Expert Answers - Nov. 21, 2008

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Answer provided by Richard Muck, agricultural engineer at the U.S. Dairy Forage Research Center in Madison, Wis.


Q: I have heard some things about a new type of plastic cover on bunker silos and pile silos that reduces nutrient loss and spoilage. What can you tell me about that?

A: The quality of the seal provided by the plastic cover is a key issue for minimizing losses in bunker and pile silos. Most bunker covers are 6- to 8-mil polyethylene sheets held in place by tires or tire sidewalls. Frequently, there are problems with spoilage at the shoulders (i.e., against the walls), and sometimes one finds spoilage immediately below the plastic across the whole top.

A covering system new to the U.S. is the Silostop system. The primary element of this system is a plastic film with an oxygen permeability 1/20 that of polyethylene. It is not only used on top of the bunker, but is also used to cover the bunker walls. After filling is done, the plastic film from the walls is lapped onto the top of the bunker and a top sheet is placed over the whole top surface. A plastic mesh tarp is used to cover the top plastic film, providing UV and animal protection, and gravel bags secure the film and tarp.

We have tested this system on four corn and two alfalfa silage bunker silos and compared it to 8-mil white plastic, covering just the top and securing it with a combination of tires and tire sidewalls. We have split bunkers in half for these trials, with one treatment on the front half and the other on the back half. We have sampled the surface 2 feet of the bunkers at 2 feet from the wall and at center locations prior to covering and then at the time the bunker was opened for feeding.

The biggest differences between the two systems have been at the wall. At 2 feet from the wall, losses in the top 6 inches were 19 percent and 1 percent for the white plastic and Silostop systems, respectively, in the alfalfa bunkers. Losses between 6 and 24 inches deep (3 percent) were similar between the two systems. Silage under the middle of the top sheet for either system was not visibly moldy, and losses were similar (0 to 3 percent) by system and depth. However, silage fermentation under the Silostop system was slightly, but consistently, better (lower pH and higher lactic to acetic acid ratio as shown in Table 4). Limin Kung at the University of Delaware has done similar comparisons and found comparable results: very significant improvement near the wall and only small effects toward the middle. Consequently, the Silostop system reduces losses at the wall and has small effects elsewhere compared to using 8-mil white polyethylene plastic with very good management. Bigger differences would have been expected if Silostop had been compared with 6-mil plastic. (We have found approximately a 5-percentage-point difference in losses in the top 6 inches of silage between 6- and 8-mil plastic.)

Could one get much of the benefit of the Silostop system by running white plastic down the walls and using the standard system of white plastic and tires on the top? I think so, but we have not done that comparison. The Silostop system would still have several advantages: 1) the tarp and sand bags do an excellent job of holding the plastic against the crop, 2) the tarp provides better animal and hail protection than plastic alone, and 3) the top sheet, tarp and gravel bags go on faster than plastic and tires. The downside of the Silostop system is that it is about twice as expensive, and you need to re-use the tarp to keep costs down (not so convenient in places like New York and Wisconsin where we have to deal with snow).

Table 4. Average silage characteristics below 8-mil white polyethylene compared with Silostop in the middle of the bunker top surface.

Silage

Plastic

Depth

pH

Lactic Acid,
% DM

Acetic Acid,
% DM

Lactic: Acetic
Ratio

Alfalfa

White

0 – 6 in.

4.89

2.5

4.0

0.6

 


Silostop

0 – 6 in.

4.82

4.5

2.2

2.1

 


White

6 – 12 in.

4.82

4.5

1.7

2.6

 


Silostop

6 – 12 in.

4.75

3.8

1.4

2.7

Corn

White

0 – 6 in.

4.02

3.2

1.6

2.0

 


Silostop

0 – 6 in.

3.98

3.0

1.2

2.6

 


White

6 – 12 in.

4.00

4.1

1.4

2.9

 


Silostop

6 – 12 in.

3.97

3.9

1.2

3

 

Silostop now has a one-step plastic film with low oxygen permeability, but made to replace normal plastic with a more impermeable film. We have done one trial, but do not have all of the results analyzed. We compared one-step and gravel bags to white plastic and tires. We saw the one-step billowing in the wind like normal white plastic that was not sufficiently covered with tires. This concerns me, especially if you get a hole in the plastic or if the gravel bags are not butted tightly to one another. I think the two-step Silostop system has worked well because you have both an excellent plastic plus a very good means of protecting and holding the plastic to the silage (the tarp and gravel bags). So, if you want to get away from tires, I think the original Silostop system is a good alternative, but the one-step system may not be adequate without some tires.


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