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AAES Research Series 488
remaining in the bags was analyzed using SAS NLIN procedures
(SAS Inst. Inc., Cary, NC) to determine degradation
rate using the nonlinear model of Mertens and Loften (1980).
Effective degradability was calculated for DM, NDF, and N
and those measurements and degradation rate constants for
those parameters were analyzed using SAS regression procedures.
Results and Discussion
Concentrations of organic matter (OM) and hemicellulose
declined linearly (P < .01) and concentrations of ADF
and fiber-bound nitrogen (ADIN) increased (P < .05) with
increasing HDD. These changes reflect oxidation of more
readily digestible carbohydrates from the forage as heating
increased. Acid detergent insoluble N concentrations at 401
HDD were 17.62% of total N concentration indicating a high
level of heat damage. The 5-HDD treatment fell within the
range of normal forages and the 201- and 273-HDD treatments
approach levels of ADIN necessary to be considered
heat damaged.
Rate of DM degradation (Fig. 1) and effective DM
degradability (Fig. 2) decreased linearly (P < .01) with
increasing HDD. Effective DM degradability decreased by
1.6 percentage units for each increase of 100 HDD. These
reductions in degradation rate and effective degradability
should result in reduced intake and utilization of the hay by
ruminant animals as the level of spontaneous heating increases.
The reduction of DM degradation in the rumen should
reduce the amount of energy the ruminant animal will be able
to derive from the heat-damaged forage.
The NDF degradation rate (Fig. 3) decreased quadratically
(P < .15) reaching a minimum degradation rate between
200 and 300 HDD. Effective degradability of NDF (Fig. 4)
decreased linearly (P = 0.01) as HDD increased. Fiber degradation
limits forage intake on higher fiber forages such as
bermudagrass. Therefore, decreasing the rate of fiber degradation
would result in lower forage intake. Since ruminants
fed bermudagrass hay derive a considerable amount of their
dietary energy from the digestion of forage fiber, the reduction
of effective NDF degradation should reduce the amount
of energy ruminants can derive from this heat-damaged hay.
Nitrogen degradation rate declined linearly (P < .01) as
HDD increased (Figure 5). Effective N degradability (Fig. 6)
also declined linearly by 2.5 percentage units for each
increase of 100 HDD. Therefore, more nitrogen will escape
from the rumen as spontaneous heating increases. This would
lead to lower total nitrogen utilization since enzymes in the
intestine can not degrade much of the heat-damaged forage
nitrogen.
Implications
Spontaneous heating decreases the availability of fiber
and nitrogen for ruminal degradation. Reductions in digestion
rate could limit forage intake and thereby limit total energy
intake by the animal. Reductions in ruminal degradability of
the fiber and nitrogen from the heat-damaged forage would
reduce the availability of both energy and crude protein for
the animal. These impacts would not be detected with standard
forage analyses. When spontaneous heating has
occurred, estimates of intake, energy, and crude protein
should be reduced prior to formulating diets for ruminants.
Otherwise, reductions in animal performance will occur.
Literature Cited
Broderick, G. A., et al. 1993. J. Dairy Sci. 76:165.
Cherney, J. H., et al. 1987. Anim. Feed Sci. Technol. 17:45.
Coblentz, W. K., et al. 1996. J. Dairy Sci. 79:873.
Coblentz, W. K., et al. 2000. Crop Sci. 40:1375.
Mertens, D. R., and J. R. Loften. 1980. J. Dairy Sci. 63:1437.
Weiss, W. P., et al. 1986. J. Dairy Sci. 69:2658.
Table 1. Composition of bermudagrass hays undergoing varying levels of
heating degree-day accumulation.
HDD a
Regression components
Item 5 119 201 273 401 slope intercept R 2 P-value
---------------------------- % ----------------------------
OM, % of DM 93.5 93.1 93.1 92.8 92.5 -0.0024 93.5 0.96