Oxytetracycline runoff: what 30 days outdoors showed
Oxytetracycline runoff showed up after every single rainfall event in a four season field trial, at 0.13 to 0.26 micrograms per milliliter. The setup was plain: 100 g deposits of fortified swine feces left on grass covered soil for 30 days per season, with passive collectors downslope taking the flow. Between 0.02 and 0.07 percent of the fortified mass left in water. All of it still ran 220 to 540 times above the concentration meant to keep resistance selection away.
Why oxytetracycline runoff matters on swine operations
Oxytetracycline came out of Streptomyces rimosus as a first generation tetracycline, and pigs still get plenty of it. Oral dosing is the default in intensive barns because medicated feed or water treats a whole group in one pass. Doses land between 20 and 40 mg per kg of body weight, depending on formulation and protocol. Bioavailability in pigs sits near 6.13 percent and drops further when divalent cations show up in feed or water. Most of what goes in the front comes out the back, as parent compound.
None of that stops at the manure pile. Tetracyclines grab onto soil particles, organic matter and multivalent cations, and what binds does not disappear. Binding slows the molecule down and, at the same time, parks a residue pool where later rain can reach it. Field studies keep landing on the same order of magnitude, under 0.1 to 0.13 percent of the applied mass moving in surface flow, and that still counts because the antimicrobial load in manure is high.
How the field trial was set up
Each deposit was 100 g of fresh feces from pigs that had not been treated, run in triplicate. The fortification level was back-calculated from a 40 mg per kg oral dose and the roughly 6.13 percent bioavailability reported for pigs, which gives the fraction the animal never absorbs. Control deposits without fortification sat alongside under the same weather. The plots were outdoors, grass covered and slightly sloped, and passive collectors downslope took the surface flow. Samples went to -20 degrees Celsius in the dark until testing.
Rain counted as its own event when at least 6 h of dry weather came first, giving 10 events in summer, 5 in spring, 4 in winter and 2 in autumn. For microbiology, five subsamples per treatment came out of the top 0 to 10 cm with an auger, were composited, and went into work within 24 h. Counts used pour plates on nutrient agar and aerobic incubation at 28 degrees Celsius for 24 to 48 h, reported as colony forming units per gram of dry soil.
HPLC with UV detection did the chemistry, and concentrations are given in micrograms per milliliter (ug/mL) or micrograms per liter (ug/L). Samples spun at 14,000 rpm for 5 min, then the supernatant passed through 0.22 micrometer nylon into vials. The column was a Luna C18, 5 micrometer, 250 x 4.6 mm, at 40 degrees Celsius, running acetonitrile:methanol 50:50 against 0.01 M oxalic acid 50:50, isocratic at 1.5 mL/min. Detection was 365 nm with a 50 uL injection, and oxytetracycline eluted near 2.07 min. Matrix matched calibration spanned 0.06 to 8.00 ug/mL with r2 above 0.999, relative error came in at -3.56, -1.76 and -3.02 percent, and the coefficient of variation stayed under 10 percent.
R version 4.5.0 ran the statistics. Most variables were not normal, so associations used Spearman rank correlation, and seasons were compared by Kruskal-Wallis with Dunn post hoc and Benjamini-Hochberg correction. Bacterial counts went through two-way ANOVA with Tukey post hoc testing, and both outcomes ended up in multiple linear regression chosen on AICc.
