Productivity and nutritive quality of dallisgrass ( Paspalum dilatatum ) as influenced by cutting height and rate of fertilization with poultry litter or commercial fertilizer

Dallisgrass (Paspalum dilatatum) is well adapted to the Black Belt region of the southeastern US, and information on its productivity and nutritive quality as influenced by fertility is needed. In each yr of a 2-yr study, an existing dallisgrass pasture that had been subdivided into 48 plots of 9.3 m each was fertilized with the equivalent of 34 (34N), 67 (67N), 101 (101N) or 134 (134N) kg N/ha from poultry litter (PL) or commercial fertilizer (CF; NH4NO3). In both years, primarygrowth and vegetative regrowth forage was harvested in mid-August and late September, respectively, and forage from each harvest was clipped to either a 5or 10-cm stubble height. Forage cut to a 5-cm height yielded 71% more (P < 0.001) DM than forage cut to a 10-cm height, but forage dry matter (DM) yields were not different between CF and PL treatments across years and fertilization rates. Concentration of crude protein (CP) was greater (P = 0.002) for CF than PL forage and increased for both fertilizer sources with increasing rates of N application. Forage concentrations of cell-wall constituents were not different between CF and PL treatments. Forage amended with CF had a higher concentration of Ca, Mg and Mn than PL-amended forage; however, forage amended with PL had a higher concentration of P and K than CF-amended forage. There was no effect of fertilizer source on forage concentration of Al, Cu or Zn. Results indicate that PL and CF were comparable for supporting productivity and nutritive quality of dallisgrass on Black Belt soils.


INTRODUCTION
Dallisgrass, Paspalum dilatatum, is a warm-season perennial grass indigenous to South America, primarily Uruguay, Argentina and southern Brazil [1].According to Chase [2], it was first reported in the USA in 1840, collected in Louisiana, and named for Abner T. Dallis of La Grange, GA [3].Dallisgrass represents just 10% of the perennial warm-season grassland acreage in the State of Alabama, where its major uses are for pasture, hay and silage [4].It responds well to fertilization with N up to 134 kg/ha, and optimally to P and K based on soil test.Furthermore, dallisgrass tolerates frequent defoliation better and maintains its forage quality longer into the growing season than many other commonly utilized perennial C 4 grasses [5,6].
The influence of sward height on ingestive behavior and intake of dallisgrass by cattle has been documented in a number of studies [7][8][9][10].Less extensively studied is the resilience of dallisgrass to forage and grazing-animal management practices that result in low stubble heights and significantly reduced photosynthetic leaf area and carbohydrate reserves for production of vegetative regrowth.
Productivity of pastureland in response to fertilization can be expected to differ for different fertilizer sources, soil types, forage species and meteorological conditions.
In the case of poultry litter, there is currently a very limited body of systems-level knowledge that producers can use in management decisions, litter application rate adjustments, and prescription techniques for controlling and maximizing nutrient-use efficiency in forage-based beef cattle production systems.In the Black Belt region of Alabama, depressed agricultural economies stem in part from oftentimes poor soil fertility in pasture, hayfields and row crops.Economical transportation of poultry litter could enable export of litter from areas of intensive poultry production to the Black Belt region for use as a cost-effective alternative to commercial fertilizer on pasturelands.For these reasons, we conducted a fieldplot study to determine the primary productivity and nutritive quality of dallisgrass as influenced by rates of fertilization with poultry litter or commercial fertilizer.

Site Characteristics
The experimental site was an existing dallisgrass pasture located at the Black Belt Research and Extension Center in Marion Junction, AL (32.5˚ lat., 87.2˚ long., 61 m elev.).The pasture had been utilized for grazing prior to 1990, and since 1990 it has been utilized for hay production.In 2001 to 2004, the pasture was over-seeded in the fall with oats and received 67 kg of N/ha in the spring and early summer of each yr prior to the experiment.The soil beneath the pasture is a clayey loam with a mean pH of 7.9.Mean annual temperature at the site is 17.6˚C, and mean annual precipitation is 1400 mm.Precipitation and temperature were recorded daily throughout the experiment.

