an epidemiological study of listeriosis in dairy cattle

suppression of the growth of Listeria organisms is lifted, resulting in rapid
multiplication of L. monocytogenes to the level of the infectious dose (Gronstol 1979a,
Anon 1983, Fenlon 1988, Husu and others 1990a, Sargison 1993). It has been
demonstrated that the multiplication of L. monocytogenes is related to pH. As pH rises
the number and frequency of isolation of L. monocytogenes also increases (Irvin 1968,
Gronstol 1979b, Fenlon 1988). As important as silage quality is the method or methods
of feeding silage. Some clinical forms of Listeriosis (encephalitis, iritis) have been
attributed to physical injuries of mucosal membranes such as buccal or conjunctival
membranes caused by rough forages (Asahi 1957, Dennis 1993). These facts prompted
us to further evaluate the role of silage by gathering information on preparation, the
method and time of feeding and quality. This was done for both grass silage and maize
silage.
The stage of growth or time of cutting grass for silage is known to be important.
(Anon 1983, Wilesmith and Gitter 1986, Gitter 1989). To ensure perfect fermentation
grass must be harvested at the right stage when it contains a sufficient level of
carbohydrates and has a Digestibility (D) value around 70%. This covers the period
between May, June and July (Anon 1983). In our study there was no association
between the month of making silage and disease. This may be explained by the fact that
most of the farms (90%) made silage within the recommended period (May and June for
grass silage or October for maize silage). However the number of times the grass was
cut to make grass silage was associated with an increased risk of Listeriosis in milking
cows. It is known that first cut grass silage achieves better fermentation (Fenlon 1988)
but second or later cuts grass may lack the necessary components (high sugar, correct
dry matter level, 25-30%) to achieve optimal fermentation (Anon 1983, Fenlon 1988).
Another important factor at cutting which helps determine silage quality is soil
contamination. Soil causes butyric acid fermentation which results in higher pH values
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