EFFECT OF EXOGENOUS SOMATOTROPIN ON HEMATOLOGICAL VARIABLES OF LACTATING COWS AND THEIR OFFSPRING

Eighty-two lactating Holstein cows in their first, second, or third la ctation received either one, three, or five concurrent i.m. injections of a unit dose (0.6 g) of zinc methionyl bovine somatotropin (bST) or five doses of the vehicle. Injections were administered at 14-d inter vals from GO +/- 3 d postpartum until the end of lactation or until ne cropsy. Thirty-eight cows were continued on the treatment for a 2nd yr . Blood samples were collected at wk -2, -1, 3, anti 7 relative to the start of treatment and then every 8 wk (yr 1) or 4 wk (yr 2) thereaft er. Untreated cows that were included in a survey of the resident herd were bled at wk 7 or 8, wk 10 or II, and wk 13 or 14 of lactation and every 4 or 8 wk thereafter. Calves were bled within 72 h of birth and at approximately 5 wk Of age. Most parameters associated with erythro cytes were decreased mildly in cows that were treated with bST. Howeve r, data remained within generally accepted reference ranges, and chang es were not of clinical importance. Decreased hematocrit was not assoc iated with increased hemolysis, hemodilution, or clinical anemia. No m orphological lesions related to treatment were noted in the bone marro w or spleen; bST did not affect the incidence of immature cell types. Energy and protein balances did Hot significantly affect the hematolog ical results of the cows. Calves generally were unaffected by bST trea tment of the dam, but heavier calves had higher parameters associated with erythrocyte and lymphocyte counts than did calves with lower body weight. Exogenous bST treatment caused predictable changes in hematol ogical parameters of dairy cows.