Treatment with 17 beta-oestradiol does not influence age and weight at puberty in Bos indicus heifers

The working hypothesis was that treatment of heifers with 17 beta-oestradio l (E-2) during specific periods of prepuberty would reduce the response of the hypothalamic-pituitary axis to E-2 negative feedback and induce an earl ier onset of puberty. The effects of chronic treatment with exogenous E-2 a dministered at specific maturational phases on the age and weight at pubert y were studied in 96 prepubertal Brahman (3/4-7/8 Bos indicus) heifers (187 .0 +/- 3.3 days of age, mean +/- SEM), weighing 149.9 +/- 2.5 kg. Heifers w ere randomly assigned to one of six groups (n = 16 per group). Groups 2-6 r eceived E-2 implants (Compudose 200(R)) for 90-day periods starting at 10, 13, 16, 19 and 22 months of age, while animals in group 1 remained untreate d. Implants were placed subcutaneously at the base of the ear. Blood was co llected for progesterone (P-4) determination by radioimmunoassay (RIA) and the animals were weighed at monthly intervals from 6 to 15 months then week ly from 15 to 28 months of age. Puberty was defined by concentrations of P- 4>1 ng/ml in plasma and identification of a corpus luteum (CL) by transrect al ultrasonography (Aloka 210DX:7.5 MHZ probe). Treatment with exogenous E- 2 at any of the ages/treatment intervals evaluated in this study did not re duce age or weight at puberty (P > 0.7). The mean age and weight at puberty of control heifers was 735.3 +/- 19.7 days (range: 597-861) and 299.2 +/- 10.2 kg (range: 233-382), respectively, which is greater than the age and w eight at puberty of 481 days and 246 kg, that was previously reported for B . indicus heifers [Post, T.B., Reich, M.M., 1980. Puberty in tropical breed s of heifers as monitored by plasma progesterone. Proceedings of the Austra lian Society of Animal Production 13, 61-62.]. The large variation in age a nd weight at puberty that was observed in the present study among heifers m ight indicate an individual animal effect to E-2 treatment among some of th e treated animals. The lengthy interval from birth to puberty observed in t his study, as compared to other studies, reflects the effects of other fact ors such as genotype, environmental or nutritional influences on puberty. ( C) 1999 Elsevier Science B.V. All rights reserved.