Background 

Ractopamine hydrochloride (RH) is a beta-agonist approved by the U.S. Food and Drug Administration (FDA) as a dietary supplement in cattle production for improved feed efficiency and growth promotion. Current maximum residue limits (MRL) for ractopamine in liver, a target tissue, are 40 ppb (Codex Alimentarius Commission) and 90 ppb (FDA). Muscle, another target tissue, has an MRL of 10 ppb (Codex) and 30 ppb (FDA). Maximum residue limits are based upon parent ractopamine rather than total ractopamine (i.e., ractopamine + ractopamine glucuronides); however, many countries have begun subjecting tissues to residue testing based upon total ractopamine assays and in off-target tissues, presenting a major challenge to the U.S. beef industry. Very little is known about the ractopamine (parent or total) depletion curves associated with off-target tissues upon withdrawal of cattle from RH. 

The objective of this study was to determine the concentration of parent ractopamine and total ractopamine in muscle, fat, rendered tallow, and large intestines from animals fed RH and subjected to 2, 4, or 7 days of RH withdrawal before harvest, as well as from animals that did not receive RH in their diet. 

Methodology   

A total of N = 75 experimental units (small pens of cattle from which individual cattle were randomly sampled for testing) were assigned to five treatment groups across separate feedlot pens. The treatment groups included: (i) a negative control (fed no ractopamine and no feed-tallow; fed from verified clean feed trucks; “Control-No Tallow” treatment); (ii) a control with feed-tallow (fed no ractopamine but received feed-tallow; “Control-With Tallow” treatment); and cattle fed RH (at approximately 250-300 mg/hd/d), but withdrawn from treatment at (iii) 2 days before harvest; (iv) 4 days before harvest; or (v) 7 days before harvest. The animals were harvested in a balanced design at a commercial beef harvest facility. Muscle, fat, and large intestine tissue samples were collected aseptically, within animal (n = 15 per treatment group), after commercial processing. For tallow, subcutaneous fat was collected from each animal for subsequent in-laboratory rendering (Executive Summary Figure 1) as the commercial beef harvest facility used did not have its own rendering facility. Collected samples were placed in Whirl-Pak bags in direct contact with ice to prevent conversion of ractopamine glucuronides into ractopamine. Samples were stored at -20°C until cryogenic processing for subsequent ractopamine extractions and analysis procedures. Parent and total ractopamine concentrations in all samples were measured using ultra performance liquid chromatography coupled with tandem quadrupole mass spectrometry (UPLC-MS/MS). The method, developed and validated at Colorado State University for each tissue/matrix type, was adapted from AOAC Official Method 2011.23. Parent and total ractopamine concentrations were analyzed separately for each tissue/matrix type using a general linear mixed model approach.

Findings   

Ractopamine (parent and total) residues were not detected (< 0.12 ppb; assay limit of detection [LOD]) in any of the muscle and fat samples from either of the control treatments (Control-No Tallow and Control-With Tallow) (Executive Summary Tables 1 and 2). Additionally, for both control treatment groups, ractopamine residues (parent and total) were either not detected (< 0.04 ppb; assay LOD) or were below the limit of quantification (< 0.14 ppb) in individual rendered tallow samples. Overall, mean ractopamine concentrations in muscle, fat, and large intestine samples collected from carcasses of animals that received RH, decreased (P < 0.05) as the RH withdrawal time was extended. Irrespective of RH withdrawal duration, mean parent ractopamine levels in muscle, fat, large intestine, and rendered tallow ranged from 0.33 to 0.76 ppb, 0.16 to 0.26 ppb, 3.97 to 7.44 ppb, and 0.05 to 0.08 ppb, respectively. Corresponding mean concentration ranges of total ractopamine were 0.41 to 1.22 ppb (muscle), 0.18 to 0.50 ppb (fat), 4.80 to 8.45 ppb (large intestine), and 0.06 to 0.07 ppb (rendered tallow). 

Implications 

The data from this study may be useful in the development of new recommendations for RH use and withdrawal to beef cattle producers in the U.S. who intend to export to global markets.