Treatments
Forage in the pasture was clipped to a height of 10 cm on July 17, 2006, and the study area was subdivided into 48 plots of 9.3 m 2 each (1.5 m × 6.1 m).Each plot received the equivalent of 34 (34N), 67 (67N), 101 (101N) or 134 (134N) kg N/ha from either poultry litter (PL; 2.75% N, air-dry basis) or commercial fertilizer (CF; 35% N as NH 4 NO 3 ).Commercial fertilizer was applied to half of the plots utilizing a tractor and spreader, and PL was applied manually to the remaining half of the plots.The PL consisted of wood shavings and manure collected from chicken houses in North Alabama.Prior to transport to the research site, PL was ground to pass a 5-mm screen in a hammer mill and stored in a sealed container under refrigeration.In order to facilitate its transportation to the research site and application to field plots, PL was pre-weighed into paper bags in quantities of 1.14, 2.27, 3.41 and 4.54 kg that corresponded to 1222 (34N), 2443 (67N), 3665 (101N), and 4887 (134N) kg poultry litter/ha, respectively.
Half of the plots within each fertilizer source × application-rate treatment (n = 3) were assigned to aboveground clipping heights of either 5 or 10 cm that simulated different intensities of grazing management.In order to minimize the influence of environmental conditions external to the study area, 12 border plots were maintained around the experimental plots.Border plots received 34N from CF and were clipped to the same height as the study plots to which they were contiguous.

Forage Harvesting, Sampling and Laboratory Analyses
Forage was clipped with a flail-chopping mower when it achieved a mean target height of 20 cm on August 21, and then again on September 25.Harvested forage was collected into plastic baskets and immediately weighed on a portable field scale.Samples of forage from each plot were placed into tared paper bags, weighed, dried at 55˚C for 72 hr and ground to pass a 1-mm screen in a Wiley mill.Forage concentrations of crude protein (CP = N × 6.25) and dry matter (DM) were determined according to procedures of AOAC [11], and concentrations of neutral detergent fiber (NDF), acid detergent fiber (ADF) and acid detergent lignin (ADL) were determined sequentially according to the procedures of Van Soest et al. [12].Concentrations of P, K, Ca, Mg, Al, Cu, Fe, Mn and Zn were measured using inductively coupled argon plasma (ICAP) spectroscopy according to the procedures of Olsen and Sommers [13].
The experiment was repeated in 2007, at which time forage in each plot was clipped to a height of 10 cm on April 23, amended with the same fertilization treatments as those applied in 2006, and harvested on August 16 and then again on September 27 at the same clipping heights as those assigned in 2006.

Statistical Analyses
Data were analyzed by analysis of variance for a completely randomized design with a 2 × 2 × 4 factorial arrangement of treatments (3 replicates/treatment) in which harvest was treated as a repeated measure using the PROC MIXED procedures of SAS and standard leastsquares model fit [14].Components of the statistical model included clipping height, fertilizer source, fertilizer application rate and their two-and three-way interactions treated as fixed effects, and year treated as a random effect.Plot was considered the experimental unit.All data are reported as least squares means ± SE, and the significance level was preset at P < 0.10 for all analyses.mated or were slightly higher than 30-yr averages for Marion Junction, AL in July, August and September of 2006 and 2007, but monthly total precipitation was 42%, 69% and 26% of average for July, August and September, respectively, in 2006, and was 55%, 75% and 67% of average, respectively, for the these three months in 2007.

Dry Matter Yield
Forage cut to a 5-cm height yielded 71% more (P < 0.001) DM than forage cut to a 10-cm height (Table 2).There was no difference in DM yield between fertilizer-source treatments; however, the 134N treatment yielded one-third more DM than the 34N (P = 0.015) and 67N (P = 0.012) treatments.

Crude Protein
There was no difference (P = 0.71) in forage concen- tration of CP between clipping-height treatments (Table 3).However, forage amended with CF had 0.8 percentage unit higher (P = 0.002) concentration of CP than PL-amended forage.Forage receiving 134N had 1.2 and 0.8 percentage units higher concentration of CP than the 34N (P = 0.001) and 67N (P = 0.035) treatments, respectively, but was not different (P = 0.21) from the 101N treatment.Forage receiving 101N had 0.7 percentage unit higher (P = 0.039) CP concentration than 34N, but was not different (P = 0.37) from the 67N treatment.

