THERE is much needless waste of silage in wrapped bales all over Australia. Needless because often only a small attention to detail or slight changes to management can stop or at least reduce substantial losses of silage dry matter (DM) and quality on many farms. There are no short cuts when wrapping round or square baled silage.
Say a 1.2 metre round bale of silage with a diameter of 1.3m weighed 300 kilograms of dry matter (kg DM), or about 600 to 650kg fresh weight. If a hole is left unattended for only a few weeks, this could result in about 10 per cent wastage of that bale, potentially representing at least $11 loss in milk production not achieved.
This figure is derived from 10 per cent of 300, which equals 30kg DM waste. With a silage quality of 10 megajoules of metabolisable energy (ME) per kilogram of dry matter (MJ ME/kg DM), the 30kg DM represents 300 MJ ME lost. It takes about 8MJ ME to produce one litre of milk, so at 30 cents a litre, the loss is about $11. Many bales have much greater losses than this.
The target for baled silage is high-quality forage ensiled into bales at the correct DM content (40-50 per cent for rounds, 40-60 per cent for squares), densely compacted and wrapped with the correct number of layers (four layers at 55 per cent stretch) over the entire bale. If achieved, the silage will have a pleasant smell with negligible mould anywhere on the bale when opened up to 12 months later.
Any mould noticed on or in the bale indicates that air has entered the bale somewhere, somehow, and the amount of mould and its location can often be an indicator to the cause (and its solution). Similarly, the amount of moisture and its location in the bale can help pinpoint the cause.
Be aware that baled silage has six to eight times the surface area in contact with the plastic film compared with conventional stack silage and about half of the silage volume is within 15 centimetres of the plastic film. Therefore, it is important that the integrity of the film is not compromised in any way until fed out.
This article discusses some of the characteristics of wrapping bales and how these and other factors cause failures in baled silage.
Pre-stretch rate: Many people think that any plastic is airproof and will prevent all air from passing through the film and entering the bale. Wrong! Most stretch wrap films have a thickness of 25 micron (Ám) before application to a bale. Most wrapping machines sold in Australia are designed to pre-stretch the stretch wrap plastic 55 per cent as it is applied to the bale. If four layers are applied, the final thickness will be about 18-20 Ám per layer, which is 74-80 Ám in total.
If stretched 70 per cent, as is common in New Zealand, thickness is further reduced, hence their recommendation to apply six layers versus our four. Compare the 74-80 Ám cover to the thickness of 120-200 Ám for plastic sheets sealing bulk cut silage.
A new stretch wrap film has recently been introduced into Australia, which is co-extruded with five layers instead of three and, as a result, is said to be much stronger and more evenly stretched. The company stipulates 70 per cent pre-stretch and to apply six layers, requiring a change of gears on the pre-stretcher.
Farmers and contractors may also remember another Australian company, with the aim to reduce plastic costs, introducing a 35 per cent pre-stretched film, which also required a change of gearing on wrappers.
There is a potential problem looming of gearing and stretch wrap mismatches once machines start to change hands or different films are used, if the gearing information is not also passed on to the new operator.
To check that the pre-stretcher is working and/or the correct cogs are in place, mark a 10-centrimetre line or trace around a match-box on the roll of film (see Figure 1). Once pre-stretched and applied to the bale, re-measure the mark. At 55 per cent stretch the line should now measure just above 15cm or the match-box increased by half its original size.
Wrapping in high ambient temperatures (above about 25 degrees Celsius) may affect the properties of some plastics, particularly the stretch rate.
Bale coverage: Every round or rectangular bale that is being individually wrapped must have at least four layers of plastic (at 55 per cent stretch) over the entire bale. This is difficult to achieve with slightly odd shaped bales resulting in underlapping (see Figure 2) and only three layers will be applied at that section causing plastic coverage to be reduced by 25 per cent on a seal where 100 per cent coverage is a must. Overlap for each layer should be at least 50 per cent, no less.
Baled crops with mature or stemmy material should have six layers applied to reduce puncturing of the film and extra care taken when placing bales onto lucerne or cereal stubbles.
Centre the bale: Larger diameter bales are becoming more common and if the pre-stretcher is aligned with the bale centre, wrapping will be uneven on the bale.
Number of layers applied: Bales that have had four layers of stretch wrap film applied at 55 per cent stretch with a 50 per cent overlap and are guaranteed to last 12 months before the film starts to degrade from solar radiation sometimes last several months longer. Practical experience in the field has shown that applying six layers at the above stretch will generally ensure bales will last a further 12 months.
Overseas research comparing two, four, six and eight layers, at 70 per cent stretch, has shown that two layers will result in huge losses of dry matter and nutritive value due to mould and yeast growth and aerobic deterioration due to air entry. Applying four layers resulted in only slightly extra surface mould (0.5-1.7 per cent of bale surface) growth compared with six layers (0.1-0.7 per cent). However, six to eight layers may be necessary if wrapping drier or more mature pastures. Six layers are recommended if bales are to be transported after wrapping.
Colour of plastic: Based on overseas research, the lighter-coloured films would be more suited to the hotter areas of Australia compared with black due to less surface heating. Black is more absorbent of solar radiation. However, in the cooler temperate areas such as southern Victoria and Tasmania, black wrap is equally effective and may be less prone to ultra-violet (UV) break down.
Plastics ain't plastics: Most films are now of good quality with adequate levels of resistance to UV light degradation. Films can vary in permeability to air, consistency in stretching capacity, degree and longevity of adhesiveness or "tackiness". They can also contain irregularities from the production line, have unevenness of colour integration and, most importantly, the quality and amount of UV light inhibitor impregnated into the plastic film can vary substantially between products.
Holes: Probably the largest cause for wasted baled silage. An Irish experiment measured 8 per cent loss of edible silage from one three millimetre hole, 15 per cent loss from 10 holes of 3mm diameter but about 32 per cent lost from one 24mm sized hole if left unpatched for five months compared with under 1 per cent in undamaged bales.
The Irish bales were only about 30 per cent DM compared with Australian bales commonly having a DM range of 40 to 50 per cent so losses can be expected to be higher here.
Bottom line, the drier the forage, more stemmy the material, warmer the weather, larger the hole and the longer before being repaired, the greater are the DM and quality losses and greater the mould growth and aerobic deterioration.
Many batches of bales are protected by single-wired electric fences, which too commonly don't do the job. Damage caused by animals breaking in and ripping large holes in the film is almost impossible to repair unless rewrapped, which is rarely done. The extra cost of more permanent fencing might be worth considering.
Birds, vermin, cats, possums are other causes of holes and most of these problems can be overcome with persistence. Regularly checking for damage and patching with similar coloured specific silage repair tape to clean, dry and cool/warm plastic will reduce losses substantially.
With a bit more effort or extra time, large losses of baled silage can be reduced substantially, resulting in more feed for animal production. D
*Senior dairy extension officer, Agriculture Victoria, Victoria
Want to read more stories like this?
Sign up to receive our e-newsletter delivered fresh to your email in-box twice a week.