Somatotropin in lactating cows: effect on response to epinephrine and insulin.

The dose dependence of plasma nonesterified fatty acids (NEFA) and glycerol responses to epinephrine was determined in lactating cows treated with bovine somatotropin (bST). Also, the effects of insulin on lipolysis and glucose clearance were examined. Lactating cows (190 +/- 24 days postpartum) received daily subcutaneous injections of methionyl bST (40 mg) or excipient during two 12-day treatment periods. Epinephrine challenges (0.1, 0.2, 0.4, 0.8, 1.2, and 1.6 micrograms/kg body wt) were administered intravenously on days 6-11 and an insulin challenge (1 microgram/kg) on day 12. Milk energy secretion increased 55% without any change in voluntary feed intake during bST treatment. Circulating concentrations of NEFA were chronically elevated and reciprocally paralleled changes in net energy balance. Maximal response of plasma NEFA and glycerol to epinephrine was markedly increased during bST treatment, whereas the dose of signal-producing half-maximal response (ED50) was unchanged. Antilipolytic effects of insulin were also enhanced, whereas the effect of insulin on the fractional rate of glucose change was reduced. Thus bST alters tissue response to homeostatic signals to allow more nutrients to be partitioned for milk synthesis. Furthermore, the alterations in response to epinephrine and insulin appear to predominantly occur at a postreceptor level.

Temporal pattern of insulin-like growth factor-I response to exogenous bovine somatotropin in lactating cows.

The effect of exogenous bovine somatotropin (bST) treatment on the temporal pattern of insulin-like growth factor-I (IGF-I) in serum of four multiparous Holstein cows was examined. Cows (190 +/- 24 days postpartum) were treated with daily subcutaneous injections of recombinant bST (40 mg) or excipient for 12-day periods in a crossover experimental design. During excipient treatment, concentrations of IGF-I in serum were relatively constant throughout the day and averaged 70 ng/ml. Following the first bST injection, serum IGF-I began increasing after a lag of 5 to 7 hr and progressively increased over the first 2 days of treatment. Serum IGF-I levels were approximately 2-fold greater than control values at the end of day 1 of bST treatment, with a 3-fold elevation observed at the end of day 2. Concentrations of IGF-I in serum plateaued by day 3 of bST treatment. Serum concentrations of IGF-I did not follow the oscillating pattern of bST in serum resulting from daily bST injections. Milk yield (3.5% fat-corrected) plateaued after 6 days of bST treatment and was increased 61% (+15.3 kg). Both IGF-I and milk yield remained essentially constant across days for the remainder of treatment. Following cessation of treatment, serum IGF-I and milk yield gradually declined, returning to control values after approximately 4 days. The temporal pattern of circulating concentrations of IGF-I is consistent with a role for IGF-I in mediating a portion of the effects of exogenous bST in lactating cows.

Response to metabolic challenges in early lactation dairy cows during treatment with bovine somatotropin.

Milk production is increased in lactating cows treated with bovine somatotropin (bST) because a greater portion of absorbed nutrients are partitioned for milk synthesis. This homeorhetic action may be caused by alterations in response of key tissues to homeostatic signals. To examine this theory, acute metabolic challenges were administered to 8 multiparous Holstein cows (61 +/- 2 days postpartum) receiving daily subcutaneous injections of pituitary-derived bST (26.3 mg) or excipient during two 14-day treatment periods (crossover experimental design). Treatment with bST increased milk yield 12%. Feed intake did not change so that net energy balance decreased (+ .5 vs. -4.3 Mcal/day). Plasma concentrations of nonesterified fatty acids (NEFA) were chronically elevated in bST-treated cows, consistent with energy balance differences. However, baseline concentrations of glucose, insulin, and glucagon in plasma did not differ. On the last 3 days of treatment, individual metabolic challenges were administered via jugular cannulas: epinephrine (700 ng/kg BW), glucose (250 mg/kg BW), insulin (1.0 micrograms/kg BW), and glucagon (175 ng/kg BW). Plasma glucose was reduced after the insulin challenge to a lesser extent during bST treatment. In bST-treated cows, the increase in plasma NEFA in response to epinephrine was greater, and NEFA concentrations were lowered to a greater extent after insulin and glucose challenges. Glucose, insulin, and glucagon removal rates were not altered, nor was plasma glucose response to epinephrine or glucagon challenges. Treatment of lactating cows with bST primarily altered the response of adipose tissue to homeostatic signals which affect lipid metabolism.

