Increased teat wall thickness in response to machine milking.

ABSTRACT

Transiently increased teat wall thickness in response to machine milking has been documented by various methods, including ultrasound. However, correlative ultrasonography and histology to detect the origin of this phenomenon is lacking. The first goal of the present study was to evaluate and compare milking-related changes of the teat tissue in 2 breeds of dairy cows (11 Simmental and 3 Holstein) using B-mode ultrasonography. Additionally, the observed changes were compared with ultrasonographic findings in a Holstein cow with periparturient udder edema. Finally, corresponding histological sections of the Simmental teats were analyzed and compared with those from a lactating nonmilked Angus cow. We hypothesized that the mechanical load of both stretching by the vacuum during phases of open teat cup liner and compression by the closed liner during machine milking results in a transient congestion of blood vessels in the teat wall. The barrel of 1 front teat of each cow was scanned immediately before and after machine milking (system vacuum 42 kPa; pulsation rate 60 cycles/min; pulsation ratio 6535). Shortly after milking (33 ± 6 min), the Simmentals were slaughtered, and their scanned teat was immediately removed and processed for investigation by light microscopy. Ultrasonography after milking revealed anechoic tubular structures mainly in the inner half of the teat wall. Histological examination revealed these structures to be thick-walled veins. The left front and hind teats of the nonmilked lactating cow, collected and prepared identically to those from the Simmental cows, showed the same histological features. Ultrasonographic measurements showed that the diameter of these veins significantly increased after milking compared with matching images before milking. This effect was most pronounced in the Holstein cows. Similarly, these veins were very prominent in the periparturient cow. However, neither the milked cows, including the periparturient cow, nor the lactating nonmilked cow provided any evidence of edematous extravasation on ultrasonography or histology. These findings corroborated our hypothesis that the increase in size of thick-walled veins in the teat tissue is the main reason for the thickening of the teat walls in response to machine milking.