Effect of oral drenching with zinc oxide or synthetic zeolite a on total blood calcium in dairy cows

Danish Holstein dairy cows in late lactation and milked in the morning only were used as a model for dry pregnant cows to determine the effect of oral drenching with zeolite A and zinc oxide, respectively, on total serum calc ium. Ten cows were assigned randomly to two groups of five cows each, given either synthetic zeolite A (group A) or zinc oxide (group B). Blood sample s were drawn daily at 10 a.m. and 10 p.m. during the whole experiment, and total serum calcium was determined. Daily fluctuations in blood calcium wer e recorded, with morning values being consistently lower than evening value s. Oral drenching with a single dose of zinc oxide of 100 mg/kg of body wei ght as well as with zeolite in doses of 500 g of zeolite/cow twice a day fo r 2.5 d was reflected in serum calcium levels. In the group given zeolite A , there was a depression in evening values of total serum calcium although the difference did not reach statistical significance. It was followed by a n increase above baseline level ("overshooting"). This was interpreted as a response from the calcium homeostatic mechanisms. In the group given a sin gle dose of zinc oxide, a decrease in total serum calcium occurred. This de crease was not followed by overshooting, indicating that the single treatme nt with zinc oxide did not stimulate the calcium homeostatic mechanisms. Th e perspective of this first attempt to reduce dry cow ration calcium availa bility may be seen in relation to difficulties in formulating dry cows rati ons from home grown forage sufficiently low in calcium to elicit a hypocalc emia protective response at calving.