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FORAGE & GRAZING LANDS

Bale Density and Moisture Effects on Alfalfa Round Bale Silage

^a Department of Agronomy, 500 S. Limestone St., University of Kentucky, Agric. Science Center-North, Lexington, KY 40546-0091
^b Department of Animal Sciences, University of Kentucky, Lexington, KY 40546

^* Corresponding author (mike2{at}uky.edu).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Moisture concentration and crop density during fermentation affect preservation of chopped silage, but these variables have not been adequately assessed for round bale silage. The effects of these factors on retention of crop dry matter (DM), silage quality, and nutritive value of alfalfa (Medicago sativa L.) preserved as round bale silage or as dry round-baled hay stored outside on the ground were determined in two field trials. Voluntary intake and in vivo digestibility were also assessed in beef cattle (Bos taurus). Silage bale weights were stable during storage but hay lost an average of 18% of its initial DM during 8 mo of storage. Prestorage alfalfa silage had lower concentrations of neutral detergent fiber (NDF) and acid detergent lignin (ADL) and higher crude protein (CP) and in vitro dry matter disappearance (IVDMD) than hay, probably because of leaf losses during hay harvest. Silage from the higher-density treatment had a pH of 4.76 compared with a higher pH of 5.01 for lower-density bales. Density did not affect lactic acid concentration in silage, but that from the higher-density treatment had more propionic acid at the higher moisture level. Average DM intakes of steers were 17.5, 20.4, and 21.0 g kg^–1 of body weight per day for hay, 594 g kg^–1 moisture silage, and 512 g kg^–1 moisture silage, respectively. In vivo DM digestibility of hay was 592 g kg^–1, lower than the 629 g kg^–1 average for two silages. Increasing bale density improved some aspects of silage quality, including a lower pH, but moisture effects on silage preservation were small. Results of two field trials indicate that the round bale silage preservation system saved a greater proportion of alfalfa crop DM and improved nutritive value and forage intake compared with hay.

Abbreviations: ADL, acid detergent lignin • CP, crude protein • DM, dry matter • IVDMD, in vitro dry matter disappearance • NDF, neutral detergent fiber • VFA, volatile fatty acids

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
ROUND BALE SILAGE shows promise as a forage preservation system for the transition zone of the eastern USA, where typical weather conditions are not conducive to hay harvest. Field-cured hay is currently the major forage preservation system in this region. Therefore, structures and equipment for preparing and utilizing chopped corn (Zea mays L.) silage are generally rare. Round bale silage takes advantage of existing hay equipment for most steps in silage production and feeding. The round bale silage system has other advantages over chopped silage such as a lower capital investment and flexibility in feeding (Holmes, 1997; Muck and Shinners, 2001). Rotz (1996) evaluated several forage preservation systems and found round bale silage to have the lowest labor cost per megagram of crop DM.

Vough and Glick (1993) observed wide variation in feed quality of round bale silages produced by farmers. Beaulieu et al. (1993) evaluated the quality of round bale silage and found it to be generally acceptable for livestock feeding. Petit and Tremblay (1992) reported higher in situ degradability of DM and CP in round bale grass silage than fresh grass or hay, while McCormick et al. (1998) reported production of fat corrected milk per unit of DM intake of Italian ryegrass (Lolium multiflorum Lam.) to be higher when preserved as round bale silage rather than hay.

The optimum moisture for crops preserved as round bale silage has not been fully elucidated. In traditional silage systems, ensiling crops with less than 300 to 350 g kg^–1 moisture resulted in mold and heating because of difficulties in air exclusion (Gordon et al., 1961), while more than 700 g kg^–1 moisture resulted in more effluent, growth of clostridia, and lower voluntary intake (Gordon et al., 1959; Moore et al., 1960; McDonald et al., 1991). Huhnke et al. (1997) reported that because of variable production conditions, moisture and pH of round bale silage produced in eastern Oklahoma showed a wide moisture range (140–717 g kg^–1) and a high frequency of high pH silages (17.9% of round bale silage had higher pH than 6.5). Davies and Nicholson (1999) suggested that round bale silage should be below 780 g kg^–1 moisture to limit effluent production. Bates et al. (1989) found moisture to be a greater source of variation in round bale silage quality than microbial inoculant addition. Field et al. (1999) found in England and Wales that the average moisture concentration of round bale silages was 636 g kg^–1 and that 77% of the silages were between 810 and 541 g kg^–1 in moisture concentration.

