Brian J. Holmes

FACTORS AFFECTING BUNKER SILO DENSITIES

High densities in bunker silos minimize losses and reduce storage costs; however, the guidelines to attain high densities are based on relatively little research. The objective of this study was to determine those practices or factors most correlated with bunker silo density. Density was measured in 175 bunker silos across Wisconsin using core samples collected at chest height (1.13 m, 3.70 ft, on average) across the feed-out face. Silo filling practices were surveyed and correlated with density. Most silages sampled were alfalfa or corn. Dry matter densities ranged from 106 to 434 kg/m 3 (6.6 to 27.1 lb/ft 3 ). The core densities were correlated with the height of silage above the core, indicating the effect of self-compaction. To adjust for this, all densities were corrected for the median depth below the surface (2.16 m or 7.09 ft) using the equations of Pitt (1983) for density with height for the center of tower silos. The adjusted dry matter densities were most strongly correlated with how thinly a load was spread (L), tractor weight (W), packing time per tonne as-fed (T), and dry matter content (D). These four factors were combined into a packing factor [W (TD)1/2 L –1 ] that explained 18.2% of the variation in dry matter density. Additional factors such as the use of dual wheels, etc. did not significantly improve the prediction of dry matter density. An equation was developed to predict average density in a bunker silo based on the packing factor plus crop height in the silo.

BAG SILO DENSITIES AND LOSSES

Bag silos made at three research farms in 2000 and 2001 were monitored at filling and emptying to determine densities and losses. A total of 47 bags (23 alfalfa, 1 red clover, 23 whole-plant corn) were made, and losses were calculated on 39 of the silos. Dry matter (DM) density ranged from 160 to 270 kg/m3. Dry matter density increased with DM content in hay crop silages on average 2.9 kg/m3-% DM, whereas the effect in corn silage varied by bagging machine. Density decreased with increasing particle size at 4.1 kg DM/m3 per mm. The operator and the bagging machine setup both affected density. A wide range (0% to 40%) of DM losses was observed. Average DM losses were 9.2% invisible plus uncollected losses and 5.4% spoilage losses, for a total loss of 14.6%. Six silos had excessive spoilage losses of more than 15% and total losses above 25% due to plastic integrity issues or overly dry silage (>40% DM) being fed out in warm weather. Invisible losses were reduced in high-porosity silages (where spoilage losses were exacerbated), greater in warm weather, and affected by emptying procedures. Spoilage losses in bags without plastic integrity issues were greater in dry silages, from emptying silos in warm weather, at lower feed-out rates, in bags stored for a long time, and if bags were emptied periodically rather than daily.

Laboratory Assessment of Bunker Silo Density Part II: Whole-Plant Corn

To better understand and predict silage density in bunker silos, chopped whole-plant corn was placed in layers of 0.15, 0.30, 0.45, and 0.60 m in a 482- . 584-mm rectangular container simulating the footprint of a tractor tire. Pressure between 20 and 80 kPa was applied to the forage by a platen. The total time of compaction varied between 1 and 10 s. A total of 25 tests were conducted with crop dry matter (DM) content ranging between 33% and 44%. The pre-compressed density of the first layer (0.30 m high) averaged 95 kg DM/m3. The highest compressed density ranged between 169 and 261 kg DM/m3 with an average of 216 kg DM/m3. After releasing pressure, the relaxed density of the first layer ranged between 117 and 153 kg DM/m3 with an average of 131 kg DM/m3. After six layers, the average relaxed density became 185 kg DM/m3, a density 14% lower than the average highest compressed density. A logarithmic model fit the data very well (R2 . 0.93 in 24 out of 25 tests), indicating that density increased continually as the number of layers increased. Model parameters were significantly affected by pressure, layer thickness, and crop processing while time of compaction had a small effect. DM content was not significant. A model based on extrapolation of laboratory results is proposed to predict density for deep bunker silos, but field data are required to validate the model under such conditions.

