**3. Contamination of animal feeds and raw milk**

Depending on climatic conditions, season, and specific feeding requirements, ruminant diet could include fresh forage during summer months or all year long under favourable climate, or be fed with hay, silage, and/or grains (including corn) during winter season in northern regions. Butyric acid spores could contaminate all of these feed types, but at different level and thus be part of a contamination cycle. Among clostridial species at farm level, species related to Cluster 1 are a major concern in relation with cheese making. However, some other species also need consideration for their potential in health problem namely *C. botulinum*.

When silage is used as a feed source on a milk farm, silage conservation is of particular importance. Different events will take place that have relevance on the number of clostridial spores that could end up in raw milk in the bulk tank. During mowing and harvesting, contamination of crops by soil particules and manure aggregates will happen. During silage fermentation, butyric acid spores could germinate and grow if conditions are met. Animal will ingest the contaminated silage and spores will be released in feces. Clostridial spores will subsequently end up in manure, where population shift will happen before being applied again on crops. It could also contaminate teats, thus facilitating contamination of milk while milking the cows. These events can be considered as the spore cycle on a farm (Figure 3) (Pahlow et al., 2003).

**Figure 3.** Contamination cycle of clostridial spores from soil to raw milk at the farm level (adapted from Pahlow et al. , (2003)).

#### **3.1. The soil, reservoir of Clostridium and organic fertilizers**

Numbers of clostridial spores in the soil following plate counts on Reinforced Clostridial Agar (RCA) lead to mean counts around 4 log10 CFU g-1 soil (Julien et al., 2008). Considering the low selective capacity of this medium in relation with the high diversity of clostridial species that could be present in the soil environment (Figure 2), this number greatly overestimate the number of strictly "butyric acid" spores present in soil. Even though, these different species could contaminate forage plant following harvesting and, cause hygienic problem in silage that could affect yield and health in some herds (see section 4. *Clostridium botulinum*). Number of butyric acid spores on plant surfaces is higher following plant harvesting since harvesting machineries, either mower, harvester or baller, contribute to the dispersion of soil dust leading to spores contamination on the plant surface. The type of mower used has a direct incidence on plant contamination. Disk mower will pick up soil is dry or ground surface is unlevelled. Speeds at which the disks rotate allow aspiration of soil. It is then very important to raise the cutting table to mow at least 7-8 cm above ground level. On a trial in an old grass stand, spore numbers was one order of magnitude lower when cutting height was set to 10 cm versus 7 cm (Drouin et al., unpublished results). This is particularly critical in a new stand or on a stand that received manure application, which could be up to 3 orders of magnitude higher (Drouin et al., unpublished results). Utilization of a windrower to accelerate forage wilting is also an operation that could contaminate the forage with dust soil.

380 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

Depending on climatic conditions, season, and specific feeding requirements, ruminant diet could include fresh forage during summer months or all year long under favourable climate, or be fed with hay, silage, and/or grains (including corn) during winter season in northern regions. Butyric acid spores could contaminate all of these feed types, but at different level and thus be part of a contamination cycle. Among clostridial species at farm level, species related to Cluster 1 are a major concern in relation with cheese making. However, some other species also need consideration for their potential in health problem namely

When silage is used as a feed source on a milk farm, silage conservation is of particular importance. Different events will take place that have relevance on the number of clostridial spores that could end up in raw milk in the bulk tank. During mowing and harvesting, contamination of crops by soil particules and manure aggregates will happen. During silage fermentation, butyric acid spores could germinate and grow if conditions are met. Animal will ingest the contaminated silage and spores will be released in feces. Clostridial spores will subsequently end up in manure, where population shift will happen before being applied again on crops. It could also contaminate teats, thus facilitating contamination of milk while milking the cows. These events can be considered as the spore cycle on a farm

**Figure 3.** Contamination cycle of clostridial spores from soil to raw milk at the farm level (adapted from

Numbers of clostridial spores in the soil following plate counts on Reinforced Clostridial Agar (RCA) lead to mean counts around 4 log10 CFU g-1 soil (Julien et al., 2008). Considering

**3.1. The soil, reservoir of Clostridium and organic fertilizers** 

**3. Contamination of animal feeds and raw milk** 

*C. botulinum*.

(Figure 3) (Pahlow et al., 2003).

Pahlow et al. , (2003)).

The conservation method used to store forages has a direct impact on the subsequent number of spores. Hay will usually not allow germination of spores and subsequent development of the organism. Silage made from grasses or legumes could lead to important population of clostridial spores according to the ensilability of the crop. Lactic acid fermentation of whole corn silage is generally fast enough to ensure a good conservation. However, under aerobic instability environments, *Clostridium* development had been observed (Vissers et al., 2007a).
