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a USDA-ARS and Dep. Crop Science and Dep. Animal Science, North Carolina State Univ., Raleigh, NC 27695
b Dep. Soil Science, North Carolina State Univ., Raleigh, NC 27695
c USDA-ARS, Watkinsville, GA 30677. Cooperative investigation of the USDA-ARS and the North Carolina ARS, Raleigh, NC 27695-7643. The use of trade names does not imply endorsements by USDA-ARS or by the North Carolina ARS of the products named or criticism of similar ones not mentioned
* Corresponding author (Joe.Burns{at}ars.usda.gov).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: ADF, acid detergent fiber • ADG, average daily gain • CELL, cellulose • CP, crude protein • EFU, effective feed unit • HM, herbage mass • HEMI, hemicellulose • IVTOD, in vitro true organic matter digestion • LOF, lack of fit • NDF, neutral detergent fiber • NIRS, near-infrared reflectance spectroscopy • OM, organic matter • T44, Tifton 44 bermudagrass
Received for publication July 3, 2008.
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Information for the Piedmont is not available on the response of bermudagrass to a range of N application rates managed at HM favorable to daily animal performance, and for effective forage utilization. Further, the movement of inorganic N in the soil profile when applied generously to bermudagrass pastures has not been well assessed. The objectives of this experiment were to: (i) compare the daily animal response and pasture productivity of Coastal and T44 bermudagrasses over a range of N rates, (ii) determine N concentrations of the pasture canopy and of the animals' diet at increased N fertility, and (iii) determine the associated N (total and inorganic) status of the soil profile after 3 yr of applying high rates of elemental N.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Pasture
Pasture Management
The six treatments consisted of Coastal and T44 each fertilized at three N rates. Nitrogen was applied as granular ammonium nitrate in two or three split applications depending on N rate. The 101 kg ha–1 N rate consisted of two split applications targeted at 50.4 kg ha–1 and applied 3 April and 5 June. The 202 kg ha–1 N rate was applied in three split applications targeted at 67.2 kg ha–1 and applied in April and June as noted for the 101 rate plus a third application 10 July. The 303 kg ha–1 N rate was applied in three split applications as noted for the 202 rate but with each application targeted at 101 kg ha–1. The soil N status was also monitored to determine potential surface accumulation. The pastures were fertilized according to soil test, receiving annually about 40 kg ha–1 of P and 149 kg ha–1 of K and pH was maintained between 6.0 and 6.6 during the study.
Variable stocking was used to control HM among all treatments. Grazing was initiated when pastures reached the targeted canopy height of 8 to 12 cm. This height was predicated on a recently completed study which compared per animal and pasture productivity of the same two cultivars over a range in HM (Burns and Fisher, 2008). The HM of about 2.4 vs. 4.0 Mg ha–1 in that study resulted in greater gain ha–1, whereas 4.0 Mg ha–1 improved steer daily gain. Herbage mass in this study was targeted at <4.0 but greater than 2.5 Mg ha–1. The intent was to favor steer average daily gain (ADG) without appreciable reduction in gain ha–1, as N rate was expected to stimulate pasture production (Doss et al., 1966). Initial stocking was dependent on environmental conditions and occurred 2 May in Year 1 (2000), 29 May in Year 2, and 9 May in Year 3. Each year, at the beginning of grazing, steers were grouped by weight with a lighter and a heavier steer paired and designated as tester one and tester two, respectively. The tester pair was then randomly assigned to a pasture treatment within each replicate. This equalized animal weight per pasture and aided in managing HM, that is, tended to group pastures needing stocking adjustment. Because of the similarity in growth between cultivars, canopy height was used as an index to maintain comparable HM among treatments. All pastures were evaluated using height measurements three times weekly during the trial and extra (put-and-take) steers added or removed weekly, as necessary. Grazing was terminated in late summer when pasture canopies could no longer be sustained above the lower target height (i.e., 8 cm). Termination occurred 5 September in Year 1, 4 September in Year 2, and 26 September in Year 3.
