Effect of bleeding stress and variable suckling intensity upon serum luteinizing hormone in Brangus heifers

In the first experiment, the effect of the stress of blood collection (via tail vessel puncture) on serum luteinizing hormone (LH) was evaluated in six nonsuckled first calf Brangus heifers. The animals were bled on days 22 and 31 postpartum at 15 minute intervals for a period of two hours. Blood was processed to yield serum and analyzed for LH via radioimmunoassay (RIA). There were no significant differences or fluctuations in serum LH levels between bleeding periods or between cows. Serum LH concentrations in nonsuckled cows were not affected by the stress of blood collection. In the second experiment, 24 first calf Brangus heifers were randomly assigned to one of four treatment groups. Treatment 1 cows were suckled once daily for approximately 30 min starting day 21 postpartum. Treatment 2 cows were suckled twice daily for approximately 30 min each time, starting 21 days postpartum. Treatment 3 cows were suckled once daily for approximately 30 min starting 30 days postpartum. Treatment 4 cows were suckled twice daily for approximately 30 min each time starting 30 days postpartum. Each cow was bled via tail vessel puncture on days one and nine following the start of each treatment. The blood sampling regime was similar to that used in Experiment 1 and consisted of four presuckling samples taken at 15 min intervals, one midsuckling sample (the calf was allowed to suckle for 15 min) and four postsuckling samples taken at 15 min intervals. Blood was collected, processed to yield serum and assayed for LH via RIA. Suckling intensity (SI) was found to have a significant effect on serum LH levels. The once daily suckled cows had higher (P<.01) mean serum LH levels than did the twice daily suckled cows (1.70 +/- .03 and 1.53 +/- .03 ng/ml, respectively). The LH concentrations decreased (P<.01) from the first to last bleeding time (BT). The mean serum LH levels for the presuckling, midsuckling and the first postsuckling samples were higher (P<.05) than the last postsuckling sample. The mean serum LH level for the first time period prior to suckling was higher (P<.05) than the last postsuckling sample. The mean serum LH level for the first time period prior to suckling was higher (P<.05) than the last two periods after suckling (1.73 +/- .08 ng/ml vs 1.51 +/- .06 and 1.41 +/- .06 ng/ml). Bleeding day (BD) and weaning day (WD) did not alter serum LH levels. The interactions found to be significant (P<.01) were SIxBD, SIxWD, BDxWD and BTxSIxBDxWD.