ESTROUS BEHAVIOR AND THE ESTRUS-TO-OVULATION INTERVAL IN NELORE CATTLE (BOS-INDICUS) WITH NATURAL ESTRUS OR ESTRUS INDUCED WITH PROSTAGLANDIN-F2-ALPHA OR NORGESTOMET AND ESTRADIOL VALERATE

Estrous behavior and the estrus-to-ovulation interval are essential fo r estimating the best time to artificially inseminate cattle. Because these parameters are not well characterized in the Nelore breed (Bos i ndicus), the main purpose of the this study was to determine the estru s-to-ovulation interval in Nelore heifers and cows with natural estrus or with estrus induced by treatments with PGF2 alpha or norgestomet a nd estradiol valerate (NEV). The cows and heifers were observed contin uously (24 h a day) to determine the onset of estrus and to study estr ous behavior in the cows. Ten hours after the start of estrus the ovar ies were scanned every 2 h by ultrasonography to monitor the dominant follicle until ovulation. Blood samples were collected periodically to determine progesterone levels by RIA. Administration of PGF2 alpha (2 injections, 11 days apart) did not induce estrus in most Nelore femal es in spite of the presence of functional CL, indicated by progesteron e concentrations above 6.0 ng/ml in 25 of 28 animals. Treatment with N EV induced high sexual receptivity in cows (10/11), but only 66% ovula ted. Cows with natural or induced estrus exhibited behavioral estrus o f 10.9 +/- 1.4 h, and ovulation occurred 26.6 +/- 0.44 h (n = 26) afte r the onset of estrus. In most of the cows (53.8%) estrus began at nig ht (between 1801 and 600 h), and 34.6% it started and finished during the night. It is concluded that in Nelore females ovulation occurs app roximately 26 h after the onset of estrus. Additionally, estrous behav ior is shorter than in European breeds, and there is a high incidence of estrus at night, which makes it difficult to detect and, consequent ly, impairs Al in Nelore cattle. The observation that a high percentag e of Nelore females with an active CL did not respond to usual dosages of PGF2 alpha warrants further investigation. (C) 1998 by Elsevier Sc ience Inc.