Laboratory Assessment of Bunker Silo Density Part I: Alfalfa and Grass

A laboratory apparatus was developed to simulate pressure, time of compaction, and layer thickness as applied in a bunker silo. Chopped alfalfa or orchardgrass was placed in layers of 0.15, 0.30, and 0.45 m in a 482- × 584-mm rectangular container simulating the footprint of a tractor tire. Pressure between 20 and 80 kPa was applied to the forage by a platen. The total time of compaction varied between 2 and 10 s. A total of 23 tests (17 with alfalfa, 3 with grass, and 3 with mixed alfalfa-grass) were conducted; dry matter (DM) concentration ranged between 20% and 54%. The pre-compressed density of the first layer (0.30 m high) averaged 72 and 55 kg DM/m3 for alfalfa and grass, respectively. The highest compressed density ranged between 138 and 339 kg DM/m3 with an average of 220 kg DM/m3. After releasing pressure, the relaxed density of the first, uppermost layer ranged between 81 and 152 kg DM/m3 with an average of 127 kg DM/m3 . After six layers, the average relaxed density was 181 kg DM/m3, 18% lower than the average highest compressed density. As successive layers were added, the cumulative DM density increased according to a logarithmic model. The model suggested that density would continually increase, slowly but without reaching a plateau, as the silo height increased. Within the experimental range, parameters of the logarithmic model were significantly affected by pressure, DM content, crop species and chop length, but not by layer thickness or time of compaction. More laboratory data are needed to understand interactions between the variables while field validation is necessary to extrapolate results to deep bunker silos.

Packing Bunkers and Piles to Maximize Forage Preservation

Forage is a valuable commodity stored on dairy farms. Bunker and pile silos have increased in use due to increasing herd size. Losses in feed value in bunker and pile silos are frequently higher than they should be because producers are not packing them sufficiently to exclude oxygen during the storage and feed out periods. The objective of this paper was to consider the recommendation of a minimum packing density of 240 kg dry matter/m3 compared to a minimum bulk density recommendation to keep porosity low. Our conclusions are that producers should try to achieve a minimum bulk density of 700 kg as fed/m3 while harvesting forage in the recommended range of 30-40% dry matter so as to limit porosity to a maximum of 0.4. This should result in lower losses of forage dry matter (DM) over a range of DM contents than when following the recommendation of a minimum packing density of 240 kg DM/m3.

Density and Losses in Pressed Bag Silos

The objective of the study was to monitor the filling and emptying of bag silos at three research farms to determine the variation in density and losses in pressed bag silos. Twenty five bags were filled over the 2000 harvest season, and 15 of those bags have been completely emptied. Dry matter (DM) densities in alfalfa silages were approximately 200 kg/m 3 when the crop was at 40% DM, and densities declined approx. 3 kg/m 3 -% DM in wetter crops. With one bagger, densities were 3 to 8% lower in corn silage whereas densities were 16 to 35% higher in another bagger. Densities declined 5 kg/m 3 -% DM in wetter corn silages. Average DM losses were 8.4% gaseous/seepage loss and 5.8% spoilage loss for a total of 14.2% loss. The average spoilage and total losses were inflated by three bags with substantial spoilage (26 to 38% total loss). Removing those from the average reduced average total losses to 9.7%. Total losses increased with low feed out rates and with drier, more porous silages.

Packing Practice Effects on Density in Bunker Silos

The objective was to compare different packing practices on density in field-scale bunker silos. In May 2003, a bunker silo (37.8 x 10 x 3.5 m) was filled with alfalfa. At filling, loads (2.6 t dry matter each) were placed on alternating sides of the open end of the silo. One tractor (6.6 t, single-wheeled) was used to spread and pack all loads. The tractor spent a similar time spreading each load (7 min) on both sides, but packing time on one side (8.3 min) was approximately twice that of the other (4.5 min). Average densities on both sides based on volume were similar, but core samples taken at the face during feedout indicated similar densities near the floor but reduced densities in the upper half of the side receiving less packing time. In September 2003, two bunker silos (21.3 x 5 x 3.5 m) were filled simultaneously with whole-plant corn. The same 6.6 t, single-wheeled tractor was used to spread loads in each silo. This tractor also packed one silo, but a 9.7 t, single-wheeled tractor was used in the other. Packing times were similar for both silos. Density was increased 27 kg DM/m3 when using the heavier tractor.

