Effect of Once Daily Milking and Concurrent Somatotropin on Mammary Tight Junction Permeability and Yield of Cows

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Abstract
Six pairs of monozygous Friesian twin cows during late lactation were used to assess the effect of once daily milking and bST treatment on yields and tight junction permeability of mammary epithelial cells. During the first 7 d, all cows were milked twice daily; on d 8 through d 21, all cows were milked once daily, but one cow of each twin pair was treated daily with 20 mg of bST on d 13 through 21; and, finally, during d 22 through 28, all cows were again milked twice daily. Once daily milking, a common management practice in New Zealand, resulted in a small (7%) but significant decrease in milk yield. Treatment with bST increased milk yield by 19%, thereby exceeding the milk yield loss from once daily milking. The integrity of mammary tight junctions was assessed indirectly by measuring concentrations of plasma lactose and milk BSA. Once daily milking temporarily disrupted tight junction integrity, based on a 4- to 5-fold increase in plasma lactose and a 42 to 55% increase in the concentrations of milk BSA. In the present study, bST did not affect the permeability of mammary tight junctions.