The measurement of the effects of inherent and environmental factors on the lactose content of the milk of individual cows and of the herd bulk milk in a number of commercial herds

Abstract

Summary(1) In a previous paper (Walsh, Rook & Dodd, 1968) a scheme was devised in which the potential milk lactose content for individual cows was predicted from the observed potassium-to-lactose ratio in the milk, and the difference between the predicted potential and the actual milk lactose contents was partitioned into effects due to age, changes with stage of lactation and inter-quarter difference. The scheme was based on repeated analyses for potassium and lactose of milk from the separate quarters of the udder of each animal at intervals throughout a single lactation, and was applied to 2 commercial herds. A simplified scheme, in which analyses are confined to milk secreted in mid-lactation and in which the measurement of the effect of changes with stage of lactation is omitted, has now been applied to a further 6 commercial herds. The results are reported here, together with those for the mid-lactation period for the 2 herds studied previously.(2) The potassium and lactose contents of the milk of uninfected quarters of the heifers in 4 of the 6 herds and of the second-lactation animals in the 5th herd, which had no heifers, were found to conform with the relationship established previously (Walsh & Rook, 1964) on which the prediction of potential lactose content is based. The values for heifers in the 6th herd did not conform, and the results for this herd are therefore not reported.(3) The range of herd mean values for SNF content was 8·17–8·62% and for lactose content 4·33–4·72%. The overall range for all herds for the predicted potential lactose content of the milk of individual cows was 5·06–5·71 (g/100g milk water). Herd mean values for predicted potential lactose content were, however, generally similar, ranging from 5·240 to 5·486 (g/100g milk water).(4) The range of values for individual cows and the herd mean values for the effects of age and inter-quarter difference showed important differences from herd to herd. Herd mean values for the effect of age varied from −0·028 to −0·266 (g/100g milk water), and there was a similar range, of −0·074 to −0·251 (g/100g milk water), for the effect of inter-quarter difference. In herds where the effect of age was high, the effect of inter-quarter difference also was high.(5) The effects of inter-quarter difference and of udder infections were similar. The loss in milk lactose content was about twice as high in quarters infected with streptococci as in quarters infected with staphylococci.(6) Estimates of herd values for potential lactose content and for the combined effects of age and inter-quarter difference based on analyses of herd bulk milk agreed with corresponding estimates based on analyses of the milk of individual animals within the herd.(7) The importance of variation in milk lactose content as a source of variation in SNF content and the relative importance of predicted potential lactose content and the effects of age and inter-quarter difference as a source of variation in milk lactose content are discussed.