| Item | Setting | Why it matters |
|---|---|---|
| Fecal deposit | 100 g fresh feces, triplicate per treatment | Sets the residue pool for transport |
| Fortification | 40 mg/kg oral dose, about 6.13 percent bioavailability | Reproduces the voided fraction |
| Exposure | 30 days outdoors per season, slight slope | Captures natural weather cycles |
| Chromatography | Luna C18, 5 micrometer, 250 x 4.6 mm, 40 degrees Celsius | Separates the analyte |
| Detection | UV 365 nm, 50 uL injection, elution near 2.07 min | Quantifies the peak |
| Calibration | 0.06 to 8.00 ug/mL, r2 above 0.999 | Sets the reportable range |
| Quality control | Relative error -3.56, -1.76, -3.02 percent | Sets accuracy limits |
Oxytetracycline runoff concentrations in all four seasons
Not one rainfall event in any season came back clean. Summer was the outlier at 0.20 to 0.26 ug/mL, or 0.03 to 0.07 percent of the fortified mass, with the peak on day 17 and 0.20 ug/mL still there on day 30. Summer differed from the other three at p below 0.001, and autumn, winter and spring did not differ from one another. Autumn sat at 0.15 to 0.17 ug/mL and stayed detectable to the last sample. Winter started at 0.15 to 0.20 ug/mL and slid to about 0.11 ug/mL by day 22. Spring barely moved, 0.13 to 0.15 ug/mL between day 4 and day 21.
Thirty days was not long enough to empty a deposit. Something in that fecal mass stayed available the whole time, even though the fraction leaving in water never passed 0.07 percent of what went in.
| Season | Concentration (ug/mL) | Share of fortified mass | Pattern across 30 days |
|---|---|---|---|
| Summer | 0.20 to 0.26 | 0.03 to 0.07 percent | Peaked on day 17, still 0.20 on day 30 |
| Autumn | 0.15 to 0.17 | 0.04 to 0.05 percent | Narrow band, detectable at final sampling |
| Winter | 0.11 to 0.20 | 0.01 to 0.06 percent | Fell from early events to about 0.11 by day 22 |
| Spring | 0.13 to 0.15 | 0.02 to 0.04 percent | Flat from day 4 to day 21, still detectable |
Rainfall intensity and the variables that actually explained transport
Concentration tracked four hydrological variables: I30 at rs 0.686, cumulative precipitation at rs 0.611, soil moisture at rs 0.477, and the E.I30 erosivity index at rs 0.374, all at p of 0.003 or better. Regression narrowed the picture. The best model, at AICc -281.4, kept cumulative precipitation positive at beta 5.72 x 10^-4 and cumulative air temperature negative at beta -2.60 x 10^-4, both at p below 0.001. Season stayed in as its own term, summer at beta 7.41 x 10^-2 and autumn at beta 4.34 x 10^-2 against winter, at p below 0.001 and p = 0.004, while spring did not separate at beta 2.19 x 10^-2 and p = 0.106.
Both I30 and E.I30 correlated on their own and still lost out in model selection. Winter oxytetracycline runoff arrived with the weakest intensities of the year and hit 0.20 ug/mL anyway. What moves the drug is water piling up over event after event rather than one hard storm, and the negative temperature term agrees, because dissipation speeds up in the heat.
| Variable | Summer | Autumn | Winter | Spring |
|---|---|---|---|---|
| Daily precipitation (mm) | 18.05 +/- 12.47 | 1.30 +/- 0.14 | 5.55 +/- 5.25 | 10.68 +/- 11.04 |
| Cumulative precipitation (mm) | 119.99 +/- 56.64 | 4.30 +/- 0.99 | 18.65 +/- 5.05 | 35.20 +/- 23.67 |
| I30, maximum 30 min intensity (mm) | 7.87 +/- 5.02 | 0.60 +/- 0.56 | 0.95 +/- 0.30 | 2.12 +/- 3.22 |
| Maximum air temperature (degrees Celsius) | 27.82 +/- 4.02 | 18.55 +/- 1.77 | 14.35 +/- 5.97 | 25.86 +/- 3.91 |
| Superficial soil temperature (degrees Celsius) | 23.44 +/- 1.78 | 10.01 +/- 0.87 | 10.10 +/- 2.09 | 20.80 +/- 1.88 |
| Soil moisture (percent) | 17.50 +/- 4.15 | 18.53 +/- 0.42 | 19.89 +/- 0.50 | 8.38 +/- 0.41 |
| Rainfall events (n) | 10 | 2 | 4 | 5 |
| Variable paired with runoff concentration | Spearman rs | p |
|---|---|---|
| I30, maximum 30 min rainfall intensity | 0.686 | below 0.001 |
| Cumulative precipitation | 0.611 | below 0.001 |
| Soil moisture | 0.477 | below 0.001 |
| E.I30 rainfall erosivity index | 0.374 | 0.003 |
Sediment and the particle bound fraction
Sediment followed erosivity. Summer collectors held 6.7 to 16.1 g, winter 4.8 to 11.6 g, spring 5.2 to 8.8 g, and autumn just 0.5 to 1.9 g, dried at 60 degrees Celsius to constant weight. Runoff from every event in a season was pooled, so each collector gives one number per season, and no season to season comparison was run. A compound that sticks to particles is exactly the one this design would miss.
