<strong>Arkansas</strong> Animal Science Department Report 2001 which created differences in the spontaneous heating characteristics <strong>of</strong> these bales. A total <strong>of</strong> 20 bales from each harvest year were selected for evaluation <strong>of</strong> rumen escape N. Initial moisture concentrations <strong>of</strong> the treatment bales ranged from 17.8 to 32.5% in 1998 (Coblentz et al., 2000) and 21.9 to 30.2% in 1999. Temperature data. Internal bale temperatures were monitored by inserting single thermocouple wires into the center <strong>of</strong> the bales. Bale temperatures were recorded at 0700 and 1600 h for the first 10 d after baling and once daily (at 1600 h) thereafter, until the end <strong>of</strong> the 60-d storage period. All temperature data were obtained with an Omega 450 AKT Type K thermocouple thermometer (Omega Engineering, Stamford, CT). For purposes <strong>of</strong> analysis, the mean internal bale temperature for a given day was considered the same as the observed temperature, except during the first 10 d, when the mean <strong>of</strong> the two observations was used. Forage Preparation and Analysis. After the storage period, at least two cores (Star Quality Samplers, Edmonton, AB, Canada) were taken from the ends <strong>of</strong> each bale (35-cm depth). Core samples were dried to constant weight under forced air at 122°F. Dry forage samples were ground through a Wiley mill (Arthur H. Thomas, Philadelphia, PA) equipped with a 1-mm screen prior to analysis. Total plant N was determined using a macro-Kjeldahl procedure (Kjeltec Auto 1030 Analyzer, Tecator, Inc., Herndon, VA). The procedures used to determine estimates <strong>of</strong> rumen escape N utilized a preparation <strong>of</strong> Streptomyces griseus protease (P-5147; Sigma Chemical Co., St. Louis, MO) and were similar to those described by Coblentz et al. (1999). Estimates <strong>of</strong> rumen escape N were expressed on the basis <strong>of</strong> total plant N and DM. All forty samples <strong>of</strong> bermudagrass hay were evaluated in each <strong>of</strong> three separate runs; therefore, estimates <strong>of</strong> rumen escape N are the means <strong>of</strong> three individual evaluations. Statistical Analysis. Estimates <strong>of</strong> rumen escape N were regressed linearly (PROC REG; SAS Inst., Inc., Cary, NC) on two indices <strong>of</strong> spontaneous heating; these included maximum internal bale temperature, and the average internal bale temperature over the first 30 d <strong>of</strong> storage. For purposes <strong>of</strong> analysis, bale temperatures were averaged over the initial 30 d <strong>of</strong> storage because little evidence <strong>of</strong> heating occurred beyond this time interval in previous studies. An independent test <strong>of</strong> homogeneity (PROC GLM) was included to determine if a common line could be used to describe the data from both years. Results and Discussion For our test forages, the mean concentration <strong>of</strong> N was 2.24 ± 1.3% in 1998 and 2.06 ± 1.2% in 1999 (14.0 and 12.9% CP, respectively). Concentrations <strong>of</strong> rumen escape N for bermudagrass hays harvested in 1998 exhibited a range <strong>of</strong> 11.3 percentage units when expressed on a total N basis and 0.41 percentage units when expressed on a DM basis (Fig. 1 and 2, respectively). Slightly smaller ranges were observed for bales harvested in 1999 (9.0 and 0.37 percentage units, respectively), which can probably be explained on the basis <strong>of</strong> the narrower range <strong>of</strong> heating characteristics in these bales. Mean concentrations <strong>of</strong> rumen escape N were numerically greater for hay harvested in 1999 than in 1998 (57.1 vs. 44.6 % <strong>of</strong> total N or 1.18 vs. 0.99% <strong>of</strong> DM). Estimates <strong>of</strong> rumen escape N for warm-season grasses can exceed 50% <strong>of</strong> the total N pool, and concentrations <strong>of</strong> rumen escape N for both harvest years were consistent with those determined for other warm-season grasses by various enzymatic and in situ methodologies (Coblentz et al., 1999). Both indices <strong>of</strong> spontaneous heating (maximum and 30-d average temperature) positively affected (P < 0.0001) concentrations <strong>of</strong> rumen escape N; this was true when rumen escape N was expressed as a percentage <strong>of</strong> total N (Fig. 1) or DM (Fig. 2). In each <strong>of</strong> these cases, there was no difference (P ≥ 0.416) between slopes for hays made in 1998 and 1999, thereby suggesting that heat affected concentrations <strong>of</strong> rumen escape N consistently across years. Across the entire range <strong>of</strong> spontaneous heating, estimates <strong>of</strong> rumen escape N were generally greater in 1999 than in 1998, and intercepts were dissimilar (P < 0.0001) for these regression lines; therefore, no single line could be used to describe the relationship between rumen escape N and indices <strong>of</strong> spontaneous heating in any case. The r 2 statistics for these relationships were substantially greater for hays harvested in 1998 (range = 0.597 to 0.706) than in 1999 (range = 0.294 to 0.439); it remains unclear why these relationships were weaker for hay harvested in 1999. These data clearly suggest that rumen escape N is increased in bermudagrass hay in response to spontaneous heating during bale storage. The rate <strong>of</strong> increase in this fraction was similar (P > 0.05) across years, implying that this response may be consistent across some storage environments. Other work has shown similar rates <strong>of</strong> increase with heat for alfalfa hay (Coblentz et al., 1997). These increases in rumen escape N are likely to occur as a greater proportion <strong>of</strong> the total N pool is associated with the cell wall; these relationships with spontaneous heating have been observed commonly in bermudagrass and other forages. Although our data is based on only two harvests, is also clear that estimates <strong>of</strong> rumen escape N for bermudagrass can be affected by other factors; an incomplete list could potentially include forage variety, climate, fertilization, frequency <strong>of</strong> contaminant species, and maturity at harvest. One or more <strong>of</strong> these factors may explain the numerically higher estimates <strong>of</strong> rumen escape N observed across all increments <strong>of</strong> spontaneous heating during the 1999 harvest year. In this study, the proportions <strong>of</strong> N from bermudagrass that would likely escape the rumen intact are considerably higher than those <strong>of</strong>ten reported for cool-season grasses and legumes; however, our estimates <strong>of</strong> rumen escape N for these bermudagrass hays are consistent with similar estimates for numerous other warmseason grasses. Implications Rumen escape N for the 40 hays ranged from 40.7 to 61.3% <strong>of</strong> total N and increased linearly with all indices <strong>of</strong> spontaneous heating in both 1998 and 1999. These data sug- 118
AAES Research Series 488 gest clearly that ruminal escape N is increased when spontaneous heating occurs in bermudagrass hay. Literature Cited Coblentz, W. K., et al. 1997. J. Dairy Sci. 80:700. Coblentz, W. K., et al. 1999. J. Dairy Sci. 82:343. Coblentz, W. K., et al. 2000. Crop Sci. 40:1375. NRC. 2000. National Research Council. 8th rev. ed. Natl. Acad. Press, Washington, DC. Sniffen, C. J., et al. 1992. J. Anim. Sci. 70:3562. 119
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