproducibility and accuracy of both tests are negligible, but the former procedure is quicker, more economical in terms of cost, and more adaptable to a national testing programme.

Measurement of the quantity of lactose passing into mammary venous plasma and lymph in goats and in a cow

Lactose in the plasma of 12 lactating goats and one cow with healthy udders has been measured by a sensitive and specific enzymic assay. Mean values (±S.E.) for arterial and venous concentrations were 52±11 µM and 70±11 µM respectively. Simultaneous measurement of mammary blood flow, milk yield and milk lactose concentration showed that 2·70±0·25% (mean±S.E.) of the total lactose production passed into the plasma. This rose to 42·7% in a goat when milking was suspended for 46 h. No lactose was found in the plasma of male and non-lactating female goats, but it was detected 4 days before parturition in a nulliparous goat.

The effect of very frequent milking and of oxytocin on the yield and composition of milk in fed and fasted goats

1. The effect of milking goats 1-4 times an hour for 3-12 hr on the yield and composition of milk has been studied in fed and fasted animals at all stages of lactation.2. It was essential to inject oxytocin (50-400 m-u. I.V.) just before each milking to remove all the milk already in the udder and then the yield was similar to that obtained on twice daily milking (105 +/- 2.1 S.E.%). There were no significant differences between goats or between the two glands of one goat, even if one had been denervated by autotransplantation. However, the variation from hour to hour was 1.5 times greater than from day to day.3. The claim of Zaks (1964) and Zaks, Natochin, Sokolova, Tanasichuk & Tverskoy (1965) that milking every 15 min always produces a large rise in milk Na and a fall in K and lactose, which is characteristic of alveolar milk, is not substantiated. In high yielding goats milking gently by hand or with a cannula caused a small change in K only, but vigorous hand milking exacerbated this fall and also caused a fall in lactose and a rise in Na and Cl. Still larger changes were produced by using excessively large doses of oxytocin (2500 m-u.) when there was also a rise in citrate and total nitrogen. Hourly milking in goats fasted for 24 hr had the same effect.4. In fasted goats the milk yield fell to 90% within 8 hr and to 56 +/- 2.1% of the previous level by 24 hr. It remained at this level for a further 10-12 hr on twice daily or on hourly milking. The yields of autotransplanted glands usually fell slightly but significantly more than that of the glands in situ. In most goats mammary blood flow was halved but in all animals there were large falls in mammary uptake of glucose, acetate and amino acids and greatly increased uptake of free fatty acids. There were significant differences between fasted goats on hourly milking.5. It is concluded that, in spite of changes in milk composition, milking hourly can be a useful technique for studying milk secretion. The striking effects of a short fast in a lactating animal are emphasized.