Neutral Detergent Fiber
Clipping forage to a 10-cm height resulted in a 0.9 percentage-unit increase (P = 0.02) in NDF concentration compared with clipping to a 5-cm height (Table 4).A clipping height × fertilizer source interaction (P = 0.06) was observed such that forage amended with PL and clipped to a 10-cm height had 1.0 and 1.7 percentage units higher NDF concentration than forage clipped to a 5-cm height and amended with CF (P = 0.06) and PL (P = 0.003), respectively.

Acid Detergent Fiber
Forage clipped to a 10-cm height had 0.8 percentage unit higher (P = 0.002) concentration of ADF than forage clipped to a 5-cm height (Table 5).Forage receiving 134N had 1.1 and 0.7 percentage units lower ADF concentration than the 34N (P = 0.001) and 67N (P = 0.06) treatments, respectively.Forage receiving 101N had 0.6 percentage unit lower (P = 0.09) ADF concentration than the 34N treatment, but was not different from the 67N (P = 0.66) or 134N (P = 0.13) treatments.

Acid Detergent Lignin
Clipping forage to a 5-cm height resulted in a 0.2 percentage-unit increase (P = 0.08) in ADL concentration compared with clipping to a 10-cm height (Table 6).A clipping height × fertilizer source interaction (P = 0.08) was observed such that forage amended with CF and clipped to a 5-cm height had 0.4, 0.5 and 0.4 percentage units higher ADL concentration than PL-amended forage clipped to a 5-cm height (P = 0.04), CF-amended forage clipped to a 10-cm height (P = 0.02), and PL-amended forage clipped to a 10-cm height (P = 0.04), respectively.

Calcium
Forage clipped to a 10-cm height had 0.05 percentage unit lower (P < 0.001) concentration of Ca than forage clipped to a 5-cm height (Table 7).There was a 0.03 percentage unit higher (P = 0.003) concentration of Ca in forage amended with CF (0.49%) than PL (0.46%), but there were no differences (P = 0.63) in forage concentration of Ca among N application-rate treatments.

Phosphorus
Forage clipped to a 5-cm height had 0.01 percentage unit higher (P = 0.01) concentration of P than forage clipped to a 10-cm height (Table 8).There was a 0.01 percentage unit higher (P < 0.001) concentration of P in forage amended with PL (0.18%) than CF (0.17%), but forage concentrations of P were not different (P = 0.68) among N application-rate treatments.A clipping height × fertilizer source interaction (P = 0.06) was observed such that forage amended with PL and clipped to a 5-cm height had 0.02, 0.03 and 0.02 percentage unit higher concentration of P than CF-amended forage clipped to a 5-cm height (P < 0.001), CF-amended forage clipped to a  10-cm height (P < 0.001), and PL-amended forage clipped to a 10-cm height (P = 0.002), respectively.Also, forage amended with PL and clipped to a 10-cm height had 0.01 percentage unit higher (P < 0.08) concentration of P than forage clipped to a 10-cm height and amended with CF.

