Changes in serum luteinizing hormone (LH) concentrations in response to luteinizing hormone releasing hormone (LHRH) in bull calves that attained puberty early or late

The objectives of this study were to determine if the response to luteinizing hormone releasing hormone (LHRH) could be used to select bull calves capable of early sexual maturation and to establish the optimum route and dose of LHRH. In Trial 1, at 4, 10 and 20 week of age, 20 calves were treated iv with 2 μg/kg body weight of LHRH 1 and 5 h after commencing a 9-h period of blood sampling. Bulls were separated into early and late maturing (n = 10), based on age at puberty (scrotal circumference (SC) of ≥28 cm). At 4 and 20 week of age, peak serum LH concentrations and area under the LH response curve in response to LHRH were lower (P < 0.05) in early- versus late-maturing bulls. In Trial 2, calves at 20 week of age were given LHRH as follows: 2 μg/kg body weight iv (n = 6), im (n = 6) or sc (n = 6); 5 μg/kg im (n = 6), or ischio-rectally (ir, n = 6) or sc (n = 6); and 10 μg/kg im (n = 6) or sc (n = 6). Serum LH concentrations were at a plateau from 30 to 165 min after treatment with 5 μg/kg of LHRH (im or ir; P > 0.05). We concluded that the LH responses to LHRH in calves at 4 and 20 week of age could facilitate the development of a simple test (one blood sample prior to treatment with LHRH and a second during the period of sustained response to LHRH) to select early-maturing bulls.