A study of diurnal rhythms in milk and fat production in the cow : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Agricultural Science in the University of New Zealand
When cows are milked under commercial conditions milk yields are normally greater in the morning than in the evening, mainly because a longer time interval precedes the morning milking. In addition the percentage of fat in the milk is generally lower in the morning and higher in the evening. The alternation of high milk yields of low fat percentage, with lower milk yields of higher fat percentage, form a diurnal rhythm. This will depend partially on the length of time between morning and evening milkings, for the difference in yield and fat percentage is reduced as the time intervals approach equal length. There is left in the udder after a normal milking an appreciable quantity of milk of high fat percentage which has been termed "residual milk". In a hypothetical case if the volume and fat percentage of this milk remained constant after each milking, then the yield obtained from the udder would represent synthesis, and any diurnal rhythm would be caused solely by synthesis. However, neither the amount, nor the fat percentage of the residual milk remains constant at morning and evening milkings. Thus the diurnal rhythm may be attributed to two general causes, firstly synthesis, and superimposed on to this a diurnal variation in residual milk.