Potassium
Forage clipped to a 10-cm height had 0.07 percentage unit higher (P = 0.001) concentration of K than forage clipped to a 5-cm height (Table 9).There was a 0.29 percentage unit higher (P < 0.001) concentration of K in forage amended with PL (1.03%) than CF (0.74%), but there were no differences (P = 0.11) among N application-rate treatments in forage concentrations of K.A clipping height × fertilizer source interaction (P < 0.001) was observed such that forage amended with CF and clipped to a 5-cm height had 0.39, 0.18 and 0.36 percentage unit lower concentration of K than forage clipped to a 5-cm height and amended with PL (P < 0.001), forage clipped to a 10-cm height and amended with CF (P < 0.001), and forage clipped to a 10-cm height and amended with PL (P < 0.001), respectively.Also, forage amended with PL and clipped to a 5-cm height had 0.21 percentage unit higher (P < 0.001) concentration of K than forage clipped to a 10-cm height and amended with CF, and forage amended with CF and clipped to a 10-cm height had 0.18 percentage unit lower (P < 0.001) concentration of K than forage clipped to a 10-cm height and amended with PL.A fertilizer source × N application rate interaction (P < 0.001) was also observed.Forage concentration of K increased as N application rate increased in forage amended with PL such that the 34N treatment (0.91%) had 0.10, 0.17, and 0.20 percentage unit lower concentration of K than the 67N (1.01%; P = 0.02), 101N (1.08%; P < 0.001), and 134N (1.11%; P < 0.001) treatments.Forage amended with PL and 67N had 0.10 percentage unit lower (P = 0.04) concentration of K than the 134N treatment.In contrast, forage amended with CF and receiving 34N (0.79%) had 0.09 percentage unit higher (P = 0.04) concentration of K than the 134N (0.70%) treatment, but did not differ from the 67N (0.72%; P = 0.13) and 101N (0.75%; P = 0.40) treatments.

Magnesium
Clipping forage to a 5-cm height increased (P < 0.001) concentration of Mg over that of forage clipped to a 10-cm height (Table 10).There was a 0.06 percentage unit higher (P < 0.001) concentration of Mg in forage amended with CF (0.25%) than PL (0.19%), but there were no differences (P = 0.70) among fertilizer-rate treatments in forage concentration of Mg.An interaction  (P = 0.06) was observed such that each clipping-height × fertilizer-source treatment was different (P < 0.001) from each other.Also, a fertilizer source × N application-rate interaction (P < 0.001) was observed.Forage concentration of Mg increased with increasing N application rate in forage amended with CF such that 34N (0.23%) had 0.02, 0.04, and 0.03 percentage unit lower concentration of Mg than the 67N (0.25%; P = 0.02), 101N (0.27%; P < 0.001), and 134N (0.26%; P < 0.001) treatments.Forage amended with CF and 67N had 0.02 percentage unit lower (P = 0.09) concentration of Mg than the 101N treatment.In contrast, concentration of Mg decreased with increasing N application rate in forage amended with PL such that 34N (0.20%) had 0.02 percentage unit higher concentration of Mg than the 101N (0.18%; P = 0.009) and 134N (0.18%; P = 0.008) treatments.

Aluminum
Clipping forage to a 5-cm height resulted in a 407 mg/kg increase (P < 0.001) in concentration of Al compared with clipping to a 10-cm height (Table 11).There were no differences (P = 0.63) in forage concentrations of Al between fertilizer sources or among N application-rate treatments (P = 0.49).

Copper
Clipping forage to a 5-cm height resulted in a 1.4 mg/kg increase (P = 0.03) in concentration of Cu compared with clipping to a 10-cm height (Table 12).There was no difference in forage concentration of Cu between fertilizer-source (P = 0.17) or among N application-rate treatments (P = 0.38); however, a fertilizer source × N application-rate interaction (P = 0.02) was observed such that forage amended with PL and 101N (9.6 mg/kg) had 3.9 mg/kg higher (P = 0.002) concentration of Cu than forage amended with PL and 34N (5.7 mg/kg).However, there were no differences (P = 0.15) among N application-rate treatments in concentration of Cu in CFamended forages.Also, a clipping height × fertilizer source × N application rate interaction was observed (P = 0.10).

Iron
Clipping forage to a 5-cm height resulted in a 273  mg/kg increase (P < 0.001) in concentration of Fe compared with clipping to a 10-cm height (Table 13).There was no difference in forage Fe concentration between fertilizer-source (P = 0.69) or among N application-rate treatments (P = 0.39).