The importance of DM density in forage preservation as round bale silage is not well understood. In bunker silos, increasing density reduces storage cost per megagram of DM by increasing storage capacity and reducing nutrient losses during storage (Muck and Holmes, 2000). The average of round bale silage density was 159 kg m^–3 and showed significant variation due to baler type (Huhnke et al., 1997). Muck and Holmes (2000) reported that averages of hay crop silage and corn silage were 237 and 232 kg m^–3 respectively.

The objective of this research was to determine bale density and forage moisture concentration effects on physical characteristics, storage behavior, nutritive value, and voluntary intake of alfalfa round bale silage. A further objective was to compare the most widely used current preservation system, round-baled hay, with round bale silage for harvesting and storing alfalfa forage in a humid region.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Two field trials were conducted beginning on 14 May and 7 Aug. 2001 in a monoculture of ‘Rushmore’ alfalfa grown on a Maury silt loam soil (Typic Paleudalf) on the University of Kentucky Spindletop Research Farm near Lexington, KY (38^°8' N, 84^°31' W). The crop was fertilized during March with 280 kg ha^–1 of K on the basis of annual soil test. In each trial, alfalfa was wilted in windrows to two target moisture concentrations (450 or 550 g kg^–1) and baled with a New Holland model 648 baler (New Holland North America, Inc., New Holland, PA). Higher or lower bale density was achieved by baling at hydraulic pressures of 842 or 421 x 10³ Pa and bales were tied with plastic twine. Bales were approximately 1.5 m in diameter and 1.2 m in length. Six bales were prepared for each treatment from randomly selected windrows. Silage bales were wrapped within 2 h after baling with four layers of 76-cm-wide stretch film (Sunfilm, AEP Industries, Inc., South Hackensack, NJ) with a New Holland B27P bale wrapper (New Holland North America, Inc., New Holland, PA). Length and diameter were measured for each bale and thermocouples were inserted into the center of each bale to monitor temperature. Holes made by insertion of thermocouples were sealed with silicon caulk and taped with bale repair tape. Hay was baled with a baler hydraulic pressure setting of 1054 x 10³ Pa and tied with plastic twine. Hay and silage bales were stored on a well-drained grass sod for 8 mo without covers.

After storage, bales were reweighed and the dimensions measured again. Film was removed and each bale was visually evaluated for mold on a scale of 1 to 5 on the basis of the percentage of the bale surface area affected (1 = none to 5 = 100% coverage) according to Bates et al. (1989). Core samples were taken before and after storage with a 25-mm-diam boring device with a serrated tip. One-half of each core sample was cooled on dry ice and the other half was dried at 60°C for 72 h. Dried samples were ground to pass through a 3-mm screen and then reground through a 1-mm screen with a Wiley mill.

Neutral detergent fiber concentrations were determined with an ANKOM Model 200 fiber analyzer (ANKOM Technology, Macedon, NY), using the sodium sulfite modification of Robertson and Van Soest (1977). Determination of ADL concentration was by sequential analysis (Goering and Van Soest, 1970). Total N was measured by the semimicro-Kjeldahl procedure of Bremner and Breitenbeck (1983) and CP was calculated as N x 6.25. Duplicate 0.25-g samples were used to determine IVDMD by the method of Goering and Van Soest (1970).

Silage extract was prepared from one-half of each core sample which had been cooled with dry ice. Silage extracts were prepared by adding water to silage (10:1 w/w) and homogenizing for 30 s. Silage pH was measured on the homogenate after filtering through four layers of cheesecloth. Aliquots of the filtrate were mixed with 0.2 mL of 25% (w/v) metaphosphoric acid and centrifuged at 5°C at 21 600 g for 20 min. Volatile fatty acids (VFA) in the supernatant were determined by gas chromatography as described by Baumgardt (1964) and Brotz and Schaefer (1986). Lactic acid was determined by an enzymatic method (Hohorst, 1963).

A feeding trial was conducted at the Univ. of Kentucky, Animal Research Center, Woodford Co., KY. Twelve ruminally cannulated Angus steers (average body weight: 340 kg) were allocated to four blocks on the basis of body weight. One steer from each block was randomly assigned to receive alfalfa hay, higher-moisture silage, or lower-moisture silage from the higher density treatment during a 21-d feeding period. Procedures used in steer cannulation and daily care had been approved by the Univ. of Kentucky Institutional Animal Care and Use Committee. Round bales were prepared for feeding with a Vermeer model S500S bale splitter (Vermeer Manufacturing Co., Pella, IA), and the chopped forage stored in containers at –20^°C before feeding. Silage was thawed and stored during the feeding process at 4°C. Silage was fed individually to steers at 115% of their 7-d average intake. After a 14-d adaptation period, feed offered, orts, and feces were weighed and subsampled each day during a 7-d collection period. Two layers of plastic film were added to cover the bottom 1-m height of wall around the floor to prevent loss of feces. Total feces were collected and weighed every morning immediately after feeding. Approximately 10 g kg^–1 of the total feces was subsampled for estimation of in vivo digestibility.

