Arkansas - Agricultural Communication Services - University of ...

Arkansas Animal Science Department Report 2001
Forage Preparation and Analysis. All forage samples
were dried to a 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
before analysis. Total plant N was determined by rapid combustion
(850°C); conversion of all N-combustion products to
N 2 ; and subsequent measurement by thermoconductivity cell
(LECO Model FP-428; LECO Corp., St. Joseph, MI). Neutral
(NDF) and acid detergent (ADF) fiber analyses were conducted
by the batch procedures outlined by ANKOM
Technology Corp. (Fairport, NY). No sulfite was included in
the neutral detergent solution. The concentrations of N in
NDF and ADF residues (NDIN and ADIN, respectively) were
determined as described previously. Cell-soluble N was calculated
as 100 - NDIN, where NDIN was expressed as a percentage
of total plant N.
Statistical Analysis. For each individual harvest,
orthogonal contrasts (PROC GLM; SAS Inst., Inc., Cary, NC)
were used to test each N fraction for linear, quadratic, and
cubic responses to N fertilization. For each individual plot,
fertilization rate (0, 50, 100, or 150 lb N/acre) was not necessarily
the same on both application dates; therefore, for the
first and second harvest, contrast statements were constructed
based on the initial (April 28) application of ammonium
nitrate. For the third harvest, contrast statements were based
on the second application of fertilizer N on July 19. A combined
analysis of all data from all three harvests at each site
was conducted by similar methods. This analysis included
seven N fertilization rates (0, 50, 100, 150, 200, 250, and 300
lb N/acre) and a test for quartic effects was included in the model.
Results and Discussion
Nitrogen. At both sites, forages from the initial harvest
on May 30 and the third harvest on August 18 exhibited a
strong positive linear relationship (P < 0.0001) between N
fertilization rate and the concentration of N in the forage
(Table 2). This is a commonly observed response to N fertilization.
The increases observed in forages fertilized with 150
lb N/acre compared with those receiving no fertilization
ranged from 0.56 to 1.11 percentage units, which corresponds
to 3.5 to 6.9 percentage units of CP. The effects of the first
application of N fertilizer on forages harvested on the second
date (July 7) were not significant (P > 0.05) at either site,
indicating that the residual effects of the first application were
not sufficient to affect concentrations of N in forages harvested
on the second date. When all data for the year was
combined, strong positive linear effects (P < 0.0001) were
found at both sites (Table 3). The higher concentrations of N
generally observed for forages grown at the Latta site probably
reflects the more recent applications of poultry waste during
the preceding year that did not occur at the Stephens site.
Cell-soluble N (NDSN). The percentage of total plant N
found in the cell-soluble portion of the plant ranged from 55.5
to 75.6% over the study and was affected by N fertilization in
a manner similar to that described for total plant N (Table 4).
Positive linear effects (P = 0.001 and 0.033 for the Stephens
and Latta sites, respectively) were observed for the first harvest;
however, no effects (P > 0.05) were observed for the
second harvest date across both sites. On the third harvest
date, linear (P = 0.047) and quadratic (P = 0.022) effects were
observed at the Stephens and Latta sites, respectively. The
overall analysis for the entire year indicated that the concentration
of NDSN was related to N fertilization rate in a positive
linear manner at both sites (P ≤ 0.023; Table 3).
Cell-wall associated N (NDIN). Concentrations of
NDIN in these bermudagrass forages ranged from 24.4 to
44.5% of total N on individual harvest dates (Table 5). At
both sites, concentrations of NDIN declined (P ≤ 0.047) with
N fertilization on the first and third harvest dates, but not on
the second harvest date (P > 0.05). Apparently, the residual or
carryover effects of the first application of N fertilizer were
not sufficient to affect the relative percentages of N partitioned
into the cell wall and cell solubles. The overall analysis
for the entire year indicated that the concentration of
NDIN was inversely related to N fertilization rate in a linear
manner at both sites (P ≤ 0.023; Table 3).
Acid-detergent insoluble N (ADIN). The N fraction that
is insoluble in acid detergent (ADIN) is generally assumed to
be indigestible; therefore the effects of N fertilization on this
fraction are of particular interest to nutritionists. Normally,
this fraction is of considerable interest when spontaneous
heating has occurred during the storage of dry hay or during
the silage fermentation process. Unlike the other N fractions
discussed in this study, substantive differences in concentrations
of ADIN were observed between sites. On the first harvest
date, ADIN decreased linearly (P < 0.0001) with N fertilization
rate from 11.48 to 8.32% of the total N pool at the
Stephens site (Table 6); however, concentrations of ADIN
and fertilization rate were not related (P > 0.05) at the Latta
site (overall mean = 4.69% of N). There was no relationship
(P > 0.05) between ADIN and N fertilization rate at either site
for the second harvest date. Linear declines in concentrations
of ADIN were observed with N fertilization rate at both the
Stephens (P = 0.002) and Latta (P = 0.011) sites on the final
harvest date. The overall analysis for the entire year indicated
that concentrations of ADIN were inversely related to N
fertilization rate in a linear manner at both sites (P ≤ 0.001;
Table 3).
Implications
The results of this study suggest that fertilization with
N may result in lower percentages of the total plant N pool
being associated with the cell wall. Theoretically, this should
increase the degradability of N in the rumen; additional studies
are being planned to verify this hypothesis.
Literature Cited
National Research Council. 2000. 8th rev. ed. Natl. Acad.
Press, Washington, DC.
Sniffen, C. J., et al. 1992. J. Anim. Sci. 70:3562.
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