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.)