Reproduction in Domestic Animals - Facultad de Ciencias Veterinarias

16 t h International Congress on Animal Reproduction
20 Workshop Abstracts
In conclusion, both the number of animals in estrus at the same time
and parity affected expression of estrus, but not timing of ovulation
after estrus. These factors are not easy to influence. In practice,
intense estrus detection protocols or the use of pedometers could
improve estrous detection and subsequent timing of insemination.
WS13-2
Behavioral and endocrine responses to estradiol-17b (E)
in Holstein cows
Silvia, WJ*, Reames, PS and Hatler, TB
Department of Animal and Food Sciences, University of Kentucky, USA
Since 1960, there has been a decline in the duration of estrus in
Holstein cows from an average of 18h to less than 8h. The endocrine
factors regulating expression of estrus in cattle have been extensively
investigated yet many critical questions remain. Estradiol (E) clearly
is the endogenous hormone that induces estrous behavior. There is
experimental evidence to support the concept of a critical ‘threshold’
of E required for the expression of estrus. Higher concentrations had
no additional effect on intensity or duration of expression. In other
studies, the duration and intensity of estrus was related to the amount
of E present. Therefore, the first objective of this experiment was to
reexamine the concept of ‘threshold’ by administering different doses
of estradiol to ovariectomized cows. This also allowed us to meet the
second objective, to determine if individual cows differed in the
amount of E required to initiate estrus. The third objective was to
determine if the effects of E dose on the occurrence, timing and
duration of estrus were associated with similar effects on the
occurrence, timing and magnitude of the LH surge. Ovariectomized
Holstein cows (n=5) were used. In each replicate of the experiment,
each cow received an i.v. injection of E followed by a continuous i.v.
infusion of E designed to maintain 0, 3, 6, 9 or 12 pg/ml in blood.
This experiment was replicated so that each cow was treated with
each dose at least once. Three of the cows showed estrus at the 6, 9
and 12 pg/ml doses. The other two cows only showed estrus at the 12
pg/ml dose. Thus, cows differ in the amount of E needed to induce
estrus. Duration of estrus was less in cows that received the 6 pg/ml
dose (8.8 h) than in those that received the 12 pg/ml dose (17.1 h;
P