| Season | Recovered sediment per collector (g) |
|---|---|
| Summer | 6.7 to 16.1 |
| Autumn | 0.5 to 1.9 |
| Winter | 4.8 to 11.6 |
| Spring | 5.2 to 8.8 |
Soil bacteria were higher, not lower, under exposure
Culturable aerobic mesophilic counts ran from 7.66 x 10^6 to 1.86 x 10^8 colony forming units per gram of dry soil, and exposed soil carried more than control soil at p = 0.021. Season mattered too, at p = 0.001, with winter on top at up to 1.86 x 10^8, and the treatment by season interaction was absent at p = 0.335. In the AICc 898 +/- 1.5 model, cumulative precipitation came in positive at beta 1.35 x 10^6 and p below 0.001, mean relative humidity negative at beta -7.03 x 10^6 and p = 0.001, lower soil temperature negative at beta -2.03 x 10^7 and p below 0.001, and oxytetracycline exposure itself positive at beta 2.53 x 10^7 and p = 0.028.
More colonies on exposed plates reads as tolerant populations taking the room, not as the drug knocking the community down. Plate counts cannot settle it, because nutrient agar only recovers part of what lives in soil.
Risk quotients: a tiny flux with a large risk signal
PNECMIC, the predicted no effect concentration for resistance selection, was set at 0.5 ug/L for oxytetracycline. Divide the measured concentration by that number and you have the risk quotient, where under 0.1 is low risk, 0.1 to 1 is moderate, and 1 or more is high. Every event in this trial was high, and oxytetracycline runoff water ran 220 to 540 times the concentration meant to keep selection away. By season, quotients were 380 to 540 in summer, 280 to 360 in autumn, 220 to 480 in winter and 260 to 320 in spring, on measured concentrations of 110 to 270 ug/L.
| Season | Measured concentration (ug/L) | Risk quotient | Risk class |
|---|---|---|---|
| Summer | 190 to 270 | 380 to 540 | High |
| Autumn | 140 to 180 | 280 to 360 | High |
| Winter | 110 to 240 | 220 to 480 | High |
| Spring | 130 to 160 | 260 to 320 | High |
How to cut oxytetracycline runoff at pen and field level
This is a storage and timing problem more than a dosing problem.
- Plan spreading around cumulative rain rather than storm intensity, because the model kept cumulative precipitation and dropped I30.
- Cover and contain what is stockpiled, since a deposit stayed a source for the full 30 days in all four seasons.
- Put sediment in the plan with buffers or settling areas, because particles carry adsorbed drug.
- Keep winter on the calendar, since it had the weakest intensities and the wettest soil at 19.89 percent.
- Sample the water and not just the soil, because what matters is the fraction already moving.
Limitations
Pooling sediment by season left one number per collector per season, so event level particulate transport was never measured, and the particle bound share of the loss stays unknown. Cold season persistence is read off temperature, moisture and radiation patterns rather than a measured dissipation rate constant. Event counts were lopsided, 10 in summer against 2 in autumn, which weakens the seasonal comparison. One soil type, one slope class, and one predicted threshold applied to every event.
