Top spoilage losses for corn and forage sorghum silages stored in bunker silos

Corn and forage sorghum silages were stored in small bunker silos for 180 days. Dry matter (DM) and organic matter (OM) losses, fermentation characteristics , in-situ DM digestibility, and temperatures were measured at 10, 20, and 30 inches from the original silage surface. Sealing the exposed surface increased DM and OM recoveries and improved fermentation quality and nutritive value in both crops, regardless of depth. The unsealed corn silages were much hotter within the top 3 ft than sealed silages, indicating aerobic losses. As expected , the unsealed silages from both crops deteriorated severely i n the top 20 inches. Placing a roof over the unsealed silos increased the silage DM content at all three depths, but did not consistently improve the storage efficiency or silage quality of either crop.


Introduction
Large horizontal silos (i.e., bunkers, trenches, and stacks) store large quantities of ensiled feeds economically, b ut by design, much of the silage is exposed to the environment.In 1,000 ton si los (100 ft long × 40 ft wide × 12 ft deep), up to 25% of the original silage mass is within 3 feet of the surface.In earlier studies, we found that DM losses in unsealed bunkers were 80.4 and 29.4% at depths of 10 and 20 inches, respectively, for corn silage after 6 mont hs of storage and 77.0 and 53.2% for forage sorghum silage at the same depths (KAES Report of Progress 651, page 131).However, sealing with polyethylene sheeting significantl y reduced the DM losses for both crops at both depths.Our ob j ectives were: 1) to measur e the rate and extent of top spoilage losses in unsealed and sealed silages and 2) to deter mine the effect of placing a roof over the silage mass on preservation efficiency and nutritiv e value.
To our knowledge, the feasibility of using a roof to protect unsealed corn or sorghum silage from rai n and snowfall has not been studied in controlled experiments.

Experimental Procedures
Experiment 1: Whole-Plant Cor n.On August 2 5 and 26, 1992, whole-plant corn (2/3 milk line maturity and 34.2% DM ) was chopped and packed into four , 16 ft long × 13.5 ft wide × 4 ft deep, bunker silos.Alternate loads were used to fill the bottom half of each silo on the first day and the top half of each silo on the second day.During fil l ing, nylon net bags, each containi ng 4.5 lb of fresh material, were placed 10, 20, and 30 inches from the surface of the origina l ensiled mass (three bags/depth/silo).Thermocouple s were placed at each bag location, and temperatures were recorded for the first 42 days.The silos contained similar amounts of fresh material and were packed with single-tire d tractors to equal densities (13.8 lb of DM/cubic ft).Treatments were: 1) silo left unsealed, no roo f; 2) sealed, no roof; 3) left unsealed, with a roof; and 4) sealed, with a roof.Both sealed silos were covered with a single sheet of .4mm polyethylene, weighted with tires.A galvanized tin roof was used for treatments 3 and 4. Bunkers were emptied 180 days postfilling.The nylon net bags were recovered , and the silage was weighed; mixed; sampled ; and analyzed for DM, ash, pH, and fermentation end products.
Experiment 2: Forage Sorghu m.On Octobe r 20 and 21, 1992, whole-plant, Northrup King 300, forage sorghum (latedough maturity and 37.1% DM) was chopped and packed to equal densities (10.2 lb of DM/cubic ft) into four bunker silos.All experimenta l procedures were the same as in Experiment 1.
In-Situ Digestibility.Ruminal DM disappearance was determined for the silages from the nylon net bags recovered from the bunker silos in Experiments 1 and 2. Three ruminally cannulated steers were used.Approximately 1 g of dried, ground silage was placed in a 5 cm × 10 cm dacro n bag with a 53 micrometer pore size.Silage s were digested for 72 hours.Once removed from the rumen, the bags were rinsed with cold wa ter until the water was clear.Bags and undiges ted material were dried in a forcedair oven at 5 5EC for 72 h and then weighed; DM disappearance was calculated.

Results and Discussion
Experiment 1: Whole-Plant Cor n. Results are presented in Table 1.The two silages from the unsealed silos had dramatically lower DM and OM recoveries at the 10-and 20-inch depths tha n the two sealed silos.Sealed silages were well preserved at all three depths, but in the unseale d silos, only the silage at the 30-inch depth was acceptable.The silage in the unsealed , roofed silo had the highest DM content at all depths, whereas the unsealed, no-roof silage had the lowest.Temperatures in the two sealed silos and the unsealed, no-roof silo peaked within the first 4 t o 12 days postfilling, but temperatures in the unsea l ed, roofed silo did not reach maximum until 40 to 44 days.Few fermentation differences were observed among the sealed silages .The poor fermentation at the 10-in ch depth in the sealed, roofed silage was caused by rodent damage to the polyethylene.The two unsealed silages were of dramatically lower quality, as indicated by increased pH values, lower lactic acid content, and decreased lactic:aceti c ratios (data no t shown), especially at the 10-an d 20-inch depths.Silages were of acceptabl e quality at the 30-inch depth in both unsealed silos.The silages in the two sealed silos and the unsealed, roofed silo had higher rumina l in-situ DM digestibilities at all three depths than the silage in the unsealed, no-roof silo.In both unsealed silos , silage at the 10-inch depth had a lowe r in-situ DM digestibility than silages at the 20-and 30-inch depths, indicating that much of the digestible organic matter had been removed by weathering and(or) spoilage.
Experiment 2: Forage Sorghu m. Results are presented in Table 2 .The unsealed, no-roof silage had the lowest DM content at all three depths, wherea s the unsealed, roofed silage had the highest.Unsealed silages had dramatically lower DM and OM recoveries at the 10-inch depth than the two sealed silages.The unsealed, roofed silage had the lowest recoveries at the 20-inch depth.The two unsealed silages had deteriorated severely at the 10-inch depth, as evidenced by high pH values and almost no fermentation end products.The silages at al l three depths in the two sealed silos and the unsealed, roofed si l o had higher ruminal in-situ DM digestibilities than that in the unsealed, no-roof silo.

Conclusions.
During the 180 days of storage , water from rain and snow percolated through the unsealed, no-roof silage for both crops, and the silages at all three depths were much wetter than the pre-ensiled forages.In contrast , the silages at the 10-and 20-inch depths in the unsealed, roofed silos were much drier than the pre-ensiled forages, because considerabl e dehydration/evaporation took place in the absence of a seal.
These data document that sealing corn or forage sorghum silages in bunker, trench, or stack silos greatly increases preservation efficiency and nutritive value in t h e original top 2 to 3 ft of ensiled material.

Table 1 . Effects of Sealing Treatment and Depth from the Original Surface on the DM Content, DM and OM Recoveries (rec.), Temperature (temp.), Fermentation Characteristics, and DM Digestibility (DMD) of Corn Silages Stored in Bunker Silos in Experiment 1
(P<.05) .Means within a sealing t reatment across depth with different superscripts differ (P<.05).x,y

Table 2 . Effects of Sealing Treatment and Depth from the Original Surface on the DM Content, DM and DM Recoveries (rec.), Fermentation Characteristics, and DM Digestibility (DMD) of the Forage Sorghum Silages Stored in the Bunker Silos in Experiment 2
(P<.05).