Manganese
Forage clipped to a 5-cm height had 28 mg/kg higher (P < 0.001) concentration of Mn than forage clipped to a 10-cm height (Table 14).There was an 18 mg/kg higher (P = 0.008) concentration of Mn in forage amended with CF (159 mg/kg) than PL (141 mg/kg), but there were no differences (P = 0.17) among N application-rate treatments in forage concentration of Mn.A clipping height × fertilizer source interaction (P < 0.001) was observed such that forage amended with CF and clipped to a 5-cm height had 47 and 37 mg/kg lower concentration of Mn than PL-amended forage clipped to a 5-cm height (P < 0.001) and CF-amended forage clipped to a 10-cm height (P < 0.001), respectively.Forage amended with PL and clipped to a 10-cm height had 46, 93 and 83 mg/kg lower concentration of Mn than CF-amended forage clipped to a 5-cm height (P < 0.001), PL-amended forage clipped to a 5-cm height (P < 0.001), and CF-amended forage clipped to a 10-cm height CF (P < 0.001), respectively.

Zinc
Forage clipped to a 5-cm height had 5.0 mg/kg higher (P < 0.001) concentration of Zn than forage clipped to a 10-cm height (Table 15).There was no fertilizer source effect (P = 0.21) on forage concentration of Zn.However, forage receiving 134N had 2.8 and 2.4 mg/kg higher concentration of Zn than 34N (P = 0.02) and 67N (P = 0.05) forages, respectively.A clipping height × fertilizer source interaction (P < 0.001) was observed such that forage amended with CF and clipped to a 5-cm height had 4.2 mg/kg lower Zn concentration than forage clipped to a 5-cm height and amended with PL (P < 0.001).Forage amended with PL and clipped to a 10-cm height had 3.9, 8.1 and 2.1 mg/kg lower concentration of Zn than CF-amended forage clipped to a 5-cm height (P = 0.001), PL-amended forage clipped to a 5-cm height (P < 0.001), and CF-amended forage clipped to a 10-cm height CF (P = 0.08), respectively.Forage amended with PL and clipped to a 5-cm height had 6.0 mg/kg higher concentration of Zn than forage clipped to a 10-cm height and amended with CF (P < 0.001).

