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Effectiveness of dairy first pond sludge as a nutrient source for perennial ryegrass pasture production

Graeme Ward and Joe Jacobs

Future Farming Systems Research, Department of Primary Industries, 78 Henna Street Warrnambool. VIC. 3280. Email graeme.ward@dpi.vic.gov.au

Abstract

First pond sludge from on-farm dairy effluent treatment systems contains large amounts of agronomically useful nutrients. As a high proportion of these are in organic form, there is uncertainty amongst dairy farmers as to the effectiveness of sludge as a nutrient source and partial replacement of fertiliser for pasture production. In this experiment, sludge was applied in autumn to an established perennial pasture and subsequent pasture DM consumed by livestock and herbage mineral content for the following year were monitored. Sludge application resulted in a 66% increase in pasture DM consumed for the 30 mm application compared to the untreated control for the 11 months following application. Responses over winter continued to be high suggesting that organic N was being mineralised and made plant available. Unlike second pond effluent, applications of first pond sludge reduced the dietary cation anion difference of herbage, and hence the risk of animal metabolic diseases, for most of the growing season.

Key Words

Dairy sludge, organic waste, recycling, nitrogen, DCAD, dairy pasture

Introduction

The accumulation of excess sludge in the first ponds of on-farm dairy waste treatment systems is emerging as a major environmental risk. Many farmers are reluctant to de-sludge their ponds due partially to the high cost and the need for specialised equipment. In addition, although this sludge contains large amounts of agronomically valuable nutrients that can be recycled onto farm land (Cameron et al. 1996), there is widespread farmer uncertainty on the effectiveness of the sludge as a fertiliser replacement for forage production on farm and the cost:benefit of applying it to land. The application of sludge directly to established pastures in the late summer-early autumn period is the most common method of sludge disposal on dairy farms in southern Victoria. The objectives of this experiment were to quantify the pasture production responses to a range of application rates of sludge to established dairy pastures, the longevity of these responses, and the effect on herbage mineral composition.

Methods

Study site and trial establishment

The trial was conducted on a commercial dairy farm (DemoDAIRY) (38o14’S, 142o55’E) in south west Victoria. The soil is a fine sandy clay loam (brown Chromosol, Isbell 1996) derived from quaternary basalt. Initial soil tests (0-0.1 m) indicated a soil pHH20 of 5.4, Electrical Conductivity of 0.24 dS/m, Olsen P of 35 mg/kg, available K of 193 mg/kg and CPC S of 35 mg/kg.

The trial was conducted on a long-term (10+ years) commercial dairy pasture. The sward was dominated by perennial ryegrass (Lolium perenne L.) with very small amounts of winter grass (Poa annua L.), capeweed (Arctotheca calendula (L.) Levyns.) and white clover (Trifolium repens L.). Dairy sludge was extracted from the bottom of the first pond of the dairy effluent treatment system using a commercial 9000 L dairy effluent vacuum tanker. The sludge was discharged under pressure and spread with a splash plate giving an effective spreading width of 6.5 m. The tanker was calibrated to give target application rates of 5 and 10 mm of sludge per pass. A representative sample of sludge was collected from each load for nutrient and DM content analysis. Application of sludge directly to the established pasture commenced on 22 March 2007 and was completed by 26 March. The trial was a randomised design with 4 replicates and treatments of a control (no sludge), 5, 10, 20 and 30 mm of sludge. No fertiliser was applied for the duration of the trial.

Site management and measurements

The treated pasture was returned to grazing on 18 May 2007. All plots within the experimental site were commonly grazed by the commercial herd when the perennial ryegrass component of the sward (in the vegetative stage of growth) reached the 2.5-3 leaf stage of development. A total of 8 grazings were conducted between May 2007 and February 2008.

Herbage DM accumulation (kg DM/ha) was estimated by measuring pre- and post-grazing pasture mass at each grazing with a calibrated rising plate pasture meter (Earle and McGowan 1979). Prior to each grazing, a representative sample of perennial ryegrass herbage cut to ground level was collected from each plot. This sample was dried at 60oC for 72 hours and ground for mineral content analysis.

The Apparent Nitrogen Recovery (ANR) was calculated for each sludge application rate using the definition of Schroder et al. (2005):

ANR = (N uptake of treated crop) - (N uptake of control crop)/Total N input from sludge.

