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a Dep. of Soil Science, 1525 Observatory Dr., Univ. of Wisconsin, Madison, WI 53706-1299
b USDA-ARS, Dairy Forage Research Center, 1925 Linden Dr., Madison, WI 53706
* Corresponding author (amebeling{at}students.wisc.edu)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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10 times higher (2.84 vs. 0.30 mg L-1) than the low P diet manure, and four times higher (1.18 vs. 0.30 mg L-1) when applied at equivalent P rates. Phosphorus concentrations in October runoff and November to July natural runoff were lower (0.02 to 1.69 mg L-1), but treatment effects were the same as for the June runoff. These results show that excessive addition of inorganic P to dairy diets increases the potential for P loss in runoff from land-applied manure, even at the same P application rate. Diet P effects on potential losses in runoff from land-applied manure should be considered in P-indexing and nutrient management planning.
Abbreviations: BAP, bioavailable P • CV, coefficient of variation • DI, water soluble P • DRP, dissolved reactive P • HPD-21, high P diet manure applied at 21 wet Mg ha-1 • HPD-56, high P diet manure applied at 56 wet Mg ha-1 • LPD-56, low P diet manure applied at 56 wet Mg ha-1 • TP, total P
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Phosphorus from land-applied manure is one of the major sources contributing to soil P accumulation in Wisconsin, and there is increasing evidence that the amount of P in manure could be substantially reduced by avoiding excess P supplementation of dairy rations (Ternouth, 1989; Morse et al., 1992; Metcalf et al., 1996; Khorasani et al., 1997). Satter and Wu (1999) reported that the average dairy diet in the USA is supplemented to contain 4.8 g P kg-1, while only 3.8 g P kg-1 is needed for optimum milk production and reproductive efficiency. This is a 25% over-supplementation of dietary P, based on National Research Council standards (National Research Council, 1989). Valk et al. (2000) discussed the possibility of decreasing P in dairy cow diets without negatively affecting performance and reproductive ability, and cited the work of Brodison et al. (1989) showing that dietary P could be lowered from 6.5 to 4.5 g P kg-1 without consistently influencing milk production or reproductive performance. One study has shown that a 40% decrease in dietary P lowered excreted P by 23% (Morse et al., 1992), and another study reported that reduced dietary P levels lowered P excretion by 30% (Metcalf et al., 1996).
Phosphorus excretion in manure depends largely on the level of P intake (Ternouth, 1989; Morse et al., 1992; Metcalf et al., 1996; Khorasani et al., 1997). If P supplementation could be reduced to the minimum concentration needed for optimum production, the amount of P in manure and in applications to farmland would also decrease. Studies are needed to determine if reducing P supplementation in dairy diets, and the resulting decrease in manure P concentrations, will reduce P concentrations in runoff when manure is land-applied. The objective of this study was to determine dairy diet P effects on the amounts and forms of P in manure as well as on P losses in runoff from land-applied manure. To relate this work to on-farm manure management practices, we included a manure application strategy simulating a N-based nutrient management approach where manures from two P diets were applied at the same manure rate. In addition, a P-based nutrient management approach was also included where manures from the differing P diets were applied to achieve the same P addition.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Dairy manures (feces only, no bedding) with differing P concentrations were hand applied to 2.4-m x 2.4-m plots on 24 May 1999. The experimental site was not tilled in 1999 and residue from the previous year's corn crop remained on the surface. As described below, simulated rain was applied from 1–4 June 1999 and again on the same plots on 30 Sept., 1, 4, and 5 Oct. 1999 after corn silage harvest. Natural runoff was collected from November 1999 to July 2000.
A randomized complete block design with four replications was used in the experiment. The four treatments were: control (no manure, no P addition); low P diet (LPD-56) manure (4.8 g P kg-1 manure dry matter) and high P diet (HPD-56) manure (12.8 g P kg-1) applied at 56 wet Mg ha-1 to provide 40 and 108 kg P ha-1, respectively; and high P diet manure applied at 21 wet Mg ha-1 (HPD-21) to provide the same P addition (40 kg P ha-1) as LPD-56.
