Abstract
1. 1. Mammary blood flow was measured during the lactation period in two low- and two high-yielding dairy goats (peak milk yields approx. 1.5 and 3.6 kg/day respectively), using the ultrasound Doppler principle for determination of blood velocity in both milk veins (subcutaneous abdominal veins) of the animals, and ultrasound scanning for measurement of cross-sectional area of the veins. 2. 2. Milk vein cross-sectional area ranged from 0.11 to 0.38cm 2 in the four goats, with a close to significant ( P = 0.06) difference between the veins in the two sides of the animals. Cross-sectional area remained constant during the lactation period. Changes in mammary blood flow was therefore caused by changes in blood velocity. 3. 3. Milk vein blood velocities ranged from 4.4 to 34.7 cm/sec independently of the time of the day, and were of the same magnitude in the two sides of the animals. Except in one goat ( P = 0.1), blood velocity decreased significantly ( P < 0.01) with progressing lactation, during which period also milk yield was declining. 4. 4. In the two low-yielding goats, a positive linear relationship ( R 2 = 0.20) was found between milk yield and milk vein blood velocity, whereas a diminishing exponential relationship ( R 2 = 0.97) was found in the two high-yielding goats. At a given milk vein blood velocity, high-yielding goats obtained a higher milk yield and also responded to changes in blood velocity (up to approx. 15 cm/sec) with greater increases in milk yield than low-yielding goats. 5. 5. The ratio of mammary blood flow:milk yield decreased significantly ( P < 0.0001) with increasing milk yield from approx. 430:1 at 1 kg of milk to 250:1 at 3kg of milk. 6. 6. The possible explanations for the differences between high- vs low-yielding goats with respect to milk vein blood velocity and the reasons for the declining ratio of mammary blood flow:milk yield with increasing milk yield is discussed.
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