Dietary cation-anion differences (DCAD) of herbage were calculated using the equation of Ender et al. (1971): DCAD = (Na+K) - (Cl+S) (mEq/100g).

Results and Discussion

Sludge composition

The chemical composition of the sludge and the nutrients applied for each application rate are shown in Table 1. More than 90% of the total N in the sludge was in organic form. Such organic N would require mineralisation in the soil for it to become plant available.

Table 1. The pH, electrical conductivity (EC), total solids, total carbon (C), total nitrogen (N), ammonia (NH3), total phosphorus (P), phosphate (PO45-), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na) and sulphate (SO42-) content of the dairy first pond sludge and the amount (kg/ha) surface applied to pasture in March 2007

 

Analysis

Application Rate

 

Mean

s.d. (n=5)

5 mm

10 mm

20 mm

30 mm

pH

7.3

0.14

        

EC dS/m

7300

141

        

Total Solids g/L

79.5

3.65

4000

8000

16000

23900

Total C %

26.5

2.12

        

Total N

12.0 mg/g

0

48

95

191

286

NH3

0.98 mg/g

0.255

4

8

16

23

Total P

2.5 mg/g

0.14

10

20

40

60

PO45-

42.5 mg/L

5.21

2

4

9

13

K

8.4 mg/g

1.21

33

67

134

200

Ca

29.4 mg/g

2.44

117

234

467

701

Mg

12.3 mg/g

1.56

49

98

196

293

Na

11.4 mg/g

1.55

45

91

181

172

SO42-

36.2 mg/L

3.51

2

4

7

11

DM herbage consumed

At all grazings (Table 2), except 30 November 2007, there were linear increases (P<0.05) in pasture DM consumed by cattle with applied sludge. Total herbage consumed over the May to February period also showed a linear increase (P<0.001) to applied sludge. Total pasture DM consumed over this period increased from 4.87 t DM/ha for the control to 5.63 (5 mm/ha), 6.60 (10 mm/ha), 7.69 (20 mm/ha) and 9.37 t DM/ha (30mm/ha). Total response over this May-Feb period was 146 kg DM/ha/mm of applied sludge. The highest individual response occurred during the spring period where a response of 45.8 kg DM/ha/mm was recorded at the 1 November grazing. During the winter growth period of 26 June to 17 August, a response of 29.7 kg DM/ha/mm was recorded. Such responses demonstrate that the sludge was an effective nutrient source for the pasture, being able to supply nutrients across the year, including the colder winter months when the mineralisation of organic materials is likely to be at their slowest. Contrary to UK findings (Smith et al. 1995), no negative effects on pasture production, due to smothering and/or scorch, was observed at the early times of grazing, with increasing sludge rates.

Table 2. Estimates and standard errors of regression coefficients (Y=A + Bx) for the relationship between herbage DM consumed (t DM/ha) by the grazing herd and sludge application rate (mm) from perennial pasture for a range of grazing dates from May 2007 to February 2008

Grazing Date

A

s.e.

B

s.e.

18 May 07

0.185

0.0280

0.0102

0.00166

26 June 07

0.223

0.0177

0.0143

0.00105

17 August 07

0.417

0.0397

0.0297

0.00235

21 September 07

0.660

0.0544

0.02807

0.00322

1 November 07

1.490

0.0488

0.0458

0.00289

11 January 08

0.949

0.0999

0.0133

0.00592

8 February 08

0.543

0.0313

0.0069

0.00185

TOTAL

4.930

0.1130

0.1464

0.0067

Apparent nitrogen recovery

The apparent nitrogen recoveries (ANR) by the pasture of the N applied in the sludge during the first six grazings after application (18 May to 30 November) were 46%, 54%, 42% and 41% for the 5, 10, 20 and 30 mm treatments respectively. These figures are substantially higher than an average 13% (Unwin et al. 1986) and the range 9 – 28% (Pain et al. 1986) on similar ryegrass pasture cut for silage for the growing season in the UK. This is also despite the slurry used in this UK work having 40-50% of the N in water soluble forms.