Feces were collected from lactating Holstein cows involved in a study designed to determine effects of dietary P levels on milk production and reproductive performance (Wu et al., 2000). For dairy, almost all P excreted is in feces with only trace amounts of P in urine (McDowell, 1992). In this paper, we use the term manure to describe the fecal portion of the dairy manure collected for use in this study. Manure was collected from the barn floor immediately after voiding from Days 148 to 155 during the 308-d full lactation study. Manure from individual cows fed either the low or high P diet was collected and composited in buckets, immediately frozen, and stored at 0°C until it was thawed at the time of field application. The low P diet group was fed no supplemental P, while the high P diet group had monosodium phosphate added to the low P diet. This resulted in low and high dietary P levels of 3.1 and 4.9 g P kg-1, respectively. The low and high dietary P levels produced manures with average P concentrations of 4.8 and 12.8 g P kg-1, respectively. These manure P concentrations, assessed from samples collected in the barn, were used to calculate manure application rates in the field experiments. Samples of manure applied to each plot in the LPD-56 and HPD-56 treatments (n = 8) were collected and analyzed to determine the actual P additions (Table 1).
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Soil samples for antecedent moisture and soil P determinations were collected from the 0- to 2-cm soil depth. This depth was used since it represents the reactive zone for the soil-simulated rain interface (Sharpley et al., 1994). These samples were obtained by compositing ten soil cores from the area immediately adjacent to, but outside the simulated rain test areas in each plot. Soil moisture was determined by weighing the samples before and after drying. Soil test P levels in each plot were obtained by analysis of soil samples dried at 32°C and ground to pass a 2-mm sieve. Surface residue cover (corn stover and manure) was measured inside each plot frame before simulated rainfall using the pin drop method (Morrison et al., 1996). The slope of each plot was also measured using a level placed on the edges of the plot frames. Soil P tests included distilled water extraction (Pote et al., 1996), Mehlich III (Mehlich, 1984), Bray-Kurtz P-1 (Frank et al., 1998), and BAP (Sharpley, 1993). Soil BAP was determined by shaking 1.0 g of soil with 40 mL of 0.005 M CaCl2 for 24 h with five 2- x 10-cm iron-oxide strips. Phosphorus in the extracts obtained by each method was determined colorimetrically by the ascorbic acid method (Murphy and Riley, 1962). Ammonium oxalate P, iron (Fe), and aluminum (Al) (Pierzynski, 2000) were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Iris Plasma Spectrometer, Thermo Jarrell Ash Corp., Franklin, MA), and P saturation percentage was calculated as the oxalate-extractable P divided by the sum of the oxalate-extractable Al and Fe content, and multiplied by 100 (Pote et al., 1996).
Simulated rainfall was applied to experimental treatments using techniques similar to those described by Zemenchik et al. (1996). A portable, multiple intensity rainfall simulator (Meyer and Harmon, 1979) equipped with a Veejet 80150 nozzle (Spraying Systems, Wheaton, IL) located 3 m above the soil surface delivered an application rate of 75 mm h-1 with a corresponding energy of 0.278 MJ ha-1 mm-1. This rainfall intensity has an approximate recurrence interval of 50 yr (Huff and Angel, 1992). Steel plot frames (91-cm length x 91-cm width x 30-cm height) were set in the soil to a 15-cm depth in a representative area of each plot before simulated rain was applied. Runoff was collected at the downslope side of the plot frame and continuously removed using a 0.02-MPa vacuum (Dixon and Peterson, 1968) and placed in a holding tank.
Runoff samples were collected and total runoff volumes were recorded 30 min after runoff initiation and 60 min after the start of rainfall. Since time to runoff initiation was not significantly affected by treatments and treatment effects were similar at both times, only 30-min data are presented in this paper. After mixing to resuspend sediment, subsamples of the runoff were obtained for sediment, DRP, BAP, and TP determinations. The subsample for DRP determination was filtered (0.45-µm pore diam.) immediately in the field. Unfiltered runoff samples were analyzed for TP using the ammonium persulfate and sulfuric acid digestion method (USEPA, 1993) and BAP using the iron-oxide strip method (Sharpley, 1993). Analysis for DRP was performed according to the ascorbic acid method (Murphy and Riley, 1962). Samples were frozen until analyses were performed.