Canopy Measurements
Herbage mass was estimated at initiation of grazing and at approximately 4-wk intervals during the trial. Each time the HM was estimated, 80 canopy and compressed-height measurements distributed throughout each pasture were obtained at each sampling with a falling plate (plexiglass) 0.5 m in diameter. To calibrate the falling plate, selected regions within each pasture representing short, medium, and tall canopy heights were harvested. After the compressed height was determined with the falling plate, the area (0.20 m2) was clipped to essentially the soil surface (about 2-cm stubble), the sample placed in a plastic bag, and transported on ice to the field laboratory. The dry matter concentration was determined by lyophilization and used to calculate actual HM dry matter harvested from each site. The HM was regressed on compressed canopy height to develop a calibration equation. An overall equation for the 3-yr trial was developed (r2 among years ranged from 0.83–0.86) and used to predict HM for the trial having an r2 = 0.85.
Detailed canopy characterization was conducted in June and August in Year 1 and in August and September of Year 2 and in concert with HM determinations. At this sampling a fresh subsample of forage harvested from a representative portion of the offered canopy was separated into bermudagrass and weeds [designated ‘other’; predominately crabgrass (Digiteria L. spp)]. The bermudagrass component was further separated into green leaf, stem (including sheath) and dead fractions. These fractions were subsequently used to calculate the whole canopy. All fractions were lyophilized, weighed to constant weight, and each expressed as a proportion of the whole canopy on a freeze-dried basis. The samples were all ground through an Udy Mill (Udy Corp. Fort Collins, CO) to pass a 1-mm screen and stored in air-tight containers in a freezer (–16°C) for laboratory analysis.
Soil Measurements
An assessment of the soil N status under bermudagrass was conducted in early- to mid-April and October of each year to a depth of 60 cm in 0- to 15-, 15- to 30-, and 30- to 60-cm increments. A 5-cm diam. Giddings hydraulic soil probe was used to obtain soil samples which were subsequently air-dried, ground to pass a 2-mm sieve, and stored in air tight plastic vials until analyzed.
Animals
Animal Management
Each year, Angus steers (Bos taurus) obtained in the spring through North Carolina's graded feeder-calf sales served as the experimental animals. Steers were treated for internal parasites before the initiation of grazing and again in July. All steers were sprayed to control flies each time they were weighed during the season. Animals had free access within each pasture to trace mineralized salt blocks, fresh water, and artificial shade.
Animal Measurements
The initial unshrunk weights of the tester steers averaged 237 kg ( ± 16 kg) during the 3-yr experiment. Unshrunk weights of all tester animals were obtained at 2-wk intervals on the same morning schedule for the duration of the trial. Tester steers remained on their assigned pasture treatment during the trial and provided an estimate of average daily gain. Unshrunk weights of extra animals, used to control the HM, were obtained when added or removed from a pasture and contributed to pasture productivity according to the methods of Petersen and Lucas (1968). Body condition scores were assigned to each animal at the initiation of grazing, mid-summer, and at termination of grazing. Animal body weights, weight changes, and body condition scores were then used to calculate EFU (Petersen and Lucas, 1968; Petersen, 1994) to estimate animal and pasture productivity.
Masticate Collection and Characterization
Seven Angus steers ( >450 kg) fitted with esophageal cannulae (Ellis et al., 1984) and maintained on Coastal bermudagrass pasture were used to collect samples of the grazing animal's diet on pasture. Masticate collections occurred by replicate in concert with HM determination and canopy characterization. All pasture treatments within a replicate were sampled concurrently on two consecutive days at two sampling dates each year (June and August in Year 1; August and September in Year 2; July and August in Year 3). Six steers from the seven steer pool were randomly assigned to treatments on Days 1 and 2. However, no pasture was sampled by the same steer on consecutive days. Masticate collection occurred between 0600 h and 0900 h. The masticate was collected by removing the cannula, discarding the first six to nine boli, and then walking beside the animal with a plastic lined butterfly net and catching the extrusa. After collecting approximately 1 kg (about 20 min), including the saliva, the sample was thoroughly mixed, placed in a zip locked plastic bag, flattened, sealed, and placed on a metal rack and submerged in liquid N (–195°C). The masticate samples were subsequently transferred to a freezer (–16°C) and stored until lyophilized. Thereafter, samples were ground in a cyclone mill (Udy Corp. Prod. No. 3010–014, Ft. Collins, CO) to pass a 1.0-mm screen and stored in a freezer (–16°C) until laboratory analysis. After laboratory analyses, data from the two consecutive day collections were averaged before statistical analyses.
