The effects of film colours and number of wrap layers on spoilage, nutrient quality and potential impact on animal production

Nutritional Values
Table 1


Average Spoilage Per Bale For Each Combination Of Wrap
Table 2


pH, Ammonia N and Sugars From Each Wrap Combination
Table 3

During 2002, Dow commissioned a study at CEDAR (Centre for Dairy Research) at Reading University in the UK, to compare the effects of film colours and number of wrap layers on spoilage, nutrient quality and potential impact on animal production. Specifically, the research assessed:

  • Dry matter losses,
  • Wastage,
  • Fermentation characteristics, and
  • Nutritive value.

Samples of harvested grass were taken at the point of ensiling to determine dry matter content and quality of the crop. The samples were sent to NRM (Natural Resources Management) for analysis.

Two colours of silage film were used — green and black — in combination with four, six or eight wrap layers. This produced six treatments. The silage film, standard 750 mm width, was supplied by Dow and was made from DOWLEX™ Polyethylene Resin.

While the silage cut was intended to take place in May, in the normal window for first cut silage, unusually poor weather conditions forced the cut into the second half of June, by which time the crop was older but higher in dry matter and sugar content and thus easier to ensile. The weather had also turned warmer.

The crop was cut, wilted for 24 hours, then baled and wrapped the same day. Ten bales were made for each treatment, all from the same field, and were stacked on a standard base for four months before opening.

In late October, five bales from each treatment were reweighed and opened. Visible wastage was removed and weighed, and the new dry matter content determined to estimate waste during the storage period. Following this, samples were taken for analysis.

Results

The most noticeable differences were in the wastage levels between green and black films, and between four, six and eight layers of wrap in the bales wrapped in black film (Table 1).

The spoilage in the bales wrapped with green film was nil, irrespective of the number of layers of wrap used. These results were unexpected and warrant further investigation. In the bales wrapped with four layers of black film, the average spoilage was 76 kg per bale on an average bale weight of 862 kg. This decreased to 2 kg per bale when the bales were wrapped with six layers of wrap, and 8 kg when covered in eight layers (Table 2).

Other results that proved of interest were significant increases in the digestibility (D Value) and metabolisable energy (ME) as the numbers of layers went up from four to six to eight in the black film. While these differences are difficult to explain, they may be associated with improvements noted in silage fermentation characteristics such as a reduction in pH and ammonia nitrogen percentage and an increase in residual sugars (Table 3).

Conclusions

In this study, the average wastage in bales wrapped with the standard four layers of black plastic was almost 9% of the total fresh weight of the bale. However, wrapping bales with six or eight layers reduced wastage to less than 1%. This indicates that there is a marked difference in spoilage when wrapping six rather than four layers.

The additional increase in digestibility and metabolisable energy also indicated a positive effect with increased layers of black film, possibly arising from the improved fermentation shown in the reduction in pH and ammonia nitrogen, and the increase in sugars.

Using green film appeared to leave no spoilage of any note, irrespective of the number of layers of film used. This could indicate a positive effect from controlling the impact of UV light on the stability of the bale or the air permeability of the film, but requires further investigation before any firm conclusions can be drawn.

At the stage when this grass was cut, the conditions and high dry matter made the fermentation process easier than it might have been during the normal first cut silage window in May. For this reason, the results may be more marked in a study carried out on grass cut earlier in the season, where the fermentation is likely to be less stable.

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