Zero-hour ruminal fluid sampling started at 0900 h on Day 5 of data-collection week, followed by administration of a pulse-dose of 0.92 g chromium (prepared as Cr:EDTA, Binnerts et al., 1968) in 350 mL of distilled water. Subsequent samples of ruminal fluid were drawn via cannula at 3, 6, 9, 12, and 24 h post dosing. Ruminal fluid pH was measured within 30 min of collection using a portable pH meter fitted with a combination electrode (IQ 150; IQ Scientific instruments, Inc., San Diego, CA) and then subsampled, combined with 25% (w/v) metaphosphoric acid (8 mL ruminal fluid to 2 mL acid) and frozen (–20°C) for volatile fatty acid and NH₃–N analyses. Additional samples of ruminal fluid (16 mL) were frozen without additives for subsequent analysis of Cr concentration. Ruminal VFA concentrations were determined by gas chromatography by the procedure described by Vanzant and Cochran (1994). Ruminal NH₃–N concentrations were determined using glutamate dehydrogenase (171-B; Sigma Chemical Co., St. Louis, MO) and a method adapted for use on a COBAS FARA II centrifugal analyzer (Roche Diagnostic Systems, Montlcaire, NJ). Chromium concentration was determined with an atomic absorption spectrophotometer with an air-acetylene flame. Fluid dilution rate was calculated by regressing the natural logarithms of Cr concentration against sampling times (Warner and Stacey, 1968).

Hay and silage data were analyzed by the MIXED procedure of SAS (SAS Institute, Cary, NC). Prestorage and poststorage silage data were analyzed as a completely random experiment with six replicates of a 2 x 2 factorial of density and moisture treatments. Bale density and moisture concentration were considered fixed effects and trial was treated as a random effect. Bale physical dimensions and weights and nutritive value data for the hay treatment were analyzed together with data for the four silage treatments for comparison of harvest systems. Data from the feeding trial were analyzed as a randomized complete block design with steer weight groups as blocks. Ruminal pH, rumen ammonia, and VFA data were analyzed by repeated measures analysis of variance procedures of SAS with sampling time as the repeated factor. The compound symmetry covariance structure was used in the repeated measures analysis. The pairwise t test was used for means comparison. Statistical significance is at the 0.05 level, unless otherwise stated.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Trial x treatment interactions were found only for prestorage bale DM density and for silage propionic and isobutyric acid concentrations; therefore, the data are presented as means of the two trials. Prestorage bale DM density was higher for the lower-moisture forage in Trial 1 and for the higher-moisture forage in Trial 2 (P = 0.03). In Trial 2, silage concentrations of propionic and isobutyric acid were higher and more variable with the higher compared with the lower baler hydraulic pressure setting (P < 0.05). Density effects on propionic and isobutyric acid concentrations were inconsistent in Trial 1.

Bale Temperatures and Physical Characteristics
Peak bale temperatures during storage were lower for silage than for hay (Table 1). This probably reflects the combined effects of oxygen exclusion by plastic film and moisture-induced heating of hay bales caused by wetting that occurred during outside hay storage (Table 2). Neither bale density nor initial moisture affected silage bale temperatures.

Minimal surface mold was observed on silage bales and was generally restricted to small areas on the north side of the bales (Table 1). Holes for insertion of thermocouple wire used in temperature measurement were sealed with UV-treated repair tape; however, some mold was noted beneath these insertion points. Surface mold was not significantly affected by prestorage moisture concentration or bale density.

Silage bale dry weights changed very little during storage. Calculated DM recoveries for silages were above 95% (Table 1). In hay bales, dry weight losses during storage averaged 18.5% of the initial weight. Moisture concentrations in hay increased during outside storage (Table 2). Moisture concentrations of silage increased slightly during storage indicating that some water was produced by respiration. Pitt (1990) estimated that stoichiometrically 60% of the mass of respired carbohydrate was retained in the silage as water.

Forage Nutritive Value
Except for poststorage concentrations of NDF and ADL, density x moisture interaction effects on nutritive value of silage were absent (P > 0.05), and except for poststorage CP, density effects on nutritive value of silage were absent (Table 2). Silage at the higher density had slightly lower (205 g kg^–1, P < 0.04) poststorage concentrations of CP than lower density silage (209 g kg^–1).

