Arkansas - Agricultural Communication Services - University of ...

Effects of Nitrogen Fertilization on Subsequent Partitioning of Nitrogen in Cell
Wall and Cell Soluble Fractions in Bermudagrass Forages
W. K. Coblentz, 1 J. L. Gunsaulis, 2 M. B Daniels, 3 J. E. Turner, 1 D. A. Scarbrough, 1 J. B. Humphry, 1
K. P. Coffey, 1 K. A. Teague, 2 J. D. Speight, 2 and M. R. Gross 2
Story in Brief
Estimates of rumen degradable or escape N are an important component of current nutritional models for feeding
livestock. One factor that can affect the proportion of forage nitrogen (N) that bypasses the rumen intact is the relative
proportion of total plant N that is associated with the cell wall. Relatively little is known about this characteristic of N
within bermudagrass [Cynodon dactylon (L.) Pers.] forages, and the effects of fertilization with N have not been evaluated
in depth. The objective of this study was to assess the relationship between N fertilization rate and the relative
proportions of plant N partitioned into the cell solubles and cell wall. Bermudagrass at two sites was fertilized in split
applications with 0, 50, 100, or 150 lb N/acre as ammonium nitrate such that the total applications for the year were 0,
50, 100, 150, 200, 250, or 300 lb N/acre. Plots were harvested on three dates (May 30, July 7, and August 18). Based
on results for the entire year (three harvests), concentrations of N in the forage increased linearly (P < 0.0001) with fertilization
rate at both sites. Concentrations of cell-soluble N (NDSN) also increased linearly (P ≤ 0.023) with fertilization
rate; however, the magnitude of change was relatively small (≤ 4.3 percentage units of the total plant N pool).
Conversely, the percentage of total plant N associated with the cell wall (NDIN) declined in a linear pattern (P ≤ 0.023)
in response to fertilization with N. Concentrations of acid-detergent insoluble N (ADIN) also decreased linearly (P <
0.001) with N fertilization rate, thereby suggesting that bioavailability may be slightly improved with fertilization.
Introduction
Bermudagrass has been described for more than a century
as one of the most important grasses grown in the southeastern
US. This warm-season grass is used widely by beef
and dairy producers for both grazing and hay production
throughout this region. Many current nutritional models for
ruminants require knowledge of the rumen degradable N concentration
in forages (NRC, 2000; Sniffen et al., 1992), and
diets are currently balanced on this basis. One factor that
affects the relative proportion of plant N that escapes ruminal
degradation is the proportion of plant N that is associated
with the cell wall (NDIN). Fertilization with N is known to
increase concentrations of total N in bermudagrass, but it
remains unclear how this N is partitioned within the plant.
Most cell soluble N (NDSN) should be available to rumen
microorganisms; however, NDIN is likely to have reduced
availability in the rumen. The N that is insoluble in acid
detergent (ADIN) typically has little or no bioavailability.
The objective of this study was to assess the relationship
between N fertilization rate and the relative proportions of
plant N that are partitioned into the NDSN, NDIN, and ADIN
fractions.
Materials and Methods
Generation of Sample Sets. Twenty-eight 10-ft x 20-ft
plots were established on two producer farms (Latta and
Stephens) located near Lincoln, AR in the early spring of
2000. Both sites had histories of poultry waste application.
Poultry waste was applied during the previous year (1999) at
the Latta site only. Concentrations of soil-test P were 305 and
571 lb/acre at the Stephens and Latta sites, respectively. The
associated levels of soil-test K at these sites were 137 and 496
lb/acre. These sites are representative of many in northwestern
Arkansas that have histories of intermittent or annual
applications of poultry waste. Nitrogen was applied as ammonium
nitrate (34-0-0) in split applications of 0, 50, 100, and
150 lb N/acre on April 28 and July 19. For the year, N fertilizer
was applied at cumulative rates of 0, 50, 100, 150, 200,
250, and 300 lb N/acre as shown in Table 1. Plots at each site
were arranged in a randomized complete block design with
four replications. Plots were clipped to a 2-in. stubble height
on May 30, July 7, and August 18 with a sickle-bar mower
and representative subsamples were retained for laboratory
analysis. The extremely droughty conditions in Arkansas during
the late summer of 2000 prevented a final (fourth) harvest
in early fall.
1 Department of Animal Science, Fayetteville.
2 Cooperative Extension Service, Washington County Office, Fayetteville.
3 Cooperative Extension Service, Environmental and Natural Resources Section, Little Rock.
107