Bird damage to silage bales:
Irelands newest bird-agriculture conflict!

By Keir McNamara, Padraig O'Kiely and Dermot Forristal, Teagasc Grange and Oak Park Research Centres

Ever since the process of wrapping bales with plastic stretch-film was launched on a large scale in Ireland in the mid 1980s, the popularity of baled silage has increased to such an extent that it is now made on two out of every three farms. It now accounts for one-third of the annual national crop.

This amounts to about 9 million bales annually and it now is the primary grass conservation system on smaller sized farms and the secondary system on larger farms. At the same time that this conservation method was becoming popular, the problem of bird attacks on silage bales, resulting in holes in the film became evident.

Establishing the scale of the problem

A national survey was undertaken by Teagasc researchers to establish the scale of the problem. Three hundred silage bale collections were surveyed along six different routes traversing 20 counties, to give a representative sample of farming systems and geographical locations throughout Ireland. Birds were found to be the animal species most responsible for damage to the plastic film surrounding the bales, with damage found on 63% of the bale collections during storage.

Birds were also found to have damaged the bales in the field just after wrapping in 53% of the collections surveyed. Domestic cats were the animals found to be the next most important cause of damage, responsible for damaging 29% of the collections, mostly in farmyards. The incidence of observed damage caused to baled silage by rats (0.003% of bales), dogs (0.007% of bales) and farm livestock (0.015% of bales) was less than for birds and cats.

The effects of holes in the film on silage quality

The preservation of grass as silage is based on the principle of limiting aerobic biological processes and promoting the conversion of sugars in the grass to acids. These conditions inhibit the growth of spoilage micro-organisms and ensures crop preservation. In a series of controlled experiments carried out at Grange, different bird damage levels were simulated on bales (0, 1, 10 or 50 small holes, or 1 large hole) and the effects of these holes on forage conservation were quantified after a number of months of storage.

The general trend was that with an increase in the number of holes made in the film, there tended to be a corresponding increase in the amount of mouldy or rotted silage on the bale surface.

It also increased the depth to which the mould penetrated the bale, in the volume loss of the bale during ensilage and in the proportion of inedible silage present. Even one small hole in the film was found to cause an 8% loss in edible forage in high dry matter bales (30% DM), while high dry matter bales with 10 or 50 small holes or one large hole suffered edible forage losses of 15, 21 and 32%, respectively.

Bales with lower dry matter levels suffered fewer losses, as oxygen could not penetrate as deeply into these bales compared to the lighter and less dense high dry matter bales. Analyses of the mouldy and rotted material in these bales showed it to be of very poor quality, and unsuitable for feeding to livestock.

Why do birds damage bales?....

Definitively answering whY birds damage the film is very difficult. Birds come into silage fields primarily to feed on the abundant invertebrate food supply that is exposed by the harvesting machinery, not to attack bales.

It was found that they didn't consume the film itself or the forage within a bale. Bales release large amounts of gas in the first hours after wrapping, which can be heard hissing from the bales.

This noise coupled with the heat and bright lustre of a newly wrapped bale may attract a bird to it and the observations of the birds walking around a bale and displaying a degree of curiosity prior to an attack may add some validity to this theory.

Corvids in urban and suburban areas are known to utilise human food waste, especially on refuse collection days, when black plastic bags of waste are left outside for collection.

It is possible that this association with black bags and food could be manifesting itself in the baled silage field. Another concept that many farmers hold it that birds carry food up onto a bale to consume it, and in the process inflict damage to the film.

No birds were ever observed bringing food onto a bale in the experiments conducted within Teagasc, and given what is known about their feeding ecology and behaviour in silage stubbles, it is highly unlikely they would do this to any great degree. Juvenile rooks, who damaged the bales most often, engage in little feeding while in the fields during the summer.

Like all juveniles they are highly inquisitive and invariably were seen to wander around silage bales, where they would probe at and/or damage the bales. On other occasions, they would fly up onto a bale to call its parent, and while there, would inflict damage to the film.

Rooks and jackdaws responsible for bale damage

Over three years, surveys on the level of use of baled silage fields by birds were carried out throughout the country, and detailed observations of bird behaviour took place at the research centre. It was found that rooks and jackdaws are the species that utilise silage stubbles in the greatest numbers. They were also the only species observed to damage bales. Juvenile rooks were found to damage the bales most often.

Ireland has one of the highest densities of these highly adaptable birds in the Europe, which, due to our climate, geography and farming systems, are distributed throughout the country. It is likely therefore, that nearly all the silage bales in Ireland are within an area frequented by these birds.

Many other bird species, especially seagulls and magpies utilise silage stubbles in small numbers and most are also blamed for damaging bales by farmers. While, they may be causing problems in localised areas (especially seagulls in coastal areas), given the small numbers of them that were seen to utilise silage stubble fields relative to rooks and jackdaws during these studies, it is unlikely that they are a major cause of bale damage.

Conclusions