Immediately after baling, alfalfa hay had substantially higher NDF and ADL concentrations and substantially lower CP and IVDMD than silage at either moisture level, presumably because of moisture effects on physical losses and respiration losses during the curing and baling processes (Table 2). The alfalfa preserved as silage had substantially higher prestorage IVDMD than hay. The lower prestorage nutritive value of hay compared with silage is probably due mainly to leaf loss caused by tedding, raking, and baling. Respiration and leaf shatter accounted for nearly 80% of the DM losses from alfalfa hay dried without rain damage (Collins, 1991). Moisture levels in hay prepared for these trials were below typical values for this area and probably contributed to the large differences observed between hay and silage in prestorage nutritive value. Alfalfa silage baled at the higher moisture level had lower (P < 0.05) prestorage concentrations of NDF and ADL and higher CP and IVDMD than lower-moisture alfalfa (Table 2) in spite of the relatively small moisture difference between the two treatments. As discussed for the comparison of hay with silage, the nutritive value difference between lower and higher moisture silage could result from respiration and leaf losses due to delayed ensiling. The delay in time of baling between the lower and higher moisture treatments in Trials 1 and 2 were 5 h and 24 h, respectively. Crude protein levels were not affected in their study, but Colombari et al. (2001) found that alfalfa harvested at 640 g kg^–1 moisture for silage had 22 g kg^–1 higher prestorage concentrations of NDF than alfalfa harvested at 420 g kg^–1 moisture.

Silage moisture treatment effects on poststorage CP and IVDMD were not significant but the ranking was consistent with prestorage differences (Table 2). The lower-moisture alfalfa silage remained higher in NDF and ADL compared with the higher-moisture silage. Huhnke et al. (1997) found little effect of initial moisture concentration in ryegrass and legume-grass round bale silage on behavior of NDF concentrations during storage. Poststorage NDF and ADL concentrations in hay stored outside remained higher and CP and IVDMD lower compared with silage. Concentrations of NDF increased more and IVDMD decreased more during storage of hay compared with silage (P < 0.05). Previous research suggests that inside storage of hay could have reduced storage losses of both DM and nutritive value (Collins et al., 1995); however, the outside storage system used was chosen to reflect typical large round bale hay storage methods in this region.

Silage pH and Fermentation Products
Silage pH and organic acid concentrations generally indicate acceptable preservation of the alfalfa silages regardless of prestorage moisture or DM density (Table 3). Averaged over moisture treatments, higher-density silage had a lower (P < 0.01) pH (4.76) than lower-density silage (5.01). Averaged over density treatments, silage at 587 g kg^–1 moisture had a slightly (P = 0.077) lower pH (4.81) than silage with 524 g kg^–1 moisture (4.97). Whiter and Kung (2001) reported a silage pH just below 4.5 for chopped alfalfa silage of comparable moisture.

Feeding Trial
Hay and silages from the higher-density treatments in Trial 2 were chosen for in vivo assessment with beef steers. Forage samples collected daily during feeding generally reflected the pre- and poststorage core sample data discussed earlier. The silages did not differ but had higher CP and lower fiber levels than hay (data not presented). Voluntary DM intake of hay was lower than that of either silage (Table 4). Because elevated temperatures and significant storage deterioration of hay were absent, the lower DM intake of hay was probably due to leaf shattering and other changes related to the harvest method. This corroborates results reported by Petit et al. (1985) of higher DM intakes of silage than of hay (54 vs. 40.9 g kg^–0.75), which they attributed to higher cell wall concentrations in hay. Gordon et al. (1964) reported a positive correlation between DM intake and lactic acid concentration and negative correlations with acetic acid or propionic acid. In the present study, in vivo DM digestibility did not differ between the silages but was lower for hay. Intake of NDF and digestibility of NDF did not differ among the three forages.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Round bale silage was greatly superior to outside-stored round-baled hay in preserving alfalfa crop DM and nutritive value. The existence of prestorage preservation system differences indicates that higher combined field and harvest losses for hay relative to silage contributes to the disadvantage of hay at feeding time as well as DM losses and changes in nutritive value that occur during storage. Increasing silage bale density by increasing baler hydraulic pressure to 842 x 10³ from 421 x 10³ Pa improved silage quality by increasing lactic acid concentrations and by lowering silage pH. However, the absence of density effects on nutritive value parameters such as CP, fiber constituents, and IVDMD suggests that further research is needed to determine whether the better silage quality characteristics of the high-density bales affects animal response through altered silage acceptability during feeding. Moisture effects on silage preservation and feeding response in this study were very small. It seems likely that the small range in moisture achieved between treatments contributed to the small magnitude of the silage response to the moisture variable.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Supported in part by USDA CSREES grant funds. Published with the approval of the Director, KY Agric. Exp. Stn. as publication 03-06-064.

Received for publication May 21, 2003.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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