Incidence of precocious puberty in developing beef heifers.

The objectives of this study were to determine the incidence of precocious puberty and effect of exposure to a bull on the incidence of precocious puberty in beef heifers. The experiment was conducted using 120 heifers during 1990 and 1991. Heifer calves were randomly assigned to be pastured in the presence (n = 30 heifers/yr) or absence (n = 30 heifers/yr) of a bull starting at 140 +/- 14 d of age for the duration of the study. Heifers were 402 +/- 14 d of age at the time the study was completed. Blood samples were collected weekly to determine the age of onset, duration and cessation of luteal function. Heifers were considered to have exhibited a precocious puberty when the onset of luteal function was before 300 d of age. There was no effect of exposure to a bull on the incidence of precocious puberty, and therefore the data were pooled. The incidence of precocious puberty was greater (P < .02; X2 = 5.5) in 1990 (25.0%; 15 of 60) compared with 1991 (8.3%; 5 of 60). There was no effect of year on age of precocious puberty (194 +/- 12.4 d of age), duration of cyclic luteal function (65 +/- 10.5 d), or the age at resumption of anestrus (260 +/- 15.3 d of age). Precocious puberty does occur in developing beef heifers; as many as 25% of heifers have transient luteal function before 300 d of age. However, exposure to a bull has no effect on the incidence of precocious puberty.