Regulation of yield loss and milk composition during once-daily milking: a review

Once-daily milking (ODM) of ruminants results in loss of milk production. In cows this loss is very variable between individuals bur in recent short-term trials, on average, the loss was 13%. Full lactation studies indicate losses of 35-50% but documentation from some commercial farms using ODM suggests that losses are lower. ODM also results in changes in mammary permeability leading to changes in milk composition through increased influx of serum proteins and ions and increased efflux of lactose and potassium. Somatic cell count is also increased by ODM. Yield loss occurs through a combination of acute and chronic changes in mammary function. Anatomical and physiological characteristics which are likely to minimise production losses on ODM include production of a concentrated milk, possession of a large cistern, good drainage from the alveolar compartment and a low initial somatic cell count. Efficient removal of strippings, use of oxytocin and/or injection of bovine somatotropin all act to restore yield during ODM. It is proposed that the mechanism of the yield loss is through changes in mammary function initiated by changes in cell shape during alveolar filling. Central to these changes is an increased permeability of the udder through impairment of tight junctions between the alveolar cells. This effect appears to be linked to a mild inflammatory response which occurs in the last 12 h of a 24-h period of milk accumulation. The ability to further reduce production losses on ODM could lead to much wider adoption of ODM. Key to this is the development of methods to identify high-producing animals tolerant of extended milking intervals. (C) 1999 Elsevier Science B.V. All rights reserved.