Natural runoff collectors (D. Côté, 1999, personal communication) were installed in all replications of the control, LPD-56, and HPD-56 treatments in early November 1999. These circular collectors consisted of aluminum edging (224-cm circumference x 10-cm high) inserted 5 cm into the soil, creating an internal area of 3971 cm2. Polyethylene funnels (12.7-cm diam.) were placed inside a 12.7-cm diam. x 30-cm long PVC pipe that was installed flush with the soil surface at the downslope side of the plot frame, and the soil inside the pipe was removed. Fiberglass screening (2 mm) was placed inside each funnel to prevent clogging. The funnels and screening were attached with silicone caulk. Polyethylene tubing (2.5-cm diam.) connected the base of the funnel to a 19-L polyethylene bucket installed in a 90-cm deep excavation, located 60 cm downslope from the plot frames. A plastic cover was placed 4 cm above the funnels and drained outside the plot frame to prevent rainfall from falling directly into the funnels. Runoff was collected from the buckets within 2 d of each runoff event, total volumes were recorded, and a subsample was filtered through a 0.45-µm filter and analyzed for DRP.
Corn was planted at a density of 72 000 plants ha-1 on all plots following the June rainfall simulation in 1999 and in May 2000. Nitrogen fertilizer (as NH4NO3) was surface applied to all treatments at a rate of 180 kg N ha-1 immediately following planting. Glyphosate [N-(phosphonomethyl)glycine] was used to control weeds. All plants within each plot frame were cut near the base at physiological maturity and weighed, chopped, and subsampled to determine plant dry matter yield in 1999. Phosphorus uptake was calculated by multiplying the individual whole plant P concentration (Schulte et al., 1987) by the corresponding dry matter yield.
An analysis of variance was performed for treatment effects on antecedent soil moisture, surface residue cover, runoff amount, sediment, DRP, BAP, and TP concentrations and loads in runoff, distilled water extraction, Mehlich III, Bray-Kurtz P-1, ammonium oxalate P, P saturation, and silage yield, P concentration, and P uptake using PROC ANOVA (SAS Institute, 1992). Significant differences among treatment means were evaluated using a protected least significant difference (LSD) test at the 0.05 probability level.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Site Characteristics and Measurements
Average time to runoff initiation in the simulated rainfall studies ranged from 5.7 to 7.2 min in June and 4.0 to 5.1 min in October, and treatments did not significantly affect time to runoff initiation (P = 0.48). Treatment effects on antecedent soil moisture content (0–2 cm) in June, 7 d following the manure application differed significantly (P = 0.01), ranging from 60 g kg-1 for the control to 205 g kg-1 for the HPD-56 treatment. The differences in soil moisture content were likely due to water added in the manure and/or decreased soil evaporation from the manured treatments due to their higher surface residue cover (Table 2). In October, antecedent soil moisture content ranged from 110 to 142 g kg-1 and was not significantly different among treatments. Soil bulk density measurements (0 to 2 cm) taken in October ranged between 1.33 to 1.46 g cm-3, and showed no significant differences (P = 0.10) between treatments. Slope measurements in simulated rainfall plots ranged from 4.3 to 5.5% and did not differ significantly among manure treatments, confirming that slope was relatively uniform within the experimental site and did not contribute to treatment effects on P losses.
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The HPD-56 and HPD-21 treatments increased DRP concentration in runoff relative to the control and LDP-56, and DRP concentration in HPD-56 was nearly 10 times higher than DRP concentration in LPD-56 (Table 2), even though the amount of P applied was only 2.5-fold greater in HPD-56. When the manures were applied at equivalent P rates, DRP concentration in runoff was about four times higher in the HPD-21 than from the LPD-56 treatment. Manure treatment effects on DRP load in runoff were similar to the concentration effects in that loads were significantly higher (11 times) in the HPD-56 compared with the LPD-56 treatment. The marked differences in runoff DRP concentrations and loads between the LPD-56 and HPD-56 treatments, even when the manures were applied at equal P rates, is likely due to differences in the P composition of the manures shown in Table 1.
The June runoff BAP data for the high P manure treatments are not shown in Table 2 because BAP values were lower than DRP values. Apparently, during the seven-month frozen sample storage period, a substantial portion of DRP reverted to P forms not measured as BAP. The runoff BAP data in Tables 2 and 3 could be affected by the same analytical problem as the omitted data. The BAP data are retained to point out the possible analytical problems with using the BAP method on stored unfiltered samples and to provide an estimate of runoff BAP levels in the experimental treatments. Treatment (control vs. LPD-56) effects on runoff BAP concentrations and loads were not significant.
