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Dep. of Animal Sci., Univ. of Arkansas, Fayetteville, AR 72701
* Corresponding author (jeturn{at}mail.uark.edu)
Received for publication January 17, 2001.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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was used to describe the changes in neutral detergent fiber (NDF), acid detergent fiber, lignin, neutral detergent–insoluble N, and acid detergent–insoluble N (ADIN) during storage. The total changes (ß) in NDF were 93.1, 69.5, and 67.8 g kg-1 for HM, MM, and LM bales, respectively. Respective asymptotic maxima for NDF (
) in these treatments were 777, 757, and 739 g kg-1. For ADIN, respective asymptotic maxima () reached 3.17, 1.83, and 1.71 g kg-1 for HM, MM, and LM bales, respectively. On Day 65, ADIN exceeded 10% of the entire N pool in both HM and MM bales. The nutritive value of bermudagrass hay baled and stored at >200 g kg-1 moisture deteriorates during storage, and the greatest deterioration occurs during the first 12 d after baling.
Abbreviations: ADF, acid detergent fiber • ADIN, acid detergent–insoluble nitrogen • DM, dry matter • HDD, heating degree-days > 35°C • HM, high moisture (302 g kg-1) • LM, low moisture (219 g kg-1) • MM, medium moisture (265 g kg-1) • NDF, neutral detergent fiber • NDIN, neutral detergent–insoluble nitrogen • NDSN, neutral detergent–soluble nitrogen • RMSE, root mean square error
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Concentrations of moisture >200 g kg-1 in alfalfa (Medicago sativa L.) and bermudagrass hays produce spontaneous heating, mold growth, and deleterious changes in forage nutritive value (Collins et al., 1987; Coblentz et al., 1996, 2000). Microbial activity and the subsequent production of heat reduce the nutritive value of hay. Coblentz et al. (1997) found that microbes preferentially oxidize nonstructural carbohydrates in alfalfa hay. Roberts et al. (1995) associated the mold growth and production of associated toxins with increased spontaneous heating in hay bales during storage. Rotz and Muck (1994) indicated that increased microbial activity and the associated heating result in greater concentrations of fiber components and heat-damaged N in hay.
Our research (Coblentz et al., 2000) has indicated that bermudagrass hay has two distinct temperature maxima; one occurs immediately after baling and the other between 5 and 20 d of storage. Similar findings have been reported for alfalfa and grass–clover hays (Coblentz et al., 1994a, 1994b; Hlodversson and Kaspersson, 1986). Previously, Coblentz et al. (1996) examined the changes in nutritive value of alfalfa hay as a function of time in storage; however, little or no information of this type is available for warm-season grass hays generally and for bermudagrass hay in particular. It is critical to develop a clear understanding of these relationships for bermudagrass, which is the most important forage in the southeastern USA (Burton and Hanna, 1995). Our objective was to determine the changes in nutritive value of bermudagrass hay at three concentrations of moisture as affected by both time in storage and spontaneous heating.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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For each moisture treatment, 12 conventional bales (average size = 0.48 by 0.38 by 0.96 m) were made from each block with a New Holland Model 320 baler. Baling treatments were stacked according to the method of Coblentz et al. (2000). Wooden pallets were placed on the concrete floor of an open-air pole barn. Six bales from each group of 12 were placed side by side (strings up) on top of the wooden pallets. The remaining six bales from each treatment were positioned in the same orientation on top of the first six bales, thereby creating stacks that were two bales high and six bales wide for each field replication of each treatment. Individual stacks containing 12 bales were surrounded on the sides and top by dry bales of wheat (Triticum aestivum L.) straw to limit the effects of diurnal variations in ambient temperature. Stacks were created within 2 h of removal from the field.
