The widespread use of antimicrobials in animal husbandry has led to an increase in concentrations of antibiotics in animal faeces. Furthermore, the spreading of this waste as fertilizer on agricultural land indirectly introduces antimicrobial residues into the environment. This study examined samples of pig slurry and poultry manure for three tetracyclines-oxytetracycline (OTC), chlortetracycline (CTC) and doxycycline (DOC) and conducted an environmental evaluation of soils. The samples were taken from manure heaps on eight broiler chicken farms and from slurry ponds on eight pig farms. The tetracycline analysis was conducted using high-performance liquid chromatography combined with tandem mass spectrometry (HPLC/MS). The risk assessment was carried out on the basis of the estimated PECsoil value and the ecotoxicological information available for terrestrial organisms. TC concentrations were between >0.01 and 1.38 mg/kg. OCT was detected in 37% of the poultry manure and in 20% of the pig slurry samples. DOC was present in 50% of pig slurry samples with an average concentration of 1.2 mg/kg, while CTC was only found in one slurry sample at a concentration of 0.56 mg/kg. Compared with published values for these two farm animals from other countries, the TC concentrations obtained in this study were low. The environmental importance of the data generated was also discussed.

The usage of antibiotics in animal husbandry has dramatically increased the concentration of antibiotic residues and has promoted the development and abundance of antibiotic resistance in manure. When it is spread onto agricultural land, both residues of antibiotics and bacteria carrying antibiotic resistance genes may be introduced into the environment. In this research, we isolated resistant gram negative bacteria from manure produced in two dairy and two beef cattle farms, located in Madrid (Spain), to determine their resistance to seventeen representative antibiotics commonly used in veterinary therapy. A total of 63 isolates were used to assess the overall bacterial antimicrobial resistance on cattle manure samples. Predominant species were Escherichia coli and Comamonas testosteroni accounting for 25% and 19.6% of the total, respectively. The most found antimicrobial resistances in gram-negative bacteria were to tetracycline (66.7%), sulphamethoxazole (55.6%), ampicillin (52.4%), cephalothin (46.0%), chloramphenicol (44.4%), nalidixic acid (39.7%) and trimethroprimsulphamethoxazole (33.3%). The mean of resistance and the percentage of multi-resistant bacteria in beef farms were higher and statistically significant when compared to dairy farms which is opposite from the findings of the previous studies. The presence of three tetracyclines in all manure samples was also examined with stable recoveries (76% -82%) and high sensitivity (limit of quantification 0.015 -0.03 μg/kg). The concentrations of tetracyclines detected (<0.015 -10 mg/kg) were consistent to the theoretical tetracycline levels in manure in Spain according to the excretion rate of these antibiotics and the values reported in scientific literature in other European countries.