Ever since the hot summer of 2003, the notion of wrapping silage in green film has become more and more accepted. Where once green or white film was preferred only for high Dry Matter haylage, and the less critical silage made do with the cheaper black alternative, things are changing rapidly.
As the prospect of further hot summers to come seems more and more probable, according to the scientists and weather forecasters, a realisation of the risks associated with black film should be considered, if not already understood, to de-mystify the subject and convince some of the doubters amongst you. Firstly, the obvious, from which all subsequent points are taken — film temperature. In June and July 2003, when daytime temperatures were at their peak, a roll of black film, in direct sunlight, could reach a surface temperature of 50 degrees or more within an hour, whilst green film, by comparison, remained below 30 degrees. The effect of this huge temperature rise on the black film had the effect of altering the performance of the film, making it, at the same time, much more pliable and easier to stretch and stickier to touch, as the active tack ingredient in the film migrated to the film surface much quicker in the warmer conditions.
These factors produced the ideal conditions for secondary-stretch to occur, with the result of increasing the film neck-down as it was stretched through the PSU, so narrowing the width of film applied to the bale. This narrower film web applied to the bale then made it impossible for each successive application of film to over-lap the previous by the required 50% of its width. From this, the classic case of too few film layers on parts of the bale would occur. However, with black film this mis-application of film was never seen, as the bale was still overall black in colour.
Another factor of increased temperatures on black film is the effect it has on the wrapped bale. With higher surface temperatures possible with black film, so the bale will expand more as the internal gases heat up quicker, causing a positive pressure situation. Consequently, when the bale contracts, at dusk or during a rain shower, so the gases contract creating a negative pressure within, which has the effect of being able to draw any moisture that might be present on a bale, in between film layers — one of the causes of water in bales and spoilage.
So it can be seen that the move from black to green is not some cynical ploy by film manufacturers to make you buy more expensive film, but actual common sense. It is fact that the base raw material for coloured film is naturally more expensive than black and, due to the traditional demand of black film in our market compared to coloured film; some producers may have weighted their production for the UK and Ireland towards black creating the situation of a greater cost differential between each colour. On the continent, the use of black film is significantly lower than the UK and Ireland, for the reasons already mentioned. But this market is ahead of us again in another area of bale wrapping, that the UK and Ireland has yet to wake up to fully — that of producing far better quality bales through the application of more film.
It is a well documented fact that increased film layers on a bale reduce spoilage and increase the value of the bale, either in nutritional terms or purely monetary value if selling at market. Two years ago CEDAR – The Centre for Dairy Research of The University of Reading, conducted extensive trials on this subject, with the conclusion that, on average, 9% of a bale wrapped with 4 film layers was lost to spoilage, through air or water contamination; compared to almost nil when wrapped in 6 layers. Calculating this out, one can see that the extra cost of film required to apply 2 more film layers is still far below the reduction in value suffered by the bale from having in-sufficient layers applied. Again, this is not some cynical ploy by film producers, but actual, scientific fact.
In Europe, it could be argued that their move to wrapping with a minimum of 6 film layers, and this is just a minimum (many parts of Europe apply 8 or even 10 film layers where crop or bale value is concerned), may have been a result of understanding the facts of wastage, but also by appreciating that often bales with 4 film layers applied were not being wrapped correctly, which was seen easily through the use of light coloured films. Remember, with black film, you are unable to see the mistakes made during film application, for what ever reason. So, by increasing the film layers to 6, the bale was definitely wrapped to provide at least 4 film layer everywhere — this is an important fact to remember when wrapping square bales, as the bale shape presented to the film web during wrapping is constantly changing, unlike a round bale — which remains constant. So, before you place your order for film this season, think why so many other end users throughout Europe choose green film and realise that is probably why your neighbour chose to do so too.