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October Simulated Rainfall Study
Results from the October simulated rainfall application showed that the higher manure rate treatments (LPD-56 and HPD-56) still had significantly higher surface residue cover than the HPD-21 and control treatments (Table 3). Runoff amounts and sediment concentrations and loads were substantially higher in October than in June. Higher runoff amounts in fall vs. spring measurements have been reported in previous work (Mueller et al., 1984), and were attributed to lower infiltration rates due to lower surface residue cover and more extensive soil surface sealing in the fall (Bundy et al., 2001). The control had significantly higher runoff amounts than both the HPD-56 and the LPD-56 treatments, but the HPD-21 treatment was not significantly different from the other treatments. The control was about two times higher than the high manure rate treatments in sediment concentration and about five times higher in sediment load. This reflects the greater residue cover (as manure) and lower sediment loss in the manure treatments and is similar to the results of Mueller et al. (1984) and Bundy et al. (2001).
The DRP concentration in runoff was significantly higher (about four times) in the HPD-56 treatment compared with the LPD-56 and HPD-21 treatments (Table 3). Manure treatment effects on DRP load in runoff followed a similar trend, except that the HPD-21 treatment was not significantly different from the other treatments. The DRP load in runoff was nearly four times higher in the HPD-56 treatment compared with the LPD-56 treatment.
Treatment effects on BAP concentration were the same as for DRP concentration (Table 3). Runoff from the HPD-56 treatment was almost three times higher in BAP concentration than from the LPD-56 and HPD-21 treatments, but treatment effects on BAP load were not significant (P = 0.08).
In October, treatment effects on TP concentrations and loads were opposite from June (Tables 2 and 3). Concentrations and loads of TP in June were higher in the HPD-56 treatment than in the control and other treatments. However, in October, the control had a significantly higher TP load than the LPD-56 and HPD-56 treatments. This probably occurred because TP includes sediment bound P as well as dissolved P, and sediment losses in October were higher in the control and HPD-21 treatments (Table 3). The higher sediment losses are consistent with lower surface residue cover in those treatments. Total P loads were about three to five times higher in October than in June except for the HPD-56 treatment. The high June TP load in the HPD-56 treatment was largely due to the relatively high June DRP losses in that treatment. Higher October TP loads in the remaining treatments are consistent with higher runoff volumes and sediment concentrations in October.
Results from the October rainfall simulation show that manures from different dairy diet P concentrations still influenced P concentrations and loads in runoff more than four months after these manures were land-applied. This supports the results from the June data indicating that excess P in diets can increase DRP losses in runoff when manures from these diets are land-applied, and that this effect persists for at least several months. The influence of dietary P levels on TP losses in runoff is complicated by manure treatment effects on runoff volume and sediment loss.
Natural Runoff Study
Treatment effects on DRP losses in natural runoff (Fig. 1)
support the simulated rainfall results. The cumulative DRP load is consistently higher in the HPD-56 treatment than the LPD-56 treatment and control. Precipitation events varied between 3 and 169 mm (Fig. 1) and runoff volumes ranged from 0.02 to 10.39 mm (data not shown). January to March runoff events were primarily associated with snowmelt, and monthly temperatures were above average for this period (Table 4). The months of May and June received unusually large precipitation amounts (Table 4), resulting in much larger runoff volumes than in previous rainfall events and treatment effects on DRP load were accentuated in these cases. Individual events did not always cause significant differences in DRP concentration and load, due to the variability of the data. The CV for cumulative DRP load ranged from 55 to 80%. There were significant treatment differences (P = 0.05) in cumulative DRP load at 67% of the individual events (12 out of 18 dates). For nine of these dates, the HPD-56 treatment was significantly higher than the LPD-56 treatment and the control, for three of those dates (10 January, 22 and 31 March) the HPD-56 treatment was similar to the LPD-56 treatment but significantly higher than the control. If significance was broadened to the 0.10 level, 15 out of 18 dates had significant treatment effects on cumulative DRP load. Overall, the data confirm that higher amounts of DRP are lost from plots amended with high P diet manure than plots amended with low P diet manure, and these effects on cumulative DRP loss persist for at least 1 yr.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication September 26, 2000.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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