While silage for sheep is not totally new, the advent of the silage bale has made it a more practical option and increased its use.

Grass cut for silage and then baled provides sheep farmers with good quality winter feed, conveniently packaged to be kept or transported where it is needed. There are fears, however, concerning the potential for Listeria bacteria to develop in the silage and thus infect the sheep when they are fed silage.

Listeria bacteria require specific environmental conditions in order to proliferate and become a threat. Inside a silage bale the bacteria have nutrients, moisture and suitable temperature. However, the absence or very low levels of oxygen, and a low pH, inhibit their development. Both these factors will change if the bale is punctured, which is a key reason for recommending four or six wrap layers to minimise this risk.

“Listeria can occur in sheep of all ages, but it tends to be seen in young lambs, causing neurological problems, and pregnant ewes, which may abort as a result of infection,” comments Dr. Colin Macaldowie, Veterinary Research Pathologist at the Moredun Research Institute in Edinburgh. “The exact precursors to infection are not fully known, but in lambs, infection may be connected with tooth eruption. Lambs which have teeth emerging have open sores in their mouths and it is thought the bacteria may enter through these and access the nervous system.

“Cases are sporadic, but occur usually in sheep that are four to six months old and during their first winter and as yearlings. However, there have been some large outbreaks when bad silage has been fed in sheep housed intensively. An obvious preventative measure is to make sure bales have not been punctured when they are fed, and that the silage has been well preserved and is not wet or rotting.”

Information gathered during the preparation of this survey shows that many farmers, particularly in Wales, are happy to feed well-made silage to breeding ewes and often have no other option. The essential point is that the silage should be made and stored correctly, with good quality forage, equipment and materials.

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