Aflatoxin M1 in milk

Definition

Aflatoxin M1 (AFM1) is the form of aflatoxin found in milk. It is not in the feed itself. Cows eat aflatoxin B1 (AFB1), a toxin made by the molds Aspergillus flavus and A. parasiticus. The liver converts part of it to AFM1, which passes into milk. AFM1 is a regulated contaminant because aflatoxins are carcinogens. Pasteurization and processing do not destroy it.

AFB1 is most common in:

• Corn grain and corn by-products, especially after a hot, dry growing season, insect damage or poor storage. Distillers grains concentrate the toxin from the corn they came from.
• Whole cottonseed and cottonseed meal, a frequent source in hot, dry regions.
• Peanut meal and some imported ingredients.
• Corn silage in drought years, although usually at lower levels than grain.

The share of AFB1 intake that reaches the milk as AFM1 is the carryover rate. AFM1 shows up in milk within hours of a contaminated feed being eaten, by the first milking afterwards (about 12 hours or less), and levels off within a few days of steady feeding. After the feed is removed, milk AFM1 falls quickly and is usually undetectable within about 1–4 days (Zentai et al. 2023 review; Masoero et al. 2007).

Regulators set limits on two things: the AFM1 level in milk, and the aflatoxin level in feed for lactating dairy cattle. Processors also screen loads and may set tighter internal limits. Requirements change; confirm the current version with the regulator or the milk buyer. This is not legal advice.

Formula

Milk AFM1 from feed intake (mass balance):

AFB1 intake (µg/day) = total ration AFB1 (µg/kg, i.e. ppb, DM basis) × DMI (kg DM/day)
milk AFM1 (µg/kg, ppb) ≈ AFB1 intake (µg/day) × carryover (fraction) / milk yield (kg/day)

LaTeX:

I_{\mathrm{AFB1}} = C_{\text{ration}} \times \mathrm{DMI}
\qquad
C_{\mathrm{AFM1}} \approx \frac{I_{\mathrm{AFB1}} \times k}{Y_{\text{milk}}}

Worked example: 25 kg DM/day of a ration at 20 ppb AFB1 is 500 µg/day. At 2% carryover, 10 µg goes to milk; spread over 35 kg (77 lb) of milk that is about 0.29 ppb AFM1.

Ingredient to ration:

ration AFB1 (ppb) = Σ (ingredient AFB1 ppb × ingredient share of ration DM)

C_{\text{ration}} = \sum_i C_i \, w_i

Units: ppb = µg/kg; ppt = ng/kg. Milk is about 1.03 kg/L, so µg/kg and µg/L are close but not identical.

How to pull it in DC305

Not in DC305. AFM1 is a bulk-milk measure, and feed aflatoxin is a feed-lot measure; neither is recorded on individual cows. The evidence lives in:

• Processor or cooperative load tests: screening results per pickup or per load, held with the bulk tank pickup data.
• Feed lab reports and supplier certificates for corn, cottonseed, distillers grains and other at-risk ingredients.
• Scale tickets and commodity receiving logs, which tie each delivered lot to its test result and to the dates it was fed (see scale tickets).
• The farm’s food safety plan (see HACCP), if mycotoxins are a listed hazard.

DC305 is only useful for the inputs to the mass balance: the number of lactating cows and the average milk per cow, e.g. SUM MILK FOR LACT>0 RC<>6 or the herd’s own production item.

Unverified

That RC<>6 (exclude dry) is how the herd separates lactating cows; some herds use a pen range or a lactating flag instead. See the herd’s private dc305-vocab mapping.

Typical range

Herd context: US dairies supplying Grade A milk; US FDA limits, with the EU limit for comparison; limits as of November 2026.

Limit	Value	Applies to
FDA action level, AFM1 in milk	0.5 ppb (µg/kg)	Milk in US interstate commerce
EU maximum level, AFM1 in milk	0.05 µg/kg	Raw milk, heat-treated milk and milk for dairy products in the EU
FDA action level, total aflatoxins (B1, B2, G1, G2) in feed for dairy animals	20 ppb	Corn, peanut products, cottonseed meal and other animal food and food ingredients
FDA action level, feed for finishing (feedlot) beef cattle	300 ppb	Corn and peanut products (not for dairy)

Notes on the limits:

• The FDA feed action level applies to each animal food and animal food ingredient intended for dairy animals (CPG 683.100), so an individual ingredient over 20 ppb is over the level, not just the total ration. FDA action levels are non-binding guidance for enforcement, but feed above them can be treated as adulterated.

• The EU limit (Regulation (EU) 2023/915, Annex I, entry 1.1.16) is 0.050 µg/kg for raw milk, heat-treated milk and milk for the manufacture of milk-based products.

• The Grade A PMO (2025 revision) sets no aflatoxin standard of its own; its Table 1 limits cover bacteria, SCC, temperature and drug residues.

• Carryover rate: about 1–2% on average, and up to about 6% in high-producing cows (Zentai et al. 2023 review). Milk yield is the main driver: high-yielding cows milked three times a day showed 5.8% carryover in mid-lactation vs 2.5% in late lactation (Britzi et al. 2013), and somatic cell count had no effect (Masoero et al. 2007).

• Rule of thumb: at steady state, feed AFB1 to milk AFM1 runs at roughly 70 to 1 (a 1985 study cited in Zentai et al. 2023), so milk AFM1 ≈ total ration AFB1 ÷ 60 to 75. At the 20 ppb feed limit this gives about 0.27–0.33 ppb in milk, inside the FDA limit but well above the EU limit. This is an empirical ratio, not a regulation; it is about 2% carryover at typical DMI and milk yield, and high producers with higher carryover can exceed it.

• Processor screening: many US buyers test loads with rapid screening kits and reject or investigate loads at or below the FDA level. Internal limits vary by buyer.

Common mistakes

• One grain sample per load. Aflatoxin occurs in hot spots. A single probe or grab sample can miss it entirely. Take many probes across the load and grind the composite before testing.

• Testing only corn. Cottonseed, distillers grains, peanut products and drought-year corn silage also carry AFB1. Mycotoxins in distillers grains can be concentrated up to about three times the level in the corn they came from (Wu and Munkvold 2008).

• Comparing ingredient ppb to the milk limit. Feed and milk limits are on different scales. Convert through the ration and carryover, or use the feed limit.

• Confusing ppb with ppt. 0.5 ppb is 500 ppt. Some lab and kit reports use ppt or ng/L.

• Blending a hot load with clean grain to get under the limit. FDA guidance says blending a feed over an action level with other feed is not permitted, and the blended product is unlawful whatever its final level (FDA action levels guidance, 2000).

• Relying on a binder alone. Clay binders reduce absorption but don’t remove the toxin, and none is approved in the US with an aflatoxin claim. Pull the contaminated ingredient first.

Unverified

The FDA blending position is confirmed. Still open: FDA’s position on binder (sequestering agent) claims, i.e. that no product is FDA-approved with an aflatoxin-binding claim; no FDA statement was found.

• No lot traceability. Without receiving records tied to feeding dates, a positive milk test can’t be traced to an ingredient lot, and the source stays in the ration.
• Expecting milk to clear the same day. After the source is removed, milk takes a few days to fall. Hold or retest loads during that window.

Related

• Bulk tank pickup data
• Scale tickets
• Grade A PMO
• Food safety and HACCP