All bales were weighed and measured for length before being placed on the pallets. The length and weight of the bales were used to determine the density of each hay package. Height and width of bales were not measured; prior observations indicated that these measurements were uniform with our baler (height = 0.38 ± 0.01 m, width = 0.48 ± 0.01 m; Coblentz et al., 2000). Samples were taken from two bales selected at random from each stack before stacking and at 4, 8, 12, 24, and 65 d after baling using a Multi-Forage Sampler (Star Quality Samplers, Edmonton, AB, Canada). Based on previous temperature vs. time in storage curves for bermudagrass hay (Coblentz et al., 2000), these sampling dates were selected to approximately coincide with the end of the initial heating period (Day 4); the onset, peak, and end of the secondary heating phase (Days 8, 12, and 24, respectively); and the end of the study (Day 65). The Day 0 sampling date served as a prestorage estimate of forage nutritive value. Eight 41-cm-long samples were taken from each bale (four samples per bale end). Bales were removed from each stack for sampling and then returned to their previous location in the stack for the remainder of the trial to maintain the integrity of the stack. All forage samples were dried under forced air at 55°C for 72 h; for bales sampled on Day 0, this technique was used to estimate the initial concentration of moisture for each baling treatment. Concentrations of moisture were expressed as a proportion of the original (wet) sample weight.
Bales sampled on Day 65 of storage were visually appraised for mold growth by the method of Roberts et al. (1987). This evaluation system uses a five-point scale: 1 = no visible mold, 2 = presence of spores between flakes, 3 = presence of spores throughout the bale, 4 = mycelial mat between flakes, and 5 = mycelial mat throughout the bale. When appropriate, scores were recorded in increments of 0.25. Recoveries of dry matter (DM) were determined from the calculated DM weight of each bale before and after storage.
Chemical Analysis of Forage
Dry forage samples were ground through a Wiley mill (Arthur H. Thomas, Philadelphia, PA) fitted with a 1-mm screen and subsequently analyzed for N, neutral detergent fiber (NDF), acid detergent fiber (ADF), lignin, neutral detergent–insoluble N (NDIN), and acid detergent–insoluble N (ADIN). The NDF, ADF, and lignin analyses were conducted using batch procedures outlined by ANKOM Technology Corporation (Fairport, NY) for an ANKOM200 Fiber Analyzer. Sulfite and heat-stable -amylase were omitted from the NDF procedure. These methods have been described and subsequently compared with conventional methods by Vogel et al. (1999) and found to give comparable results. Previously, similar agreement between conventionally filtered and filter-bag procedures was reported for NDF (Komarek, 1993) and ADF (Komarek et al., 1994). Nitrogen was quantified by a modified Kjeldahl procedure (Kjeltech Auto 1030 Analyzer, Tecator, Herndon, VA); the concentration of N in NDF and ADF residues (NDIN and ADIN, respectively) was determined by the same modified Kjeldahl procedure used to determine total N. Procedures for determination of NDIN and ADIN were consistent with the guidelines established by Licitra et al. (1996), with the exception that the ANKOM filter-bag method was used for digestion of forages in neutral and acid detergent. Neutral detergent–soluble N (NDSN) was calculated as total N - NDIN.
Temperature Analysis
Before being placed in the stack for storage, each bale assigned to the 24- and 65-d sampling times had single thermocouple wires inserted into its center. Bale temperatures were recorded twice daily (0630 and 1500 h) during the initial 14 d of storage and once daily (1500 h) during the remainder of the storage period. The temperature data were collected with an Omega 450 AKT Type K thermocouple thermometer (Omega Engineering, Stamford, CT). The observed temperature was considered to be the mean internal bale temperature for a given day, except during the initial 14 d when the mean of the two observations was used. Heating degree-days > 35°C (HDD) were calculated by subtracting 35°C from the daily recorded mean internal bale temperature and summing these differences for the entire 65-d storage period. Other temperature-related response variables included maximum temperature, minimum temperature, 30-d average temperature, and the average temperature for the entire 65-d storage period.
