Weaning may be associated with negative energy balance and body weight loss when calves are still immunologically immature, predisposing them to infectious diseases. The aim of the present experiment was to investigate the effects of treatment of preweaning dairy calves with recombinant bovine somatotropin (rbST) on the somatotropic axis, selected immune parameters, and hematology of calves around weaning. Thirty-six Holstein female calves were randomly assigned to receive 1.5 to 1.8 mg of rbST (Posilac, Elanco Animal Health, Greenfield, IN) per kilogram of body weight or to receive injections of saline (saline solution 0.9%, Valley Vet Supply, Marysville, KS) every 7 d from 21 to 63 d of life. Calves were fed milk replacer ad libitum from birth to 38 d of age (d −11), when progressive weaning started, and calves were weaned at 49 d of age (d 0). Calves were weighed at birth and weekly from 21 to 63 d of age, when wither height also was measured. Calves were vaccinated with 0.5 mg of ovalbumin on study d −28 and −7. Blood samples were collected on d −28, −25, −21, −11, 0, 3, 7, and 14. Polymorphonuclear leukocytes were isolated and challenged ex vivo with Escherichia coli to determine phagocytosis and oxidative burst capacity. Additionally, expression of cluster of differentiation (CD)62L and CD18 by granulocyte, lymphocyte, and CD14+ monocyte were determined. Blood samples were also used to determine hematological parameters and concentrations of growth hormone, insulin-like growth factor-1, insulin, glucose, fatty acids, β-hydroxybutyrate, haptoglobin, and anti-ovalbumin IgG. Calves treated with rbST had greater concentrations of growth hormone and insulin-like growth factor-1 from d −25 to 14 than control calves, whereas insulin, fatty acid, and β-hydroxybutyrate concentrations did not differ. On d −11, glucose concentration was greater for rbST-treated calves. Treatment did not affect polymorphonuclear lymphocyte phagocytosis and oxidative burst, but intensity of expression of CD62L and CD18 by granulocytes tended to be increased by rbST treatment. Treatment did not affect the concentration of anti-ovalbumin IgG in serum. Haptoglobin concentration was reduced in rbST treated calves on d 3 and we noted a tendency for hematocrit to be lower in rbST-treated calves. Treatment did not affect body weight, wither height, and average daily gain, despite the fact that rbST-treated calves had lower daily milk replacer intake. The relatively minor improvements in immune responses resulting from rbST treatment of weaning calves may not be sufficient to reduce the incidence of infectious diseases.

当犊牛在免疫学上还不成熟时，断奶可能与负能量平衡和体重减轻有关，使它们容易感染传染性疾病。本实验的目的是研究用重组牛生长激素（rbST）处理断奶前的犊牛对生长轴，所选免疫参数和断奶前后犊牛血液学的影响。将三十六只荷斯坦雌性小牛随机分配为每公斤体重接受1.5至1.8 mg rbST（Posilac，Elanco Animal Health，Greenfield，IN），或注射生理盐水（盐溶液0.9％，Valley Vet Supply，Marysville） ，KS）从21到63 d每隔7 d。小牛从出生开始到断奶38 d（d -11）（开始进行断奶）时随意喂养代乳品，小牛在49 d（d 0）断奶。小牛在出生时和从21到63 d时每周称重，同时也测量了凋零的身高。在研究d -28和-7时，给小牛接种0.5 mg卵清蛋白。在d －28，－25，－21，－11、0、3、7和14上采集血样。分离多形核白细胞，并用大肠杆菌确定吞噬作用和氧化爆发能力。另外，确定了由粒细胞，淋巴细胞和CD14 +单核细胞表达的分化簇（CD）62L和CD18的表达。血样还用于测定血液学参数和生长激素，类胰岛素生长因子-1，胰岛素，葡萄糖，脂肪酸，β-羟基丁酸酯，触珠蛋白和抗卵清蛋白IgG的浓度。用rbST处理的犊牛在d -25至14时具有比对照犊牛更高的生长激素和胰岛素样生长因子-1浓度，而胰岛素，脂肪酸和β-羟基丁酸酯的浓度没有差异。在d -11时，rbST处理的犊牛的葡萄糖浓度更高。治疗未影响多形核淋巴细胞的吞噬作用和氧化爆发，但是通过rbST处理，粒细胞表达CD62L和CD18的强度趋于增加。治疗不影响血清中抗卵清蛋白IgG的浓度。在第3天，用rbST处理的牛犊的血红蛋白浓度降低，并且我们注意到用rbST处理的牛犊的血细胞比容降低的趋势。尽管经rbST处理的小牛的每日代乳品摄入量较低，但治疗并未影响体重，凋零的身高和平均日增重。rbST处理断奶小牛引起的免疫反应相对较小的改善可能不足以降低传染病的发生率。在第3天，用rbST处理的牛犊的血红蛋白浓度降低，并且我们注意到用rbST处理的牛犊的血细胞比容降低的趋势。尽管经rbST处理的小牛的每日代乳品摄入量较低，但治疗并未影响体重，凋零的身高和平均日增重。rbST处理断奶小牛引起的免疫反应相对较小的改善可能不足以降低传染病的发生率。在第3天，用rbST处理的牛犊的血红蛋白浓度降低，并且我们注意到用rbST处理的牛犊的血细胞比容降低的趋势。尽管经rbST处理的小牛的每日代乳品摄入量较低，但治疗并未影响体重，凋零的身高和平均日增重。rbST处理断奶小牛引起的免疫反应相对较小的改善可能不足以降低传染病的发生率。