Bottom line
Oxytetracycline runoff carried off 0.02 to 0.07 percent of the fortified mass and returned risk quotients of 220 to 540 in all four seasons. Two findings should change practice: the residue lasted 30 days, and it appeared after every rainfall event. Percentage of dose lost is the number to stop watching.
FAQ
How much oxytetracycline runoff does swine manure produce?
Runoff water carried 0.13 to 0.26 ug/mL across four seasons, which is 0.02 to 0.07 percent of the fortified mass in the deposit. Summer produced the highest values and every season still showed detectable residue at the final sampling event. The moving fraction is small, but the residue pool in manure is large enough to keep concentrations measurable.
Why is oxytetracycline runoff highest in summer?
Summer brought the most rainfall events at 10, the deepest cumulative precipitation at 119.99 mm, and the highest 30 minute intensity at 7.87 mm. Those conditions generated more surface flow and more sediment, and the regression model put cumulative precipitation at beta 5.72 x 10^-4 with p below 0.001. Summer also separated from the other three seasons at p below 0.001.
Does oxytetracycline reach runoff for the full 30 days?
Yes, residue was present at the last sampling event in all four seasons, including 0.20 ug/mL still there on day 30 in summer. Winter fell from early values of 0.15 to 0.20 ug/mL to about 0.11 ug/mL by day 22, so transport slowed without stopping entirely. Nothing in the data suggests a deposit exhausts its mobile residue within one month.
What risk quotient did the measured concentrations produce?
Risk quotients ran from 220 to 540 across every precipitation event, and all of them sat above the high risk threshold of 1. The quotient divides the measured concentration by a resistance selection threshold of 0.5 ug/L set for oxytetracycline. Measured values were 220 to 540 times that threshold, so no season fell below the high risk band.
Why are soil bacterial counts higher under oxytetracycline exposure?
Culturable aerobic mesophilic counts in exposed soil exceeded controls at p = 0.021, with the regression term for exposure at beta 2.53 x 10^7 and p = 0.028. The pattern fits selection for tolerant populations rather than a simple drop in total biomass. Plate counts on nutrient agar cannot separate resistant from susceptible colonies, so the mechanism remains an inference.
Can oxytetracycline runoff happen during low intensity winter rain?
Yes, winter had the lowest 30 minute intensity of the year at 0.95 mm and still produced 0.15 to 0.20 ug/mL in early events. Cold soil, high moisture and reduced radiation slow dissipation, so residue stays available in the soil and manure matrix between events. Soil moisture correlated with concentration at rs 0.477 with p below 0.001.
How is oxytetracycline measured in runoff water?
Samples were centrifuged at 14,000 rpm for 5 min and filtered through 0.22 micrometer nylon before injection. Separation used a Luna C18 column at 40 degrees Celsius with isocratic elution at 1.5 mL/min and UV detection at 365 nm. Calibration covered 0.06 to 8.00 ug/mL with r2 above 0.999 and a coefficient of variation under 10 percent.
What does PNECMIC mean in a runoff risk assessment?
PNECMIC is the predicted no effect concentration for resistance selection, set at 0.5 ug/L for oxytetracycline. It comes from modeled minimum inhibitory concentration distributions and marks the level below which selection is not expected. Dividing a measured concentration by it gives the risk quotient, where 1 or above counts as high risk.
Does particle bound transport matter for oxytetracycline loss?
Sediment recovery tracked rainfall erosivity, with 6.7 to 16.1 g per collector in summer against 0.5 to 1.9 g in autumn. Tetracyclines bind soil particles and organic matter, so a meaningful share of the loss probably travels attached to sediment. The trial pooled sediment across a whole season, so that pathway was not quantified at event level.
How can a swine operation reduce oxytetracycline loss in runoff?
Keep deposits and stored manure away from the wettest windows, because cumulative precipitation was the strongest predictor in the model. Cover stockpiles and intercept flow with buffers or settling areas, since sediment carries part of the load. Treat runoff collection as a monitoring point, because residue persisted through 30 days in every season.
