Milo:

I enjoyed reading your comments. It appears a number of important 
papers are on the horizon regarding follicular and luteal development 
that will give us additional insight into luteal function.

Regarding the necessity of a future ovulatory follicle for the PGF
to induce a good CL we did inject the cows on Day 7 of the cycle. 
Cows were also injected with PGF on Day 14 of the cycle.  There was
no difference in the plasma progesterone (daily sampling day 2 to 14 
of the cycle). This would suggest that on both days injected there 
was a robust follicle, at least sufficient to produce a CL that 
maintains plasma progesterone. However, animals injected on day 7 
came in estrus 24 hours earlier than animals injected on day 14 
indicating a more developed follicle as you suggested...but no 
difference in progesterone. These data are from 60 animal bleeds from 
day 2 to 14 of the induced or natural cycle in Brahman heifers in the 
one experiment.  


However, there is a contradiction in our data.  Heifers injected on
day 7 and 14 resulted in 50% and 100% of the animals 
expressing estrus. One might expect a larger response 
from day 7 if the follicle was more robust as indicated by the 
interval to estrus.  Based on your comments it would have been 
informative to have determined if there were differences in plasma 
estrogen between the responders and nonresponders...did the 
nonresponders have a less robust follicle? It may be in our study the 
responders had a robust follicle and thus no difference in plasma 
progesterone.  

What happened to the 50% of heifers that did not respond to PGF. The
plasma progesterone decline after injection was not different from
the responders (those that expressed estrus). However, the plasma
progesterone did not quite reach a low baseline, it subsequently
recovered to pretreatment concentrations and the heifer had a normal
21 day cycle expressing an estrus as if there was no day 7
injection. We solved the practical problem by administering two
injections of PGF at a 24 hour interval resulting in 100% of the
animals expressing estrus.  The approach has worked well in the 
field.

Any suggestions from yours or other studies as to why the 
low-incomplete response was seen when animals were injected on day 7?

michael j fields


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Dr. Michael J. Fields
Professor of Physiology
Animal Science Department             Homepage:  http://gnv.ifas.ufl.edu/~irbweb/
459 Shealy Drive                      E-mail  :  fields@animal.ufl.edu
University of Florida                 Phone   :  (352)392-9566
Gainesville, Fl 32611-0910            FAX     :  (352)846-1073                                                                         
                                                                                                                   
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