Laboratory Analysis
All canopy and masticate samples were scanned in a near-infrared reflectance spectrophotometer (NIRS), and the H statistic (0.6) used to identify samples, by spectra, for laboratory analyses. The results of the laboratory analyses with the selected samples were used to develop NIRS prediction equations for all samples.
In vitro true dry matter disappearance was determined by 48 h fermentation in a batch fermentation vessel (Ankom Technology Corp., Fairport, NY) with artificial saliva and rumen inoculum according to Burns and Cope (1974). The data were corrected for ash and expressed on an organic matter basis as in vitro true organic matter disappearance (IVTOD). Ruminal inoculum was obtained from a mature Hereford steer fed a mixed alfalfa (Medicago sativa L.) and orchardgrass (Dactylis glomerata L.) hay. Total N was determined by auto-analyzer (AOAC, 1990), and crude protein (CP) was estimated as 6.25 times total N. Nitrate ion of all canopy and masticate samples was determined on a water extract using an auto analyzer III (Bran + Luebbe, Inc, Buffalo Grove, IL) according to Burns (2005).
Fiber fractions, consisting of neutral detergent fiber (NDF), acid detergent fiber (ADF), cellulose (CELL), and sulfuric acid lignin, were estimated in a batch processor (Ankom Technology Corp., Fairport, NY) using reagents according to Van Soest and Robertson (1980). Hemicellulose (HEMI) was determined by difference (NDF– ADF). Laboratory values (except nitrate ion) were used to develop NIRS calibration equations to predict concentrations of each variable in each sample from the reflectance spectrum (Table 1 ).
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Statistical Analysis
Animal performance, masticate composition, HM, and canopy composition were analyzed after averaging multiple subsamples (animal or canopy samples within each pasture) for the year to obtain a single observation for each experimental unit. These observations were analyzed using PROC MIXED of SAS (SAS Institute, 2004) with block (2) and years (3, animal and HM data; 2, canopy data) considered to be random effects and forage cultivar (2) and N rate (3) as fixed effects (i.e., six treatments). A repeated measures analysis was conducted with years being the repeated factor and with covariance modeled by either compound symmetry or first degree autoregressive depending on the fit parameters. Orthogonal contrasts (5 df) were used to test for the linear effect of N rate, the lack-of-fit (LOF) to linear for N rate, the cultivar effect, the linear interaction between N rate and cultivar, and the LOF interaction between N rate and cultivar. The least squares means are reported.
The analysis of the soil pH and N data was similar to that for the herbage and animals except in addition, multiple depths were sampled within the soil profile. Soil pH for the initial spring sampling was analyzed separately for the initial status and for the subsequent springs and falls as repeat measures (2 yr). Soil N was analyzed differently. Instead of using years as a repeated measure, winters were used to provide two estimates of the contrast between fall and spring soil N concentrations. Consequently, there were the random effects of winter (2) and replication (2) along with the fixed effects of plot treatments, that included N treatment and forage cultivar (6), fall and spring sampling (2), data collected at three soil depths (3), and the interactions of those effects. Further, soil inorganic N status was analyzed at the termination of the experiment.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Steer Performance and Stocking Rates
Average daily gain was greater for T44 compared with Coastal and increased linearly with increased N rate (Table 3). The cultivar x N rate interaction was not significant. The greater ADG from T44 compared with Coastal is consistent with much of the previous research (Utley et al., 1978; Burns and Fisher, 2008), whereas other grazing studies (Utley et al., 1981; Greene et al., 1990) reported no difference between cultivars. Both cultivars were stocked similarly, averaging 11.8 steer equivalents ha–1 (Table 3). Canopy height was adequately controlled among treatments as there was no correlation between ADG and the canopy when expressed as either HM (r = –0.27; P = 0.61), compressed height (r = –0.26; P = 0.61) or as extended height (r = –0.52; P = 0.30). Stocking rate increased linearly from 10.1 to 13.5 steers ha–1 as N rate increased and is consistent with well established forage yield responses to increased N application (Prine and Burton., 1956; Burton et al., 1963; Mathias et al., 1978; Osborne et al., 1999; Silveira et al., 2007).
