Postpartum ovarian activity and response to progestagen treatment in rustic beef cows (Spanish Retinta) underfed to simulate their typical ranging conditions

Retinta cattle, a rustic beef breed raised in free-ranging conditions in the Southwest of Spain, are adapted to seasonal pasture scarcity but exhibit a prolonged calving interval. This study characterized the ovarian activity and response to progestagen treatment (Synchromate-B type) in feed-restricted, postpartum Retinta cows. Forty cows were fed a restricted diet so that competition among cows resulted in a range of body condition scores (BCS) of 2 through 6 on a 1-9 scale. On the 90th postpartum day, a Norgestomet ear implant was administered to Treated cows (n = 20) for 9 days, followed by a 48-hour period of calf withdrawal. The remaining 20 cows served as Controls. For both groups, daily to weekly blood samples were collected from the 2nd to the 9th month postpartum and assayed for 17-[beta]-estradiol and progesterone. Intact bulls were used for breeding and estrous detection. Calves were weaned at 7-7.5 months of age. Twenty-seven cows showed estrus (on the 16-35th week postpartum) and 24 cows were confirmed pregnant (on the 19-35th week postpartum) by the end of the experiment. Treatment neither affected the conception rate nor the interval from calving to first estrus or to conception. High-BCS cows had greater conception rate and resumed estrous cyclicity and conceived earlier than low-BCS cows. Cows remaining nonpregnant by weaning had a lower and more rapidly decreasing BCS than cows conceiving before weaning. The weekly mean of cumulative progesterone release up to the 3rd week of pregnancy was not affected by treatment but it was greater and increased more rapidly in high- versus low-BCS cows. Peak progesterone values were considerably higher than the values usually reported for other beef cattle breeds. In summary, the Retinta cows in this study exhibited a marked refractoriness to estrous induction/synchronization treatment and had a prolonged postpartum anestrus either at low or adequate BCS levels. Further, undernutrition prolonged the postpartum interval to estrus and conception