DISCUSSION
Information on forage yield is used by the resource manager to establish forage allowance, and for this reason it is an especially important factor influencing graz- ing-animal performance [15].Also, yield is directly related to sward density, structure and height, all of which have been shown to be key determinants of grazing behavior and voluntary forage intake by cattle [7,9,10].
Because of the truncated experimental period utilized in each year of the present study, cumulative production of dallisgrass was somewhat less than more typical seasonal production reported by other investigators [1,[16][17][18].Also, dallisgrass is best adapted to regions that receive more than 900 mm of annual rainfall [19], and lack of rainfall may partially explain why forage in the present study did not develop to its full production potential, especially in 2007.
The influence of sward height on ingestive behavior and intake of dallisgrass by cattle has been documented in a number of studies [7][8][9][10].In general, these authors have reported that cattle modify their bite mass, defoliation area and depth of grazing in the forage canopy in response to changes in sward height, forage density, and relative proportions of leaf and stem tissue.Less extensively studied is the resilience of dallisgrass to forage and grazing-animal management practices that result in low stubble heights and significantly reduced photosynthetic leaf area and carbohydrate reserves for production of vegetative regrowth.Clipping dallisgrass to a 5-cm height resulted in an increase of more than 70% in DM yield over clipping to a 10-cm height, which is considerably greater than the 11.5% increase in DM yield reported by Holt and McDaniel [17] for dallisgrass clipped to a 5-cm compared with a 15-cm height.Dallisgrass clipped to a 5-cm height yielded 1239 and 552 kg DM/ha for first and second harvests, respectively, across both years of the study; however, dallisgrass clipped to a 10-cm height yielded only 592 and 455 kg DM/ha for first and second harvests, respectively.Because regrowth DM yield compared favorably between the cuttingheight treatments, cutting primary growth to the lower stubble height evidently did not compromise its regrowth potential compared with that of primary growth clipped to the higher stubble height.Watson and Ward [20] reported higher daily and total seasonal regrowth yields with reductions in clipping height, and suggested that dallisgrass could tolerate clipping to stubble heights as low as 2.5 cm as long as at least 10% of tillers were left intact.
Yield of dallisgrass DM increased as a result of increasing N application from 34N and 67N to 134N.Similarly, Robinson et al. [18] reported an increase in dallisgrass DM yield from 5330 kg to 15,340 kg/ha when N fertilization rate was increased from 0 to 896 kg/ha.Likewise, Pizarro [1] reported increases in DM production from dallisgrass ranging from 2400 to 9000 kg/ha over a 5-yr period with increasing N fertilization from 0 to 500 kg/ha in increments of 100 kg/ha.Jones and Watson [21] reported increases in yield of dallisgrass-bermudagrass pasture with increasing rates of fertilization with N, but no yield response to fertilization with either P or K alone in the absence of added N. Brown and Rouse [22] also reported increases in yield of dallisgrass DM with increasing rates of N fertilization in a greenhouse study with white clover-dallisgrass cultures.
Forage protein is an important source of N for ruminal microorganisms, and an important goal of forage management is to derive as much of the N requirement as possible from forage in order to limit or eliminate the need for supplementation.The range of forage concentrations of CP observed in the present study was similar to that observed by Venuto et al. [19], who reported concentrations of CP in dallisgrass of 9.8% to 11%, and lower than that observed by Baréa et al. [23], who reported a wider range of concentrations of CP in dallisgrass of 10.7% to 18.6%.Using prediction equations of Linn and Marten [24], dallisgrass in the present study would be expected to have approximately 87% the relative feed value (RFV) of a mature, medium-quality alfalfa Clipping-height mean 29.5 g 24.5 h a,b,c Within a row, means without a common superscript differ (P < 0.001; SEM = 1.5; n = 48).d,e,f Within a column, means without a common superscript differ (P = 0.08; SEM = 1.5; n = 48).g,h Within a row, means without a common superscript differ (P < 0.001; SEM = 1.3; n = 96).
hay; i.e., ~60% TDN.Values for CP concentration and RFV of dallisgrass in the present study may be compared with those required by a growing beef steer of 340 kg liveweight (8.5% CP and 60% TDN, DM basis) from a daily DM intake of 9.2 kg to achieve an ADG of 0.80 kg [25].
There was no difference in forage concentration of CP between the two clipping-height treatments.Nutritive quality varies within the forage canopy such that stems and younger leaves in the upper canopy are of higher quality than stems and older or dead leaves in the lower canopy [26,27].Results of the present study are interpreted to mean that quality of available forage in the lower canopy would not be expected to differ between grazing management intensities that produce variable stubble heights below 10 cm.
Forage concentration of CP was greater for CF than PL treatments.Wood et al. [28] observed no difference between N-source treatments in CP concentration of "Tifton 44" Bermudagrass amended with either CF or PL; however, there was an increase in CP concentration with increasing rates of fertilization with N. Similarly, forage concentration of CP increased in both CF-and PLamended dallisgrass with increasing rates of N application in the present study, in agreement with other published reports of dallisgrass response to fertilization with N [3,29,30].According to Gunter et al. [31], dallisgrass typically has higher CP concentration and in vivo DM digestibility than bermudagrass (Cynodon dactylon), and supports greater liveweight gain in stocker cattle.
Concentration of NDF is negatively correlated with voluntary intake of forage DM [32].The NDF fraction represents the recalcitrant fibrous components (primarily cellulose, hemicellulose and lignin fractions) of the plant cell wall that are negatively correlated with forage density and in turn form the physical basis of its utility as a predictor of DMI [33].On average, concentrations of NDF in dallisgrass in the present study were slightly lower than those observed by Venuto et al. [19], who reported concentrations of 70.7% for dallisgrass grown in Texas and 69.5% for dallisgrass grown in Louisiana.However, concentrations of NDF in the present study were very similar to those observed by Acosta et al. [29], who reported a mean value of 67.6% for dallisgrass in the spring in Buenos Aires, Argentina.Clipping at 10-cm height resulted in a slightly higher (<1 percentage unit) NDF concentration than clipping at 5-cm height, but this difference would not be expected to have a measurable effect on voluntary DM intake by a free-grazing ruminant animal.