Pasture Production
Examination of animal days ha–1, animal gain (kg ha–1) and EFU (kg ha–1) showed only animal days ha–1 differed between cultivars. Under the management used, Coastal provided 1449 animal days ha–1 compared to 1303 animal days ha–1 for T44 (Table 3). No difference was found in stocking rate but this may have been a Type II statistical error (P = 0.08). Animal days ha–1 also increased linearly at about 250 animal days ha–1 for every 100 kg ha–1 of N fertilizer (Table 3).
Coastal and T44 produced similar animal gains averaging 884 kg ha–1 and both showed a linear increase from 723 kg ha–1 from the 101 N rate to 1073 kg ha–1 from the 303 N rate. It is noted that in a previous study comparing Coastal and T44 at a seasonal rate of 347 kg N ha–1, Coastal produced 1110 kg ha–1 of gain and T44 1010 kg ha–1 when HM was maintained at 2.4 Mg ha–1 (Burns and Fisher, 2008). That compares closely with 1111 kg ha–1 from Coastal and 1034 kg ha–1 from T44 at the 303 N rate in this study (Table 3). Overall pasture productivity, as measured by EFU (kg ha–1), did not differ between Coastal and T44 but EFU increased linearly from 3978 to 5523 kg ha–1 with increasing N rate.
In other trials comparing Coastal and T44, weight gain ha–1 were similar averaging 969 kg when topdressed with 343 kg of N ha–1 (Greene et al., 1990) and 659 and 742 kg ha–1 when topdressed with 160 to 168 kg of N ha–1 (Utley et al., 1978; 1981, respectively). The latter N rates were intermediate between the 101 and 202 N rates evaluated in this study but weight gain ha–1 is similar to the 723 kg ha–1 we obtained from the 101 kg ha–1 N rate.
It is worth noting that increased N application, up to 303 kg ha–1, linearly increased productivity of these pastures. Examination of their productivity on a per unit of applied N basis show that for each kg of N applied in this study weight gain declined linearly from 7.1 kg gain kg–1 N to 3.5 kg gain kg–1 N (Table 3). This response was also noted for EFU. Both cultivars responded similarly for both variables. Because elemental N cost and beef prices (live weight basis) are in the same general range, that is, $1.95 to $2.20 kg–1 it is evident from just the N perspective that the 303 kg ha–1 rate remains positive. On the other hand, the extent to which N may have become a part of the soil environment at the greater rate, and hence a potential runoff pollutant with this practice warrants consideration. Before the initial application of N in this study, soil analysis showed the experimental site to be uniform with the mean and associated error mean square for pH of 6.32 (0.307), inorganic N of 3.9 mg L–1 (0.957) and total N of 1.01 g kg–1 (0.023). There were differences, however, among soil depths in inorganic N (P = 0.03) averaging 6.3 mg L–1 in the top 15 cm, then decreased to 3.0 mg L–1 at the 15- to 30-cm depth and to 2.5 mg L–1 at the 30- to 60-cm depth.