Dietary cation-anion differences

There were significant effects on the dietary cation-anion difference (DCAD) of the ryegrass herbage throughout the year (P<0.001) and between rates (P<0.05) (Table 3). At the first grazing (18 May), the DCAD of the 20 and 30 mm treatments were comparatively low at 2.6 and 5.5 mEq/100g DM respectively compared to 25.6 mEq/100g DM for the control, with the 5 and 10 mm treatments intermediate to this. These trends continued throughout the winter and early spring until the 1 November grazing when all treatments had similar DCAD values. Such reductions in herbage DCAD with sludge applications are likely to have beneficial animal health outcomes. This would make such sludge treated pastures suitable for grazing by cows in the final weeks of gestation, which if fed a diet with a high positive DCAD, run a high risk of inducing hypocalcaemia (Horst et al. 1997). This is in contrast to the effect of applying second pond effluent. In earlier work using second pond effluent from the same dairy treatment system as used in the current study, Ward and Jacobs (2007) found that second pond effluent applied to pasture substantially increased the DCAD of herbage making it unsuitable for grazing by cows in late gestation.

Table 3. The effect of first pond dairy sludge surface applied to perennial pasture in March 2007 on herbage dietary cation-anion difference (DCAD) (mEq/100g) across a range of rates and grazing dates.

 

Sludge Rate (mm)

 

0

5

10

20

30

  

l.s.d. (P=0.05)

DCAD

20.9

12.9

14.3

9.3

9.5

  

6.28

 

Grazing Date

 

18 May

26 June

17 Aug

21 Sept

2 Nov

30 Nov

l.s.d. (P=0.05)

DCAD

13.2

23.9

21.1

14.7

8.2

-0.8

6.76

Conclusion

Despite having a high proportion of its total N in organic form, autumn applications of dairy first pond sludge were found to be effective in promoting perennial pasture growth throughout the year. Responses over winter continued to be high suggesting that the organic N was still able to be mineralised and made plant available under winter conditions. Unlike second pond effluent, applications of first pond sludge reduced the DCAD of herbage, and hence the risk of animal metabolic diseases, for most of the growing season.

Acknowledgments

The authors gratefully acknowledge the Geoffrey Gardiner Foundation and the Victorian Government for providing financial assistance for this study. We also wish to thank DemoDAIRY for the use of land on their farm to undertake the experiment. The technical support of Stewart Burch, Troy Jenkin, Robyn Bush and Paul Maloney are also acknowledged.

References

Cameron KC, Rate AW, Noonan MJ, Moore S, and Kerr LE (1996). Lysimeter study of the fate of nutrients following subsurface injection and surface application of dairy pond sludge to pasture. Agriculture, Ecosystems and Environment 58, 187-197.

Earle DF and McGowan AA (1979). Evaluation and calibration of an automated rising plate meter for estimating dry matter yield of pasture. Australian Journal of Experimental Agriculture 19, 337-343.

Ender F, Dishington IW and Helgebostad A (1971). Calcium balance studies in dairy cows under experimental induction and prevention of hypocalceamic paresis puerperalis. Zeitschrift Fur Tierphysiologie, Tierernanhrung and Futtermittelkunde 28, 233-256.

Horst RL, Goff JP, Reinhardt TA and Buxton DR (1997). Strategies for preventing milk fever in cattle. Journal of Dairy Science 80, 1269-1280.

Isbell RF (1996). “The Australian soil classification”. CSIRO Publishing, Melbourne.

Pain BF, Smith KA and Dyer CJ (1986). Factors affecting the response of cut grass to the nitrogen content of dairy cow slurry. Agricultural Wastes 17, 189-202.

Schroder JJ, Jansen AG and Hilhorst GJ (2005). Long-term nitrogen supply from cattle slurry. Soil Use and Management 21, 196-204.

Smith KA, Jackson DR, Unwin RJ, Bailey G and Hodgson D (1996). Negative effects of winter and spring applied cattle slurry on the yield of herbage at simulated early grazing and first cut silage. Grass and Forage Science 50, 124-121.

Unwin RJ, Pain BF and Whinham WN (1986). The effect of rate and time of application of nitrogen in cow slurry on grass cut for silage. Agricultural Wastes 15, 253-286.

Ward GN and Jacobs JL (2007). Effect of second pond dairy effluent applications on pasture productivity, herbage nutritive value and mineral content. Proceedings of the Australasian Dairy Science Symposium 2007, Melbourne. National Dairy Alliance. 440-448.

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