Forage concentration of ADF is negatively correlated with its digestibility in vivo, and comprises the lignin, cutin, cellulose, indigestible N and silica fractions of the plant cell wall [34].In the present study, concentration of ADF was slightly higher (<1 percentage unit) in dallisgrass clipped to a 10-cm than 5-cm height, but this increase would not be expected to have a measurable effect on digestibility in vivo.Values for ADF were slightly below those observed by Ayala Torales et al. [30], who reported concentrations of ADF in dallisgrass ranging from 35.2% to 39.5%, and intermediate to those observed by Acosta et al. [29], who reported values ranging from 31.3% in the winter to 39.7% in the summer in Argentina.Higher rates of fertilization resulted in lower concentrations of ADF in dallisgrass in the present study, in contrast to findings of Wood et al. [28] who reported increased concentration of crude fiber with increasing rates of N fertilization in "Tifton 44" Bermudagrass.
Plant cell wall availability to herbivores is limited by different factors, one of the most important being lignin [34].Concentration of lignin increases and digestibility of plant cell-wall constituents and total plant DM decreases with advancing forage maturity [35].Clipping to a 5-cm height resulted in a higher concentration of ADL than that in forage clipped to a 10-cm height, which can be explained by the fact that younger leaves and stems are located in the upper stratum of the forage canopy, and therefore lignin concentration is expected to be higher in the lower stratum where the more mature steams and leaves are located.However, it is unlikely that the small difference in concentration of lignin between clippingheight treatments in the present study would be sufficient to result in a measurable difference in cell-wall or wholeplant DM digestibility in vivo.
Forage concentration of minerals is dependent upon numerous factors, including plant development stage, climatic conditions, soil characteristics and fertilization regime [36].Among these, soil fertilization can be manipulated by the resource manager in order to provide different types and quantities of nutrients for plants; generally, it is more economical to fertilize plants in order to achieve maximum growth, and then supplement as necessary to meet requirements for animal production [36].
Forage concentrations of Ca in the present study were, on average, less than half of those reported by Brown and Rouse [22] for dallisgrass cultivated in a greenhouse in association with white clover.Concentrations of Ca were higher in dallisgrass amended with CF than PL in the present study, in contrast to the study by Wood et al. [28] in which Ca concentration in "Tifton 44" Bermudagrass amended with PL was higher than in unfertilized forage or forage amended with ammonium nitrate.Results of the present study are similar to those of Robinson et al. [18], who reported Ca concentration values for dallisgrass of 0.39% to 0.48%.
Phosphorus is arguably the single mineral element that is most commonly deficient for meeting animal requirements from grazed forages.Because of its importance in various metabolic processes in animals, notably energy metabolism, dietary P deficiency can very likely result in a deficiency of energy [36].In the present study, concentration of P was higher in dallisgrass amended with PL than CF, in contrast to the study by Wood et al. [28] in which there was no difference in concentrations of P between "Tifton 44" Bermudagrass amended with CF or PL.Also, concentration of P was higher in dallisgrass clipped to a 5-cm than 10-cm height.In general, values were lower than the range of values (0.27% to 0.29%) reported by Robinson et al. [18].
Concentration of K was higher in forage amended with PL than CF, similar to results reported by Wood et al. [28] for "Tifton 44" Bermudagrass; however, there was an increase in K accumulation with increasing rate of N fertilization with PL and a decrease in K concentration with increasing rate of N fertilization with CF in their study, in contrast to the present study in which rate of N application had no effect on K concentration in dallisgrass.Forages normally contain sufficient K for meeting grazing animals' requirements; however, high (>2.5%)forage concentration of K may interfere with bioavailability of Mg [36].Concentration of K in dallisgrass averaged 0.89% in the present study, well below the threshold at which it can potentially be problematic for Mg absorption, and less than half of that in the study by Robinson et al. [18], who reported concentrations of K in dallisgrass of 2.04% to 2.24%.Potassium concentrations in this study, on average, were similar to those reported by Brown and Rouse [22] for dallisgrass grown in a greenhouse in association with white clover.Concentration of Mg was higher in dallisgrass amended with CF than PL, and increased with increasing rate of N application from CF, in agreement with Robinson et al. [18] who reported an increase from 0.19% to 0.36% Mg when N application rate was increased from 0 to 896 kg/ha.
Utilization of PL as a fertilizer source has an advan-tage over synthetic fertilizers of providing trace minerals that are important for plant and animal nutrition.However, it is important to recognize the potential for toxicity to livestock that may result from repeated land-application of PL and possible accumulation of certain trace minerals in soil and grazed forage.Franzluebbers et al.
[37] reported 4.1 and 7.8 mg/kg greater concentrations of extractable-soil Zn and Cu, respectively, in the upper 15-cm horizon of a Piedmont soil at the end of a 5-yr period of land-application of PL at a rate of 196 kg N•ha −1 •yr −1 .Gascho and Hubbard [38] reported a fourand five-fold increase in concentrations of Cu and Zn, respectively, in the surface of a Tifton soil in the Coastal Plain of Georgia following land-application of PL at a rate of 2812 kg N/ha over a 5-yr period.
Iron concentration in forages grown in the US typically meets or exceeds animal dietary requirements [36].Concentration of Fe in dallisgrass in the present study was well above the dietary requirement (50 mg/kg DM) for beef cattle [25], and was higher in forage clipped to a 5-than 10-cm height.Some trace elements are not required or may be required in small amounts, and if ingested and absorbed in excessive amounts can be toxic to cattle.Aluminum is one such trace mineral for which the maximum tolerable concentration (MTC) for beef cattle is 1000 mg/kg DM [25].Dallisgrass clipped to a 5-cm height had a higher concentration of Al than forage clipped to a 10-cm height and, with the exception of forage amended with PL at the 34N application rate had concentrations of Al that were below the MTC for beef cattle.
Suboptimal Cu status in ruminants may be caused by low forage concentration of Cu, high concentration of a Cu antagonist such as Fe, or a combination of both [36].Concentration of Cu in the present study was higher for dallisgrass clipped to a 5-than 10-cm height, and on average was below the concentration required (10 mg/kg DM) by beef cattle [25].Concentration of Mn, which is normally higher in forage than required by the animal [36], was higher in dallisgrass amended with CF than PL in the present study.Zinc and Cu are often deficient in warm-season grasses, and normally are the most limiting trace minerals in both warm-season and cool-season forages [36].Deficiencies of trace minerals in grazed forage require supplementation in order to meet animal requirements for maximum performance and optimal health.Zinc is one such trace mineral for which deficiency in forages is not uncommon in the US [36].Concentration of Zn in dallisgrass was below that required (30 mg/kg DM) by beef cattle [25], and was not different between clipping-height and fertilizer-source treatments or among N application-rate treatments in the present study.
OPEN ACCESS