During this study shifts in soil N status between October and the following April showed a significant linear trend and LOF for inorganic N with the top 15 cm containing the greatest concentration and accumulation over winter with much less effect at the at the 15- to 30-cm depth and even less effect at the 30- to 60-cm depth (Table 4 ). The LOF is associated with the proportionately larger reduction in inorganic N between the 0- to 15-cm relative to the other depths. Total N was also effected by depth being reduced between the 0- to 15- and 15- to 30-cm depths, but was <0.4 g kg–1 in the 30- to 60-cm depth and not quantifiable by the method used (Table 4).
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Canopy Characteristics
Whole Canopy
The IVTOD and estimates of nutritive value of the whole pasture canopy generally differed between cultivars and among N rates. Both cultivars, however, responded similarly to increased N application (no cultivar x N rate interaction). The IVTOD, CP, NO3– and HEMI were greater and ADF and lignin least for T44 compared with Coastal (Table 5
). However, NDF concentration was similar between the two cultivars. Greater NDF concentration was also reported by Burns and Fisher (2007) for T44 compared with Coastal, with T44 more digestible and consistent with the greater ADG from T44 noted in this study. Correlation analysis showed little relationship in this study between ADG and NDF (r = –0.10; P = 0.76) of the whole canopy. Based on these results one could speculate that the NDF fraction of T44 may have been altered in chemical composition during development as noted for ‘Tifton 85’ bermudagrass when compared with Coastal (Mandebvu et al., 1999). Nitrogen application increased IVTOD, CP, and NO3– but decreased HEMI and is in general agreement with the literature (Bergareche and Simon, 1989; Johnson et al., 2001). Positive correlations were also obtained between ADG and canopy IVTOD (r = 0.90; P = 0.01) and CP (r = 0.89; P = 0.02). These IVTOD values, as estimates of digestibility and the fiber concentrations of the offered canopy (Table 5), would not support the observed ADG in this study (NRC, 1996) as reported in Table 3. Consequently, further examination of the canopy is warranted.
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The stem fraction, with appreciably less nutritive value than the leaves, had similar IVTOD between cultivars, but CP and NDF of T44 was greater compared with Coastal. Both IVTOD and CP increased linearly in both cultivars with increasing N rate (Table 5).
The dead fraction, representing a sizable portion of the canopy dry matter, was least in nutritive value having least IVTOD and CP and greater NDF. This fraction for T44 had greater IVTOD and CP and less NDF compared with Coastal. Nitrogen application linearly increased IVTOD and CP and reduced NDF in both cultivars.
The uniformity in leaf composition (Table 6) is reflected in the lack of relationship between any constituent and steer ADG as no strong correlations were found. The stem and dead fractions, however, were influential as both composed a greater proportion (40–42%) of the canopy compared with leaf tissue (18%). Steer ADG was positively correlated with the IVTOD of stem and dead tissue (r = 0.74, P = 0.09 and r = 0.93, P < 0.01, respectively) and with their CP (r = 0.88, P = 0.02 and r = 0.91, P = 0.01, respectively). Steer ADG was also positively correlated with NDF concentrations in the stem (r = 0.79, P = 0.06) but negatively correlated with NDF of the dead (r = –0.88, P = 0.02) tissue.
Nitrate was present in all three canopy fractions averaging 2.39 g kg–1 for leaf, 2.34 for stem, and 1.96 g kg–1 for dead tissue. Tifton 44 accumulated greater concentrations of nitrate than Coastal and nitrate concentrations increased linearly with increasing N application and consistent with the literature (Murphy and Smith, 1967; Hojjati et al., 1972). Cultivar interacted with N rate (P = 0.03) for the leaf tissue. This resulted from the disproportionate increase in nitrate concentration at 303 N rate for T44 compared with Coastal (Table 6). There may have been a Type II statistical error in the tests for this effect in the stem and dead fraction (P = 0.07). The nitrate concentration of the leaf, stem, and dead fractions (ranging from 4.50 to 5.54 g kg–1) of T44 topdressed with 303 kg N ha–1 places the canopy into the nitrate toxicity range (4.4–6.6 g kg–1) if consumed by pregnant animals (Wright and Davison, 1964; Parson, 1974). In essence, NO3– concentrations in the canopy, as well as in the leaf, stem, and dead fractions, was consistently correlated with steer ADG (r = 0.74 to 0.75, P = 0.09) as was soil inorganic N at the 30 to 60 cm depth (r = 0.82, P = 0.05). To this extent NO3– is a useful nutrient as it contributes to the animal's N status. It can become, however, an antiquality constituent if concentrations in the diet enter the toxic range mentioned above.
