Cylindrical rumen boluses of a size (55 mm length x 18 mm diameter, de
nsity 2.9 g cm(-3)) appropriate for ruminating calves over 75 kg livew
eight were constructed. The mean daily nutrient releases from two rume
n boluses measured in rumen-cannulated cows were 62 mg Cu, 0.56 mg Se,
1.08 mg Co, 1.14 mg I, 38 mg Mn and 55 mg Zn with vitamins 3140 IU A,
628 IU D-3 and 9 IU E. In a 123-day experiment with housed dairy calv
es given hay and a barley/soyabean meal mix, administration of two rum
en boluses significantly increased plasma copper and blood glutathione
peroxidase levels compared with unsupplemented control calves. There
were similarly significant responses in a 143-day experiment with suck
led beef-cross calves treated with two rumen boluses at grass. In both
circumstances the basic diets provided inadequate allowances of both
copper and selenium.