|Spiers, Don - UNIVERSITY OF KENTUCKY|
Submitted to: Kentucky Beef Report
Publication Type: Other
Publication Acceptance Date: June 9, 2006
Publication Date: N/A
Interpretive Summary: Tall fescue grass is a major component of the grazing lands in the upper transition zone. Cattle grazing tall fescue will demonstrate a variety of manifestations of fescue toxicosis. The level of toxicosis in cattle varies from lost performance to sloughing of hooves and tails. A major component of fescue toxicosis is the vascular constriction of peripheral blood vessels and the subsequent reduction in blood flow. Consequently, the ability of the animal to dissipate heat and regulate body temperature is restricted, especially during heat stress. This inability of peripheral vessels to dilate will lead to hyperthermia that may lead to reduced grazing activity and productivity. The use of Doppler-image ultrasound technology has been a mainstay in vascular assessment in human medicine for years and be used as a quick, efficient, and non-invasive way of evaluating the effects of fescue toxicosis on the peripheral vessels of cattle. An experiment was conducted with two groups of steers in separate environmental chambers to determine if Doppler ultrasonography can detect vascular changes in cattle placed under routine environmental stresses of different ambient temperatures in environmental chambers. Changes were observed in the cross-sectional area of the caudal artery, which supplies blood to the tail, for cattle subjected to either heat (89OF) or cold (4689OF) challenges. Increases in blood flow rate were detected for heat challenged cattle and blood flow for cold challenged cattle tended to decrease. Results indicated that Doppler ultrasonography has the sensitivity to study alterations in hemodynamics of cattle grazing toxic tall fescue and, therefore, serve as a valuable research tool in developing technologies in overcoming this malady that annually costs the cattle industry approximately 1 billion dollars.
Technical Abstract: Two preliminary studies were performed to determine if Doppler-image ultrasound can be used to document the vascular changes of cattle under hot and cold conditions. Three calves per study (320 ± 38 kg) were acclimated to thermoneutrality (17.5oC). Three ultrasound measurements per animal were taken of the median caudal artery at the 4th coccygeal vertebrae (Cd4) using a 13 MHz transducer. The animals were then exposed to air temperatures above (32oC) and below (8oC ) this neutral level. The animals were housed under these conditions for 24 h and then returned to the original thermoneutral level. Ultrasound scans were repeated at -2 (2 h prior to temperature change), 5, 23, 24 and 28 h (4 h after returning to thermoneutrality). Blood flow rates of heat stressed cattle significantly increased from 39.12 (-2 h) to 59.87 cm/min at 23 h of exposure (P < 0.05). The vessel diameter tended (P> 0.10) to increase (0.047 to 0.063 cm2) with the onset of the stress. Heart rate was unchanged throughout the study, but respiration rate did increase from 82 to 120 breaths/min during heat stress (P < 0.05). Cattle at 8°C showed lower flow rates to the caudal artery after 23 h of exposure (P < 0.10) (30.73 at -2 h to 18.44 cm/min at 23 h). Blood flow rates of these cattle initially increased after the challenge. At 5 h post-temperature reduction, flow rates were 53.9 cm/min, increasing over the baseline of 30.7 cm/min (P < 0.07). At 8°C, heart rate was unchanged and averaged 79 beats/min across all five time periods, while respiration rate decreased from 62 to 36 breaths/min (P< 0.05) when under stress. Arterial area increased from 0.057 to 0.072 cm2 at 5 h (P < 0.07) exposure to 8°C then decreased to 0.043 cm2 at 23 h. The ability of the Doppler-image ultrasound to detect vascular changes due to environmental variation demonstrates its use as a tool for assessing environmental stress effects on hemodynamics.