**1. Introduction**

Animal feed ingredients are at risk of mould contamination with subsequent mycotoxin production during preharvest, harvest and postharvest times [1–3]. The sources of the individual components used in the formulation of dairy feeds are quite diverse ranging from cereals, cereal products, oil seeds as well as hay and forages [3, 4]. Also the high cost of feed has led to the addition of stale bread, kitchen and bakery wastes to the feed. Furthermore scarcity of protein sources for animal feeds has led to the use of alternative protein sources such as brewers' spent grains (BSG) [5]. These waste products are usually tainted with fungus and may be a contributing factor in mycotoxin production in cattle feed. Aflatoxins are the most toxic mycotoxins produced by members of

the genus *Aspergillus* [6], and their presence in animal feedstuffs has become a potential health hazard to both animals and humans [7]. Toxic effects of aflatoxins in ruminants include liver damage, diminished growth efficiency, diminished milk production and quality and impaired resistance to infectious diseases [7–9].

In dairy farming, depending on the farming system adopted, the diet consists of the concentrates, alternative protein sources as well as forage; hence the animals are exposed to more than one type of mycotoxins [4]. Although there are more than 20 aflatoxins known, only four of these occur naturally, namely, aflatoxins (AF) B1, B2, G1 and G2, based on their fluorescence under UV light (blue or green) [10–12]. The most abundant aflatoxin in cow feeds and rations is aflatoxin B1 and is also the most potent of them all [13, 14].

Animals differ in their sensitivity to mycotoxin toxicity [15] with ruminants being more resistant than the monogastrics [16] mainly because they have microorganisms in their rumen which play significant roles in the deactivation and degradation of the aflatoxins as well as alteration of the binding of the aflatoxins to some essential nutrients [17, 18]. However, aflatoxins are poorly degraded by ruminants as most of the rumen microbiota are inhibited by AFB1 concentration of 10 μg/ml [16]. The aflatoxins will get to the bioconversion sites of nutrients and xenobiotics like the intestinal epithelium, liver and kidneys unaltered [16]. In the liver, AFB1 is bio-transformed to AFM1 which enters the circulatory system or is conjugated to glucuronic acid. The conjugated AFM1 is excreted through the biliary system, and the one in circulation may be excreted through urine and milk. It has been shown that AFM1 retains some carcinogenic activity resulting in its reclassification by IARC as a group 1 carcinogen [19–21]. Consumption of AFB1-contaminated feed by lactating cows results in its metabolism into AFM1 subsequently secreted into milk thereby making milk a source of aflatoxin contamination in humans. In this study the extent of aflatoxin contamination of feeds used in different feeding systems adopted by dairy farmers was assessed.