IMPLICATIONS
Results indicate that dallisgrass can withstand defoliation to a 5-cm stubble height, thereby increasing DM yield compared with defoliation to a 10-cm stubble height, without compromising forage quality or capacity for regrowth.Also, dallisgrass amended with PL or CF was comparable in productivity and nutritive quality as determined by laboratory analysis.Dallisgrass amended with PL had higher concentrations of P and K than CF-amended dallisgrass, but trace-mineral profiles were not markedly different between dallisgrass amended with PL or CF.Results are interpreted to mean that poultry litter may offer potential as a safe, cost-effective alternative to commercial fertilizer for supporting productivity and nutritive quality of dallisgrass on Black Belt soils.

Table 1 .
Monthly mean air temperatures for 2006 and 2007, and 30-yr averages at Marion Junction, AL.

Table 3 .
Concentration of CP (%, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 5 .
Concentration of ADF (%, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 6 .
Concentration of ADL (%, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 7 .
Concentration of Ca (%, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 8 .
Concentration of P (%, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 9 .
Concentration of K (%, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 10 .
Concentration of Mg (%, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 11 .
Concentration of Al (mg/kg, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 12 .
Concentration of Cu (mg/kg, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 13 .
Concentration of Fe (mg/kg, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 14 .
Concentration of Mn (mg/kg, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.

Table 15 .
Concentration of Zn (mg/kg, DM basis) in dallisgrass amended with commercial fertilizer (CF) or poultry litter (PL) at 4 rates of N application and clipped to a 5-or 10-cm height.