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Arkansas Animal Science Department Report 2001
which created differences in the spontaneous heating characteristics
of these bales. A total of 20 bales from each harvest
year were selected for evaluation of rumen escape N. Initial
moisture concentrations of 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 of 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 of 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 of 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 of 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 of 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 of rumen escape N utilized a preparation
of Streptomyces griseus protease (P-5147; Sigma
Chemical Co., St. Louis, MO) and were similar to those
described by Coblentz et al. (1999). Estimates of rumen
escape N were expressed on the basis of total plant N and
DM. All forty samples of bermudagrass hay were evaluated
in each of three separate runs; therefore, estimates of rumen
escape N are the means of three individual evaluations.
Statistical Analysis. Estimates of rumen escape N were
regressed linearly (PROC REG; SAS Inst., Inc., Cary, NC) on
two indices of spontaneous heating; these included maximum
internal bale temperature, and the average internal bale temperature
over the first 30 d of storage. For purposes of analysis,
bale temperatures were averaged over the initial 30 d of
storage because little evidence of heating occurred beyond
this time interval in previous studies. An independent test of
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 of N was
2.24 ± 1.3% in 1998 and 2.06 ± 1.2% in 1999 (14.0 and
12.9% CP, respectively). Concentrations of rumen escape N
for bermudagrass hays harvested in 1998 exhibited a range of
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
of the narrower range of heating characteristics in these bales.
Mean concentrations of rumen escape N were numerically
greater for hay harvested in 1999 than in 1998 (57.1 vs. 44.6
% of total N or 1.18 vs. 0.99% of DM). Estimates of rumen
escape N for warm-season grasses can exceed 50% of the
total N pool, and concentrations of 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 of spontaneous heating (maximum and
30-d average temperature) positively affected (P < 0.0001)
concentrations of rumen escape N; this was true when rumen
escape N was expressed as a percentage of total N (Fig. 1) or
DM (Fig. 2). In each of these cases, there was no difference
(P ≥ 0.416) between slopes for hays made in 1998 and 1999,
thereby suggesting that heat affected concentrations of rumen
escape N consistently across years. Across the entire range of
spontaneous heating, estimates of 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 of 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 of 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 of increase with
heat for alfalfa hay (Coblentz et al., 1997). These increases in
rumen escape N are likely to occur as a greater proportion of
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 of
rumen escape N for bermudagrass can be affected by other
factors; an incomplete list could potentially include forage
variety, climate, fertilization, frequency of contaminant
species, and maturity at harvest. One or more of these factors
may explain the numerically higher estimates of rumen
escape N observed across all increments of spontaneous heating
during the 1999 harvest year. In this study, the proportions
of N from bermudagrass that would likely escape the
rumen intact are considerably higher than those often reported
for cool-season grasses and legumes; however, our estimates
of 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% of total N and increased linearly with all indices of
spontaneous heating in both 1998 and 1999. These data sug-
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