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Effect of different rates of dairy effluent on turnip DM yields and nutritive characteristics

Joe Jacobs and Graeme Ward

Natural Resources and Environment, 78 Henna Street, Warrnambool, Vic 3280
joe.jacobs@nre.vic.gov.au

Abstract

A study determined the effect of rate of dairy effluent application on soil nutrient status, dry matter (DM) yield, nutritive characteristics and mineral concentrations of turnips. Rates of effluent applied were 0, 16, 23 and 33 mm/ha. Analysis of the effluent applied indicated high levels of potassium (426.7 kg/ML) and sodium (551.4 kg/ML) and moderate levels of nitrogen (122.1 kg/ML). Application of effluent at 23 and 33 mm/ha resulted in higher (P<0.001) leaf, root and total DM yields than where effluent was not applied. Crude protein concentrations in both leaf and root were increased (P<0.001) by effluent application. Magnesium content of turnip leaves was reduced (P<0.05) at higher effluent application rates, whilst both potassium and sulphur content of roots increased (P<0.01).

Results from this study indicate that there is potential to use dairy effluent to increase forage crop DM yield during summer in dryland areas of southern Australia. However, further research is required to determine longer term sustainable practices for the use of effluent in terms of achieving a balance between production and environmental objectives.

Key Words

Nitrogen, potassium, sodium, forage crops

Introduction

It is estimated that only 50% of dairy farms in the dryland regions of Victoria have suitable dairy effluent systems and of these only 25% are managed effectively (1). With such a low proportion of dairy farms with effective systems, there is an urgent need to improve effluent management on farms if future environmental standards are to be met. In Victoria, the State Environment Protection Policy (SEPP) states that all dairy waste must be retained within the boundary of the property. To achieve this objective dairy farmers will require clear guidelines on the most appropriate and sustainable systems and practices to use dairy effluent. There is now considerable information available on dairy effluent collection systems. However, little information exists on the most sustainable ways to use dairy effluent on farm.

This paper reports a study comparing turnip forage crop DM responses and changes in nutritive characteristics and mineral content to a range of dairy effluent application rates.

Methods

This study was conducted on a commercial dairy farm near Terang (3814’S, 14255’E) in western Victoria on a basalt derived fine sandy clay loam soil. A paddock was sprayed with a knockdown herbicide (Roundup CT (3 L/ha)) on 12 December. On 20 December the paddock was chisel ploughed and 4 days later (24 December) power harrowed and sown to turnip (Brassica rapa cv Barabas) forage crop at a rate of 2 kg/ha and rolled. Fertiliser was applied at sowing as Triple Super at a rate of 20.2 kg phosphorus (P)/ha and 1.0 kg sulphur (S)/ha. Within the paddock four plots (50m x 22m) were randomly allocated to one of four treatments (0, 16, 23, 33 mm effluent/ha). Prior to effluent application soil tests (0-10 cm) of the experimental area were undertaken (8 February), showing soil pH (H20) 6.23, Olsen P 25.5 mg/kg, available potassium (K) 192 mg/kg and CPC S 28.4 mg/kg. Effluent application commenced on 21 February and was completed by 25 February with 2 pooled effluent samples collected on each day for subsequent analyses. Effluent was applied via a ‘Vaughn’ travelling effluent spray irrigator with both the 16 and 23 mm undertaken in one pass whilst the 33 mm was applied on two separate days.

Final harvest measurements were undertaken on 2 April. Ten randomly selected quadrats (1.0 m2) were collected per plot, plants separated into leaves and roots, weighed individually and sub sampled into five samples per plot. These subsamples were further divided with one portion being used to determine DM yield by drying at 100C for 24 h. The remaining sample was dried at 60C for 72 h, ground through a 1mm screen (Tecator Cyclotec 1093 sample mill) and used to determine nutritive characteristics and mineral content. Analysis of leaf and root samples for nutritive characteristics was undertaken at FEEDTEST, Agriculture Victoria, Pastoral and Veterinary Institute, Hamilton using near infrared spectroscopy. Metabolisable energy (ME) (MJ/kg DM) values were calculated from predicted DM digestibility values (2). Mineral analysis of leaf and root was by a microwave digestion (3,4) followed by Inductively Coupled Plasma - Optical Emission Spectroscopy (5). Statistical analyses for DM yields, nutritive characteristics and mineral content was undertaken using a one way analyses of variance (6). Effluent analyses are presented as means with standard deviation.

