Increase of plasma eCG binding rate after administration of repeated high dose of eCG to cows.

Equine chorionic gonadotrophin (eCG) is still used to promote follicular growth in cattle and, more recently with an increased frequency of administration, in ovum pick-up protocols. The aim of this experiment was to verify the possible effect of high frequency of administration on the immune response to eCG. The profiles of eCG binding rate, in the blood of two groups (A, B) of 4 primiparous cross breed beef cows (3-3.5 years old) submitted weekly for 5 to 10 weeks to repeated high doses (1000-2000 IU) of equine chorionic gonadotrophin, are presented in this paper. A sensitive radiometric method was used to detect antibodies in plasma. The profiles clearly indicated a marked increase of eCG binding rate after 3 to 5 injections of the exogenous hormone to the females. The statistical analysis of the results established that treatments induced a significant increase (P < 0.01) in binding rates after 6 and 3 injections in group A and B respectively. These binding rates remained elevated for at least 1 week following the last injection and decreased afterwards. The values of plasma binding rates following repeated eCG administration differed significantly between groups (0.90+/-1.04 and 1.04+/-0.11 for groups A and B before treatment versus 11.77+/-0.92, 6.70+/-0.85 for groups A and B after treatment, P < 0.01) and from one cow to another (P < 0.01) with some cows presenting no significant immune response while others were more reactive against the hormone (at least 3 injections).