Statistical Analysis
Initial bale characteristics were analyzed by PROC ANOVA of SAS (SAS Inst., 1989) as a randomized complete block design with three replications. The mean square for the bale moisture x block interaction was used as the error term. Fisher's protected least significant difference test was used to compare the actual treatment means of bale characteristics. A similar model was used to test DM recovery, visual mold score, and indices of spontaneous heating for significant treatment effects. Changes in forage nutritive value over the six sampling dates were tested for treatment effects using a split-plot model. Concentrations of initial bale moisture served as the whole-plot term, and sampling dates were evaluated as the subplot term. Whole-plot treatment effects were tested for significance with the initial bale moisture x block interaction mean square as the error term. The effects of sampling date and the bale moisture x sampling date interactions were tested for significance with the residual error mean square. For concentrations of N and NDIN, ADIN, and NDSN expressed as a proportion of total N, Fisher's protected least significant difference test was used to separate treatment means.
Previously, Coblentz et al. (1997) used a nonlinear model to describe changes in concentrations of ADIN with storage time for alfalfa hay bales packaged at 297 and 202 g kg-1 moisture. Other data (W.K. Coblentz, unpublished, 1997) indicated that this model could also be used to describe changes in concentrations of ADF and NDF during storage of alfalfa hay. For the present study, a detailed analysis of the interaction of initial concentrations of bale moisture and time in storage for bermudagrass hays was conducted to determine if this approach could be applied to forages other than alfalfa. Mean concentrations of NDF, ADF, lignin, NDIN, and ADIN on each sampling date were regressed against the associated time in storage using nonlinear techniques (PROC NLIN; SAS Inst., 1989). The nonlinear model was
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= asymptotic maximal concentration of each index of nutritive value, ß = the increase in concentration of each index of nutritive value with time in storage, and e-kt2 = fractional proportion of ß made available at a given time in storage (t, expressed in days). Two measures of variability were reported for each nonlinear regression equation; goodness of fit (r2) was calculated as 1 - [residual error sum of squares/corrected total sum of squares], and the root mean square error (RMSE) was calculated as the square root of the residual error mean square. In addition, the PROC REG procedure of SAS (SAS Inst., 1989) was used to evaluate the relationship between HDD and changes in nutritive value of the bermudagrass hay made at three concentrations of moisture. Regressions contained observations based on all three initial concentrations of bale moisture and six sampling dates (n = 54). Before the regression analysis, a test of homogeneity (PROC GLM; SAS Inst., 1989) was used to determine if a common regression equation could be used across all baling treatments to explain changes in nutritive value in response to spontaneous heating.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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0.008 m3). Bale densities were generally comparable to those reported for alfalfa and bermudagrass hays made with comparable equipment at similar moisture concentrations (Buckmaster and Rotz, 1989; Coblentz et al., 1996; Coblentz et al., 2000). Bale weight and density (as-is basis) decreased (P < 0.05) as the forage became drier at baling. Bale weight (DM basis) was greatest (P < 0.05) for HM bales.
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Visual Mold Appraisals
Elevated concentrations of moisture in hay bales and the subsequent associated increase in internal bale temperatures provided a favorable environment for microbial growth (Roberts, 1995). Visual appraisals of mold increased (P < 0.05) with moisture content at baling. The LM bales exhibited some presence of spores between the flakes while the HM bales had spores throughout the bale and evidence of a mycelial mat between the flakes.