A Hazard Analysis of Three Silage Storage Methods for Dairy Cattle

Objectives: The study objective was to characterize work methods, hazards and annual hazard exposure hours of three silage storage methods. Methods: A telephone questionnaire was designed and administered to 24 Wisconsin dairy managers. The exposure durations reported were scaled to a standard-sized dairy herd with equal dry matter tons of silage stored and fed each year. Results: Managers reported no silo gas hazards with either bunker or bag silos. Compared to tower silos, managers reported reduced fall hazards with bunker silos and no fall hazards with bag silos although both introduced front end loader operation injury hazards. Compared to bunkers, managers who used bag silos reported no exposure to tractor overturns. Conclusions: Although some hazards are clearly present using any ensling method, dairy farms that adopt silage bag technology can reduce many of the hazards traditionally associated with silage work.

Bag Silo Densities and Losses

Bag silos made at three research farms in 2000 and 2001 were monitored at filling and emptying to determine densities and losses. A total of 47 bags (24 alfalfa, 23 whole-plant corn) were made, and losses were calculated on 39 of the silos. Dry matter density ranged from 160 to 270 kg/m3. Dry matter density increased with DM content in hay crop silages on average 3.0 kg/m3-% DM whereas the effect in corn silage varied by bagging machine. Density decreased with increasing particle size at 4.1 kg DM/m3 per mm. The operator and how the bagging machine was set up affected density. A wide range of DM losses was observed, 0 to 40%. Average DM losses were 9.2% invisible plus uncollected losses and 5.4% spoilage losses for a total loss of 14.6%. Six silos had excessive spoilage losses of more than 15% and total losses above 25% due to plastic integrity issues or overly dry silage (>40% DM) being fed out in warm weather. Invisible losses were reduced in high porosity silages (where spoilage losses were exacerbated), greater in warm weather, and affected by emptying procedures. Spoilage losses in bags without plastic integrity issues were greater in dry, porous silages, from emptying silos in warm weather, and at lower feed out rates.

DENSITY AND LOSSES IN PRESSED BAG SILOS

The objectives of this study were to measure densities and losses in bag silos made at three research farms and to determine potential factors affecting both. The primary bagging machines were a 2.44 m Ag Bag model G6000, a 2.74 m Kelly Ryan model DLX shared by two stations and a rented 2.74 m Ag Bag machine. All loads of forage entering the bags were weighed and sampled. At emptying, all silage removed (both good and spoiled) was weighed and sampled. Across 47 bags, dry matter (DM) density ranged from 160 to 270 kg/m3. At all three farms and across crops, DM density increased linearly with DM concentration except with the Kelly Ryan in corn silage where density was constant. Kernel processing appeared to reduce density in corn silage. The bagging machine, operator, and crop also affected average DM densities. Within bags, density was highly variable. Densities at the top and sides were approximately 40% of densities at the bottom, center of the bag. Losses of DM were measured on 24 bag silos and were highly variable (0 to 40%). However, except for six bags with considerable spoilage loss, total losses averaged 11%. Significant spoilage losses were essentially confined to crops ensiled above 400 g DM/kg. Spoilage was also worse in bags fed out in summer. Gaseous and seepage losses were higher at low DM contents and by feeding out at low rates (200 mm/d). While more research is needed to study bagging machines with different systems of filling, the current study suggests that pressed bag silos can do an excellent job of preserving a crop provided 1) crops are ensiled between 300 and 400 g DM/kg, 2) the bagging machine is set up properly to obtain a smooth bag of high density, 3) feedout rates are a minimum of 30 to 60 cm/d and 4) the farmer routinely monitors for and repairs punctures in the bags.