Opposite direction for seasonal variation of aflatoxin M1 in bulk milk and aflatoxin B1 in rations: results from a prospective study in selected dairy farms of Qazvin province, Iran

In the present study, aflatoxin M1 (AFM1) in bulk milk (n=72) and aflatoxin B1 (AFB1) in concurrent rations (TMRs; n=48) and feed ingredients (n=230) were assessed in 12 dairy farms in winter and summer. Bulk milk was sampled on days 1, 15 and 30 of the study. Feeds were sampled at days 1 and 30. Aflatoxin was measured using ELISA kits (detection ranges: 1-81 ngkg−1 for milk, 1.25-101.25 ngkg−1 for feeds). AFM1 was identified in all milk samples (range: 2.03 to >81 ngkg−1; median: 70 ngkg−1). Overall, 76% of milk samples (n=55/72) had AFM1 levels <81 ngkg−1 (Iranian limit:100 ngkg-1). Contaminations >81 ngkg−1 (n=17/72; 24%) were more frequent in winter (n=15/36 vs. 2/36). Sixty-nine percent of winter (n=25/36) and 31% of summer samples (n=11/36) had contaminations above the median. The chance of contaminations above the median was higher in winter (OR=5.33, P=0.007). All TMRs and ingredients had higher contaminations in summer. Seventy percent of summer and 30% of winter TMRs had contaminations above median (716 ngkg-1). The chance of TMR contamination above median was higher in summer (OR=5.57, P=0.002). The lower AFM1 levels in summer could be due to reduced hepatic AFB1 metabolism and lower dry matter intake induced by heat stress. Grain mix (rs=0.90; P=0.001), corn silage (rs=0.66; P=0.001) and wet beet pulp (rs=0.68; P=0.005) were the most prominent contaminants of TMRs. Due to the limitations of the diagnostic kit and different year-round nutritional conditions, higher or lower AFM1 contaminations are probable. With the current nutritional practices, higher summer contamination may happen if heat stress is efficiently controlled.