Results

The initial soil tests prior to effluent application indicated that P, K and S should not be limiting for forage growth. The composition of effluent is presented in Table 1 and indicates that the effluent had a high content of both K and sodium (Na).

Table 1. pH (H20), electrical conductivity (EC) (dS/m), sodium adsorption ratio (SAR), phosphorus (P), potassium (K), sulphur (S), nitrogen (N), calcium (Ca), magnesium (Mg), sodium (Na) (mg/L) of effluent

 

pH

EC

SAR

P

K

S

N

Ca

Mg

Na

Mean

7.9

4.3

6.4

34.6

426.7

14.1

122.1

227.4

207.2

551.4

s.d

0.07

0.23

0.31

1.54

72.51

2.40

27.37

7.63

32.51

10.47

Application of effluent led to an increase (P<0.001) in leaf and root DM yield at application rates of 23 and 33 mm/ha (Figure 1), whilst total DM yield was increased (P<0.001) at all levels of effluent application. Leaf to root ratios were lower (P<0.01) for the intermediate effluent application rates (16, 23 mm/ha).

Figure 1. Effect of effluent application on turnip leaf, root and total dry matter yield (t DM/ha)

Crude protein (CP) content of both leaf and root were increased (P<0.001) by effluent application irrespective of application rate (Table 2). There was no effect of effluent application on ME, neutral detergent fibre (NDF) content of leaf and roots or upon water soluble carbohydrate (WSC) of the leaf component. Starch content of roots was decreased (P<0.01) when effluent was applied.

Table 2. Metabolisable energy (ME) (MJ/kg DM), crude protein (CP), neutral detergent fibre (NDF), water soluble carbohydrate (WSC) and starch (%DM) content of turnip leaf and roots receiving different rates of effluent

 

Leaf

Root

mm

ME

CP

NDF

WSC

ME

CP

NDF

Starch

0

12.9a

13.6a

27.1a

16.9a

14.0a

9.2a

20.4a

21.4a

16

12.1a

20.4c

28.4a

8.8a

14.0a

14.0b

22.0a

13.3b

23

12.4a

17.9b

27.5a

13.3a

14.1a

12.8b

21.9a

15.2b

33

12.4a

18.5bc

28.1a

12.1a

14.0a

13.1b

21.6a

14.7b

l.s.d.

0.73

2.23

3.00

5.11

0.14

1.78

1.26

2.48

Different superscripts denotes significant difference. (l.s.d. P=0.05)

Magnesium content of turnip leaves was reduced (P<0.05) at higher effluent application rates, whilst both potassium and sulphur content of roots increased (P<0.01) (Table 3) irrespective of level of application.

Table 3. Phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na) and sulphur (S) content (%DM) content of turnip leaf and roots receiving different rates of effluent

 

Leaf

Root

mm

P

K

Ca

Mg

Na

S

P

K

Ca

Mg

Na

S

0

0.22a

5.32a

2.18a

0.67a

0.70a

0.44a

0.27a

3.30a

0.45a

0.17a

0.82a

0.36a

16

0.26a

6.18a

2.38a

0.71a

1.18b

0.56b

0.30a

3.84b

0.40a

0.16a

1.14a

0.44b

23

0.23a

5.56a

2.06a

0.57b

0.78a

0.49ab

0.30a

3.94b

0.45a

0.16a

0.89a

0.45b

33

0.26a

6.84a

1.98a

0.47c

0.80a

0.49ab

0.39b

4.18b

0.47a

0.16a

1.00a

0.42b

l.s.d.

0.027

1.302

0.324

0.085

0.196

0.075

0.056

0.480

0.091

0.025

0.309

0.039

Different superscripts denotes significant difference by analysis of variance. (l.s.d. P=0.05)