Effect of somatotropin on kinetics of nonesterified fatty acids and partition of energy, carbon, and nitrogen in lactating dairy cows.

The utilization of energy, carbon, and nitrogen, and irreversible loss rates of nonesterified fatty acids were examined in lactating cows during treatment with bovine somatotropin. Six multiparous Holstein cows at approximately 62 d postpartum received daily intramuscular injections of somatotropin (29.4 mg/d) or excipient during two 21-d treatment periods in a crossover experimental design. Milk yield was increased 11% (37.1 versus 41.3 kg/d), whereas intake and digestibility of DM, energy, nitrogen, and carbon were not affected. Cows remained near zero net energy balance during both treatments (1.31 versus -1.52 Mcal/d for control and somatotropin, respectively). The major effect of somatotropin was to partition net energy into milk while tissue energy accretion was reduced. Postabsorptive use of nitrogen and carbon was altered by somatotropin as reflected by greater secretion in milk. Urinary losses of nitrogen and tissue use of carbon were reduced. Plasma concentrations and irreversible loss rates of nonesterified fatty acids did not differ between treatments. Somatotropin treatment of lactating cows results in partitioning of nutrients away from tissue deposition toward milk synthesis. The extent to which different metabolic processes are altered depends upon the energy balance of the somatotropin-treated animal.

Prolactin, estradiol, and progesterone changes in paretic and nonparetic cows during the periparturient period.

We studied the relationship of serum prolactin, estradiol-17 beta, and progesterone concentrations to plasma calcium, phosphorus, and free hydroxyproline concentrations, as well as to dry matter intake, in 14 aged dairy cows (mean of 4.5 parities), 7 of which became paretic, from 28 days before to 4 days after calving. Plasma calcium and phosphorus concentrations and dry matter intake decreased more at parturition in paretic cows than in nonparetic cows. Prolactin concentrations were not different between paretic and nonparetic cows, but were variable. Concentrations of estradiol were higher in paretic cows from 15 to 5 days before parturition, whereas hydroxyproline concentration was lower in paretic cows on days 10 through 3 before parturition. Progesterone concentration was lower in paretic cows and decreased earlier at parturition, compared with that in nonparetic cows. The findings suggested that high estradiol concentrations in late pregnancy inhibit bone resorption and predispose aged cows to parturient paresis. The earlier decrease in progesterone concentration at parturition and lower concentrations throughout late pregnancy might have contributed to the greater inappetence in paretic cows at parturition. The importance of prolactin in the pathogenesis of parturient paresis is not clear.

Sources of variation and prospects for improvement of productive efficiency in the dairy cow: a review.

In this review, "productive efficiency" in dairy cows is defined as the yield of milk obtained in ratio to the nutritional costs associated with maintenance, milk synthesis and loss of body condition during lactation. Improvements in efficiency could occur as a result of changes in digestion and nutrient absorption, maintenance requirement, utilization of metabolizable energy for production or nutrient partitioning. Digestibility can be greatly enhanced by appropriate dietary manipulation. Likewise, it may be possible to reduce maintenance requirements and improve the efficiency with which metabolizable energy is used for milk synthesis by manipulation of the pattern of nutrients presented to tissues. However, these factors apparently do not respond to selection for increased milk yield, and little variation is observed among cows. In contrast, individual cows differ substantially in feed intake and in the partitioning of nutrients among body tissues. Techniques associated with genetic engineering and the early prediction of genetic merit have the potential to improve productive efficiency by manipulation of these processes. However, changes in nutrient partitioning and feed intake during lactation are coordinated by a complex network of controls that accommodate the nutrient requirements of each tissue while maintaining homeostatic balance. Future improvements in productive efficiency will therefore depend on our ability to understand the manner in which these controls operate.