Diet Selection
The diet selected from the experimental pastures by esophageally cannulated steers was appreciably greater in nutritive value than the mean value for the whole canopy. For example, the IVTOD of masticate averaged 754 g kg–1 (Table 7
) compared with only 513 g kg–1 (Table 5) for the whole canopy. Further, the nitrate concentration of masticate was only 67% of that in the whole canopy and 62% of that in the leaf and removes it from the toxic level (Table 6). The nutritive value of T44 was greater than Coastal in IVTOD and nitrate and lesser for NDF with similar concentrations of CP (Table 7). The influence of N application on the whole canopy (Table 5), as well as the plant fractions (Table 6), was reflected in the masticate with linear increases in IVTOD, CP, and nitrate, but decreased NDF. The IVTOD and CP concentrations of the diet are adequate to support the maximum daily gain (0.65 kg) obtained by steers in this study, providing dry matter intake was not limited (NRC, 1996).
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It is worth noting that the nitrate concentration of the leaf, stem, and dead fractions of the T44 canopy from the 303 N rate (Table 5) had concentrations which ranged from 4.50 to 5.50 g kg–1 compared with 2.55 g kg–1 in the corresponding masticate (Table 6). This discrepancy may be associated with grazing behavior in which animals generally graze across the top of the canopy with minimal penetration below the upper leaves (Burns and Sollenberger, 2002). These leaves, generally exposed to full sunlight and readily consumed by steers, may have had lesser concentrations of nitrate compared with lower shaded leaves (Schmidt and Blaser, 1969). No attempt was made in this study, however, to distinguish between leaf positions in the canopy.
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Examination of the canopies showed T44 to be greater in the proportion of the DM that is leaf (20.6 vs. 14.5%), least in stem (38.6 vs. 46.0%) and both similar in dead tissue (40.2%). Increasing N from 101 to 303 kg ha–1, however, decrease the proportion of leaf (18.8 to 16.7%), but increased the stem (39.1 to 44.4%). In general, the leaf tissue of Coastal and T44 were similar in IVTOD (726 g kg–1) and CP (170 g kg–1), but T44 had greater concentrations of NO3– (3.03 vs. 1.76 g kg–1) and NDF (662 vs. 637 g kg–1). This general relationship held for stems, whereas the dead fraction was least in IVTOD, CP, and NO3–, but greatest in NDF. Nitrogen application generally increased linearly IVTOD, CP, and NO3– concentrations with a subsequent reduction in NDF.
The diet selected from the canopies increased linearly in IVTOD, CP, NO3–, and NDF with increasing N rate, with T44 having greater IVTOD (764 vs. 743 g kg–1), NO3– (1.62 vs. 1.33 g kg–1) and least NDF (596 vs. 605 g kg–1) than Coastal. Nitrate concentrations of the diet were well below toxicity levels. Masticate IVTOD was consequently positively associated with steer ADG (r = 0.82, P = 0.05). Examining the weight gain response to N application showed a linear decline from 7.1 to 3.5 kg for each kg of N applied. With N cost kg–1 and gain kg–1 valued similarly (from 1.95 to $ 2.20 kg–1) a positive N response even at the 303 kg N ha–1 rate was evident. Both Coastal and T44 are responsive to N application achieving maximum gain ha–1 of 1651 and 1514 kg, respectively, when HM was maintained at 3.0 to 3.5 Mg ha–1. Pastures of T44 were greater in nutritive value than Coastal and T44 has potential as a perennial warm-season forage in the Piedmont.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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