Changes in Forage Nutritive Value with Time
Fiber Components
Measurable changes in concentrations of fiber components were expected in hays baled at all three concentrations of moisture because the driest hay in this study exceeded the 200 g kg-1 threshold for acceptable storage (Collins et al., 1987). Typically, fiber components are not lost during hay storage; concentrations are thought to increase because of preferential oxidation of nonfiber components, particularly nonstructural carbohydrates (Rotz and Muck, 1994). Sampling date x bale moisture interactions were found (P < 0.05) for concentrations of fiber components; therefore, main effects are not presented or discussed. Generally, concentrations of all fiber components (NDF, ADF, and lignin) increased over time in storage for hays baled at all concentrations of moisture (Fig. 2, 3, and 4
, respectively). In most cases, large increases (P < 0.05) in the concentrations of fiber components were observed during the first 12 d of storage, but these fractions typically stabilized thereafter. This pattern was observed consistently for all fiber components across all concentrations of moisture at baling. Similar trends with respect to storage time have been observed previously for alfalfa hay (Coblentz et al., 1996). In that study, the concentrations of NDF and ADF increased by 119 and 44 g kg-1, respectively, by Day 11 of storage for hay baled at 297 g kg-1 moisture in conventional rectangular packages. In the present study, increases in concentrations of NDF (88 g kg-1), ADF (65 g kg-1), and lignin (16.3 g kg-1) for HM bermudagrass hays were observed by Day 12 of storage. In both studies, further increases (P < 0.05) in the concentrations of fiber components were sometimes observed; however, changes in concentration observed after the initial 12 d of storage tended to be relatively small compared with the rapid changes that occurred initially.
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- ße-kt2. For all three initial concentrations of bale moisture, changes in NDF content were adequately described (r2 
0.91; Fig. 2). As expected, the asymptotic maximum concentration of NDF () was greater for HM bales than in the drier baling treatments. Although interpretation of was complicated slightly by small variations in initial concentrations of NDF (range = 22 g kg-1), these findings were largely explained on the basis of greater overall change in response to treatment (ß) for HM bales that heated extensively during storage. The regression parameter ß ranged from a low of 68 g kg-1 in LM bales to 93 g kg-1 in HM bales, which is consistent with expectations based on observed heating patterns. For ADF, the same nonlinear model was used to describe changes in concentration for HM, MM, and LM bales (r2 0.85; Fig. 3). The asymptotic maximum concentration of ADF () ranged from 363 g kg-1 in the LM bales up to 391 g kg-1 in the HM bales. Similarly, the total change (ß) in ADF concentration was nearly double for HM bales (61 g kg-1) compared with LM bales (33 g kg-1). Generally, concentrations of lignin increased during storage in patterns similar to those observed for NDF and ADF. However, the total change (ß) for the LM bales was numerically greater than that observed for the MM bales (14.5 vs. 12.6 g kg-1).
Nitrogen Components
For most N components, the interaction of initial bale moisture x sampling date was either significant (P < 0.05) or exhibited a strong tendency (P < 0.10); therefore, means for main effects are not presented and discussed. Concentrations of total N increased slightly (P < 0.05) in MM and HM bales (Table 3). In the short term (<60 d), concentrations of N are known to increase in heated hays because nonstructural carbohydrates are preferentially oxidized by plant enzymatic processes and microorganisms associated with storage (Rotz and Muck, 1994). Concentrations of N in LM bales did not increase (P > 0.05) between Days 0 and 65.
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The sensitivity of forages to nonenzymatic browning is an important consideration in the nutrition of ruminants. The ADIN in forages is considered to be ruminally undegradable (Sniffen et al., 1992) and to have very low bioavailability (Licitra et al., 1996). Broderick et al. (1993) reported a negative apparent digestibility of ADIN (-12.2%) for unheated alfalfa hay offered to dairy cows, but the apparent digestibility of ADIN for steam-heated hays evaluated simultaneously was strongly positive (35.8%). Similarly, a positive linear relationship was observed between the apparent digestibility of ADIN and HDD accumulated during bale storage for lambs (Ovis aries) consuming bermudagrass hays that heated spontaneously (McBeth et al., 2001). Apparent digestibilities of ADIN ranged from -1.7 to 42.3% for bermudagrass hays that accumulated between 5 and 401 HDD, respectively. These findings suggest that the effects of spontaneous heating may be sufficient to limit solubility of N in acid detergent but may not necessarily render this N totally unavailable to the animal.