Discussion and Conclusion

A recent survey (unpublished) in south-west Victoria found concentrations of N in dairy effluent ranging from 5-777 mg/l, for P, 1-193 mg/l, and K 44-1180 mg/l. The concentrations observed in the present study also fell within these ranges. Studies in New Zealand (7,8) have compared the effect of different rates of effluent on pasture DM production and shown considerable DM responses to various application rates. To the authors knowledge there are no other studies that have investigated the response of a brassica forage crops to effluent. The responses observed in this study are likely to be the result of a combination of factors including both nutrient (N, P, K, S) and water responses. Although not presented, soil moisture content (27% volumetric) prior to effluent application was close to the refill point for this soil type, indicating a moisture deficit. Previous information has indicated that only ca 50% of N in effluent is in a readily form (9), however, given both the DM yield response and increase in CP content where effluent was applied in the present study it is proposed that the majority of N applied was readily available for plant uptake. Increases in forage CP during the summer period in dryland areas of Victoria are of particular importance. At this time of the year, available pasture often has a CP content of <14%, and conserved forages <15% CP and cereal grains with 12% CP, leading to a ration that is potentially deficient in CP for lactating dairy cows in mid lactation. This lack of CP in the diet is often overcome by purchasing more costly protein feeds such as lupins or pellets. The use of dairy effluent at moderate levels may alleviate the need to purchase such feeds. Although application of effluent had some impact on the mineral content of turnips, the values found were generally within ranges quoted elsewhere (10). Leaf K content tended to be higher than other quoted values (10) and some consideration should be given if such a feed is to be used for dry cows close to parturition due to likely metabolic problems such as hypocalcaemia.

Using current fertiliser prices for P, K, S and N, the nutrient value of the effluent used was approximately $560/ ML. Using the 23 mm application rate as an example, the additional feed grown was 4.1 t DM/ha, with a higher CP and an ME similar to cereal grain. Assuming that only 80% of the crop is utilised and a current grain price of $250t, the equivalent value of the extra feed is $3565/ML. Including the fertiliser value the total potential value equates to $4125/ML. Given the average Victorian dryland dairy farm produces about 3ML of effluent per annum, these results indicate that effluent can provide a fertiliser and feed resource worth about $12,000 per annum when used solely for brassica crops. Other alternatives for effluent use may be application following silage harvests to soils where high levels of K in effluent can be used to replace the high removal incurred with forage conservation.

The concentration of Na and the SAR of the effluent used in this study were higher than the averages found in the survey (Na 349 mg/l; SAR 5.1). A combination of high Na levels and a high SAR can lead to increases in the exchangeable sodium percentage (ESP) within the soil and as a consequence the risk of deterioration of soil physical properties is increased (11). Bond (12) comments that with increases in ESP comes the risk of dispersion of clay with subsequent breakdown of soil structure, blocking of soil pores and a decrease in soil permeability, which in turn may lead to waterlogging, impaired plant performance, increased run off, decreased leaching and salinisation. Although the study precluded any soil measurements, if effluent is to be used as a means to increase forage, production systems need to be developed to achieve this goal in a sustainable manner.

In conclusion, dairy effluent has the potential to increase turnip DM yields during the summer period where feed is often limiting on dryland farms in southern Victoria. However, further work is required to determine long term sustainable practices for the use of effluent in terms of achieving a balance between production and environmental implications.

References

(1) IRIS Research (2000). A survey of Natural Resource Management on Australian Dairy Farms. Technical report.

(2) SCA (1990) Feeding standards for Australian Livestock. Ruminants. CSIRO Publications, Melbourne, Australia.

(3) Lautenenschlaeger W (1989). Spectroscopy 4(9), 16-21.

(4) Nackashima S, Sturgeon E, Willie SN, Berman SS (1988). Analyst. 113 159-163.

(5) State Chemistry Laboratory (1987) Method 013. DNRE, Werribee

(6) Genstat 5 Committee (1997) 'Genstat 5.41 Reference Manual'. Oxford Science Publications, Oxford, UK.

(7) Goold GJ. (1980). New Zealand. J. Exp. Agric. 8, 93-99.

(8) Roberts AHC, O'Connor MB and Lonhurst RD (1992). Occasional Report No.6, Fertliser and Lime Research Centre. Massey University, Palmerston North, NZ. pp44-55.

(9) Crocos, A. and Wrigley, R. (1993). Dairy Waste Management. VCAH, Warragul

(10) Jacobs, J.L. and Rigby, S.E. (1999). Minerals in Dairy Pastures in Victoria. DNRE, Warrnambool.

(11) Balks MR, Bond WJ and Smith CJ (1998). Aust. J. Soil. Research. 36, 821-30.

(12) Bond WJ (1998). Aust. J. Soil. Research. 36, 543-55.

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