One of the hotter topics of discussion in the beef industry during the last two or three years has been the use of a class of feed additives called Beta Adrenergic Agonists (βAA) in feedlot cattle. These feed additives, Optaflexx® and Zilmax®, are fed to improve weight gain, feed efficiency and carcass weight during the last 20-42 days of the feeding period. These drugs produce these results by changing cell metabolism, especially muscle cells, to increase protein production and reduce protein breakdown. However, these drugs also affect many other systems in the body including the cardiovascular system. These drugs have been used successfully for decades in human medicine and have been a key component in management of asthma in people through the use of inhalers.
Research conducted by the BCI, and sponsored by the Beef Checkoff, investigating the effect of βAA on the heart of feedlot cattle was presented at the Joint Animal Science meeting in Kansas City in July. Beta-Adrenergic agonists have been shown in other veterinary species, such as dogs and horses to increase heart rate and occasionally heart rhythm, but no investigation into these effects in cattle has been conducted prior to this study. Heart arrhythmias have been well documented as a health risk in many species.
This research investigating the effect of βAA on the electrocardiogram (ECG), or heart rhythm was the first study conducted to look at the heart rhythm in feedlot cattle. The proper rhythm of the heart is extremely important for the normal function of the heart and some abnormalities, called arrhythmias, can be a serious health risk. To investigate for these arrhythmias at the BCI, we used Holter Monitors, a small portable recorder that we adapted to be placed on cattle to record the heart’s activity at different points in the normal feeding period for these two drugs. The monitors are similar to one a doctor would use for a human heart patient to wear for evaluation of the heart at home and outside of the doctor’s office.
Our findings in this study showed a mild increase in heart rate for both Optaflexx® and Zilmax ® compared to control cattle that received no drug. However, the rate of arrhythmias of the heart was not different between treatment and the control animals (see Figure) We also investigated for microscopic changes in the heart, lung and liver in this study and did not find any differences or evidence of pathology between these groups.
This study shows there aren’t any large differences in heart rhythm caused by these drugs in feedlot cattle. However, due to the small size of this study, completely ruling out the all health risk of these drugs to feedlot cattle is not possible.