Expressed as a proportion of total N, concentrations of NDIN increased (P < 0.05) during the 65-d storage period for all hays while concentrations of cell-soluble N (NDSN) decreased (P < 0.05) during this same period (Table 3). Overall, NDIN accounted for about 50% of the total plant N on Day 0 (mean = 507 g N kg-1), but this proportion increased (P < 0.05) to 598, 654, and 690 g N kg-1 in LM, MM, and HM bales, respectively, after 65 d in storage. The magnitude of these increases clearly reflects the differences in the heat increments accumulated by each treatment.
Nonlinear regression analysis was used to more thoroughly examine the interaction of sampling date x initial bale moisture for NDIN and ADIN, expressed as a proportion of total DM (g kg-1 DM). We used the same nonlinear model that was used in the analysis of fiber components; previously, this model effectively described changes in concentrations of ADIN in alfalfa hay as a function of time in storage (Coblentz et al., 1997). Concentrations of NDIN in these baling treatments were adequately explained (r2 0.80) by the nonlinear model (Fig. 5)
. The asymptotic maximum concentration () of NDIN was numerically greatest for HM bales, and overall changes in the concentrations of NDIN (ß) were 4.0, 3.2, and 2.3 g kg-1 DM, for HM, MM, and LM bales, respectively. These responses were clearly expected based on the heating that occurred in these baling treatments.
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0.71; Fig. 6)
for all baling treatments. Estimates of ß increased from 0.54 g kg-1 DM for LM bales to 1.97 g kg-1 DM for HM bales, which again reflects the relative heat increments incurred by these treatments during storage.
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Linear Regressions on Heating Degree-Days Greater than 35°C
Fiber Components
Linear regressions of NDF, ADF, and lignin on the increment of heat accumulated by the associated sampling date (HDD) were significant (P < 0.0001) in all cases (Table 4). This agrees with previous work for bermudagrass (Coblentz et al., 2000) that was based on heat increments accumulated over a 60-d storage period and sampled only at the conclusion of a 60-d storage period. Regression lines for NDF, ADF, and lignin were not homogeneous across initial concentrations of bale moisture. Regression lines for lignin and ADF had dissimilar (P 0.04) intercepts, whereas linear regressions for NDF had dissimilar slopes (P = 0.009). The NDF regression line for LM bales had the greatest slope (0.42 g kg-1 HDD-1; Table 5), indicating that concentrations of NDF changed more rapidly per HDD than in the other baling treatments. This can probably be explained by the nature of HDD estimates; some respiration and concurrent reductions in concentrations of nonstructural carbohydrates occur when internal bale temperatures are less than the arbitrary threshold of 35°C. Under these conditions, concentrations of NDF can increase indirectly without a concurrent accumulation of HDD, thereby affecting the slope in a positive manner.
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0.69) and a total N (r2 = 0.82) basis (Table 5). A positive relationship between the concentration of N and HDD was significant (P < 0.001) but much poorer (r2 0.26). This supports discussions by Rotz and Muck (1994) that suggest that concentrations of N in heated hays can increase during short-term storage.
Unlike other N fractions, a common regression equation that included observations from all treatments could not explain the relationship between ADIN (expressed on either a DM or total N basis) and HDD. Tests of homogeneity (Table 4) indicated that slopes were similar across baling treatments (P 0.24), but intercepts were not (P 0.006). This suggests that concentrations of ADIN were related to HDD in positively sloped, parallel relationships. However, this may be related simply to slightly variable estimates of ADIN across baling treatments on Day 0 (range = 0.19 or 8.0 g kg-1 N; Table 3). Overall, slopes relating concentrations of ADIN to HDD were approximately two to four times greater than those reported for alfalfa hay bales (Coblentz et al., 1996); this suggests that N in bermudagrass hays may be more sensitive to nonenzymatic browning than N in alfalfa. We do not know why the relationships between concentrations of ADIN and HDD were poorer than those observed in several other studies (Coblentz et al., 1996, 2000).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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