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Dairy calf weaning is associated with ketone concentrations that exceed the levels occurring in adults, and weaning represents a potential energy loss that may be mitigated by ionophore supplementation. To assess the effects of weaning and ionophore supplementation on net nutrient flux across portal-drained viscera (PDV) tissues in dairy calves, concentrations of glucose, acetoacetate (ACAC), beta-hydroxybutyrate (BHBA), nonesterified fatty acids, volatile fatty acids, lactate, pyruvate, insulin, and glucagon and PDV flux rates were determined in Jersey bull calves (n = 19) at 35, 56, 84, and 112 d of age. Calves were randomly assigned at birth to either a commercial pelleted starter without (CON) or with lasalocid (TRT; 83 mg/kg of dry matter). Calves were fed only milk replacer from d 3 to 34 (d 3 to 20 = 454 g/d; d 21 to 34 = 568 g/d). After blood sampling on d 35, calves received replacer (d 35 to 41 = 454 g/d; d 42 to 48 = 227 g/d) and had free access to the CON or TRT starter, and from d 49 to 112 they received CON or TRT ad libitum. Catheters were implanted in the portal vein and in the mesenteric vein and artery between d 21 and 28. Blood flow was measured by continuous infusion of p-aminohippurate into the mesenteric vein. Six serial samples were taken at 30-min intervals from the arterial and portal vein catheters simultaneously. Portal blood flow increased with age but did not differ between CON and TRT calves. Glucose was released preweaning and was extracted postweaning by PDV, but was not affected by ionophore. The portal flux of nonesterified fatty acids was not different from zero during any of the 4 sample ages. Fluxes of ACAC and BHBA in CON and TRT calves went from no measurable flux preweaning to a postweaning PDV release that peaked at d 84, but the d-84 release of ACAC and BHBA was lower in TRT calves. The portal flux of volatile fatty acids increased with age, and PDV release of both butyrate and propionate was lower at d 84 in TRT than in CON calves. However, TRT calves had a greater PDV release of lactate on d 84, partially compensating for the lower release of propionate. Glucagon was greater in CON than in TRT calves at d 84 and could be a response to the elevated ketogenesis observed in CON calves during this period. Changes in the metabolic profile and nutrient flux of transition calves were demonstrated in response to weaning and ionophore supplementation. Inclusion of an ionophore appeared to moderate alimentary output at a postweaning period (d 84) at which ketone concentrations have the potential to exceed the whole animal capacity for utilization.

The objective was to compare blood flow to the hindquarters of steers measured by transit time ultrasound with blood flow determined by indicator (p-aminohippurate) dilution. Five Hereford steers had ultrasonic flow probes on the abdominal aorta and catheters in the abdominal aorta and inferior vena cava inserted through both sets of circumflex iliac vessels. Indicator was infused continuously into the abdominal aorta through both arterial catheters simultaneously, then through each of the arterial catheters in succession. Samples of blood from the inferior vena cava and jugular vein were taken during infusion for measurement of p-aminohippurate. Blood flow determined by the ultrasonic flow probe was averaged over each blood sampling interval. Compared with the ultrasonic flow probe there was no difference in mean blood flow measured by p-aminohippurate, regardless of method of infusion. Correlation of individual values between ultrasound and p-aminohippurate was .87 when p-aminohippurate was infused into both arterial catheters, .44 when p-aminohippurate was infused into the left arterial catheter, and .78 when p-aminohippurate was infused into the right arterial catheter. The respective ranges for ultrasonic measurements and p-aminohippurate were 3.62 to 10.99 L/min and 2.25 to 30.43 L/min. Although means by the two methods do not differ, there is a greater range and incidence of occasional high values with p-aminohippurate dilution.

A useful approach of the study of nutrient absorption and metabolism is in vivo measurement of blood flow across portal-drained viscera and liver, and flux of bloodborne metabolites, successful application of the approach requires correct placement of chronic catheters in appropriate blood vessels. Additionally, catheters must stay patent long enough to allow the animal to recover from surgery and to complete an experimental protocol. This paper describes surgical techniques to install chronic catheters in mesenteric veins, the hepatic portal vein, and an hepatic vein of cattle. Techniques for access to arterial blood are described also. Materials, equipment, and supplies required for surgery, blood sampling, and blood flow determination are described. Commercial sources of supplies are suggested. Blood flow is measured by downstream dilution of para-aminohippurate, which is infused into a mesenteric vein. Examples of blood flow data for three types of cattle are provided.

In ruminants, mammary supply of substrate varies with rate of mammary blood flow and concentrations of blood substrates. Blood concentrations of most mammary substrates, except acetate and tryptophan, do not vary greatly with feed intake, short term. Fasting reduces mammary blood flow, whereas milking and injection of growth hormone or thyroxine increase flow. It is proposed that the fraction of cardiac output that perfuses the udder of lactating ruminants plays a role in regulation of nutrient partitioning between milk and body tissues. In fed animals this fraction is 15 to 16% of cardiac output, which declines on fasting to 8 to 9% and increases slightly following growth hormone treatment to 17.6%. Following realimentation of fasted cows or goats, mammary blood flow takes several hours to return to normal. Investigation of the mechanism of this response, in terms of the ability of the animal to recognize its nutritional status and partition nutrients accordingly, should prove fruitful to understanding causes of variations of milk production in response to feed quantity and quality. Several substrates show increased mammary arteriovenous difference with increasing blood concentrations. This may reflect differing ratios of blood flow:milk yield. The steep gradient of concentration of substrates across the mammary epithelial cell membrane suggests that a major impediment to substrate supply for milk synthesis is the rate of substrate transport across the membrane.

Lactating cattle were used to validate a transit time ultrasonic blood flow metering system for measuring mammary gland arterial blood flow. Blood flow probes were surgically placed around the right external pudic artery. An electromagnetic flow probe was implanted in tandem with the ultrasonic probe in two cows for comparative measurements. The absolute accuracy of the implanted flow probes was assessed in vivo by mechanical means on anesthetized cows after 2 to 3 wk of implantation. The zero offset of the ultrasonic probes ranged from -12 to 8 ml/min. When the ultrasonic probe was properly implanted, the slopes of the calibration curves were linear and ranged from .92 to .95, tracking absolute flow to within 8%. The transit time instrument's performance was examined under a variety of physiological conditions. These included milking and hormone injections. The transit time ultrasonic flow meter accurately measured physiological changes in mammary arterial blood flow in chronically prepared conscious cattle. Blood flow increased 29% during milking. Epinephrine decreased mammary blood flow by 90 to 95%. Oxytocin doses increased mammary blood flow by 15 to 24%.