LC methods will often allow you not only to detect and quantify the parent compounds, but also microbiologically relevant metabolites.

Antibiotic residues in edible animal products are of great concern to regulatory agencies and consumers, so reliable screening methods for rapid, selective and sensitive detection of these residues are necessary to ensure food safety. 

http://www.sciencedirect.com/science/article/pii/S0165993610001809

Hi Dear

https://en.wikipedia.org/wiki/Antibiotic_use_in_livestock

Dear Barbara

Degrading antibiotics discharged in the livestock manure in a well-controlled bioprocess contributes to a more sustainable and environment-friendly livestock breeding. Although most antibiotics remain stable during manure storage, anaerobic digestion can degrade and remove them to various extents depending on the concentration and class of antibiotic, bioreactor operating conditions, type of feedstock and inoculum sources. Generally, antibiotics are degraded during composting > anaerobic digestion > manure storage > soil. Manure matrix variation influences extraction, quantification, and degradation of antibiotics, but it has not been well investigated. Fractioning of manure-laden antibiotics into liquid and solid phases and its effects on their anaerobic degradation and the contribution of abiotic (physical and chemical) versus biotic degradation mechanisms need to be quantified for various manures, antibiotics types, reactor designs and temperature of operations. More research is required to determine the kinetics of antibiotics’ metabolites degradation during anaerobic digestion. Further investigations are required to assess the degradation of antibiotics during psychrophilic anaerobic digestion.

for  qualitatif  your  microbiology method, for quantify your Spectrofometric method and GCMS

I think Highly Performance Liquid Chromatography (HPLC), will do, however, I will suggest you try qualitative method first using the microbiological method of detecting the presence of veterinary residues, then you can go ahead with quantification.

Thank you all for your answers.

Awosile Babafela:  Do you have a protocol for the microbiological detection of the presence of veterinary residues?

@Barbara please find attached some articles that may help in this regards. However, the method uses agar based testing like disk diffusion method of testing for antimicrobial susceptibility testing. In this case a standardized bacteria named Bacillus stearothermophilus is used. Please find more inforamtion as attached

GCMS method you are wiil often organic acid degradation from product metabolism , and you will detection reaction from enzim metabolite