SummaryBetween March and August 1968 to 1970, Chlormadinone acetate (CAP), 10 mg/head/ day, was administered to 668 Zebu cows and heifers for oestrous cycle synchronization for 14, 12, or 9 days, the latter schedule supported by an injection of 5 mg estradiol valerate on day 2 to achieve luteolysis. Applied orally over a period of 14 days, CAP provided a reliable method for cycle synchronization. The severe anoestrous rate experienced in all herds reduced the overall percentage of synchronized animals. Fertility from insemination during the first synchronization period (day 2 to 10 after treatment) was slightly, yet insignificantly reduced, but was completely restored during the second synchronization period (day 20–30). Insemination only during this second synchronization period resulted in conception and pregnancy rates similar to those combined from the first and second synchronization period.Of factors tested for influencing synchronization and fertility, presence or absence of penis-deviated teaser bulls (psycho-stimulation) had no effect on the percentage of animals synchronized, or on post-treatment fertility. In well-managed herds, the percentage of animals responding to treatment was predetermined by the number of animals already cycling, reflecting their phase of reproductive life. Levels of management had an important impact on the success of cycle synchronization combined with artificial insemination. Both methods are managerial tools for improvement of already good management. Their failure under insufficient management conditions has diagnostic value.Possibilities and limits shown for oestrous cycle synchronization in Zebu cattle were similar to those reported in European cattle. Within these limits, CAP can be employed successfully as a means for cycle synchronization, providing sufficient synchronization and acceptable fertility and allowing economical use of artificial insemination.
Global risk of heat stress to cattle under climate change.