Method 82 Determination of moisture in silage

See Chapter 4, 'Water content in silage'. The subjection of silage to oven drying causes loss of volatile components as well as water, resulting in overestimated moisture content. One way of minimizing this effect is to distil the fresh silage in the presence of toluene. Titration of the acidic distillate with 0.1 M NaOH enables a correction to be made for the volume of the volatile acids. A suitable method is given in MAFF/ADAS (1986, pp. 85-87) where about 70 g silage plus 400 ml toluene is heated in a 1-l round-bottomed flask, the distillate collecting in a specially made Dean and Stark receiver. The modified procedure is described below and in Faithfull (1998); it uses only 10 g samples, which are distilled with 100 ml toluene from a 250-ml flask. The aqueous distillate is collected in a standard Quickfit® Dean and Stark receiver fitted with a Rotaflo® stopcock permitting easy release of the water before titration. The apparatus is shown in Fig. 8.1, and captions refer to Quickfit® Part Nos.

Reagents.

• Phenolphthalein indicator, 0.1% (m/v) - dissolve 0.1 g phenolphthalein in

100 ml 95% ethanol.

Procedure.

distillation. Weigh 10.0 g fresh or frozen silage into a beaker. Immediately replace the rest of the sample into the deep-freeze to prevent loss of volatiles. Transfer to a 250-ml round-bottomed flask, supported on a cork or rubber ring. Add 100 ml toluene, place in a heating mantle of the correct size, and connect to the 10-ml Dean and Stark pattern receiver/condenser assembly. Turn the energy controller on full to bring to boiling, then adjust to give a steady boil. After 20 min, and then after 5-min intervals, record the volume of aqueous phase in the receiver until two identical values are obtained. Dislodge any droplets adhering to walls with a glass rod or gentle rocking. Switch off the heater and allow the receiver to cool before recording the final volume. Put a black card behind the receiver to show the meniscus clearly. Run off most of the aqueous phase into a 50-ml beaker. Pipette 5 ml of the aqueous phase into a 50-ml conical flask for titration to determine the acid content.

titration. Add 20 ml ethanol to the 5 ml aqueous phase and 5 drops phe-nolphthalein indicator. Pipette into the flask 10 ml 0.05 M sodium hydroxide and titrate with the same solution from a 10-ml burette after noting the initial volume. Note: Read the burette to 0.02 ml accuracy. Titrate to the first permanent pink colour; this fades due to CO2 from the air, so don't delay. Add the 10 ml initially pipetted into the flask to the volume delivered from the burette to obtain the total titre (probably in the range 13-18 ml).

Calculation. The dry matter (g kg-1) is calculated according to the formula:

DM = dry matter g kg-1 m = mass of silage sample (g)

V = volume of aqueous distillate f = factor (0.00555)

t = titre 998 = acid:water density correction

Microsoft® Excel Program. It is usual to use PC software such as Microsoft Excel to perform repetitive calculations such as the above. This enables a tabular printout and the generation of charts. The layout of the data sheet is shown in Fig. 8.2. The first four columns of data are entered directly. The fifth column is a correction of the titration using the actual strength of the sodium hydroxide used compared to what the result would have been if the concentration had been exactly 0.05 M. Thus, the equation to be entered in the cell to convert the previous column's value is in our case:

where G2 is the previous column's cell, 0.048 is the molarity of the acid used, and 0.05 is the molarity to which the titre is being corrected.

Sample No. Pit 1/

Mass of Silage(M)

Volume of Water (V)

Vol. ~.05M NaOH

Vol. .05M NaOH (T)

DM of Silage g/kg-

Moisture in Silage %

1

10.00

7.70

21.30

20.45

240.26

75.97

2

10.00

7.25

19.02

18.26

283.78

71.62

3

10.00

7.30

19.48

18.70

279.02

72.10

4

10.00

7.25

19.76

18.97

284.07

71.59

5

10.00

7.25

22.50

21.60

285.12

71.49

6

10.00

7.40

17.20

16.51

268.25

73.18

7

10.00

7.70

18.70

17.95

239.20

76.08

8

10.00

7.50

14.26

13.69

257.19

74.28

9

10.00

7.60

14.70

14.11

247.46

75.25

10

10.00

7.35

16.64

15.97

272.97

72.70

11

10.00

7.20

15.68

15.05

287.44

71.26

12

10.00

7.60

18.68

17.93

249.07

75.09

13

10.00

7.90

17.66

16.95

219.00

78.10

14

10.00

7.70

15.66

15.03

237.95

76.20

15

10.00

7.50

16.92

16.24

258.25

74.18

16

10.00

7.15

17.94

17.22

293.25

70.67

17

10.00

7.50

15.16

14.55

257.55

74.25

18

10.00

7.45

14.64

14.05

262.29

73.77

19

10.00

7.65

14.72

14.13

242.52

75.75

20

10.00

7.80

16.38

15.72

228.35

77.16

21

10.00

7.45

15.20

14.59

262.51

73.75

22

10.00

7.75

16.08

15.44

233.18

76.68

23

10.00

7.60

13.54

13.00

246.99

75.30

24

10.00

7.30

15.00

14.40

277.28

72.27

Penglais

First Cut,

First Sam

pling

DM (dry matter g/kg)

38 cm

76 cm

114 cm

152 cm

190 cm

228 cm

Sampling

1

240.26

283.78

279.02

284.07

285.12

268.25

Position

2

239.20

257.19

247.46

272.97

287.44

249.07

3

219.00

237.95

258.25

293.25

257.55

262.79

4

242.52

228.35

262.51

233.18

246.99

277.28

Moisture%

38 cm

76 cm

114 cm

152 cm

190 cm

228 cm

Sampling

1

75.97

71.62

72.10

71.59

71.49

73.18

Position

2

76.08

74.28

75.25

72.70

71.26

75.09

3

78.10

76.20

74.18

70.67

74.25

73.77

4

75.75

77.16

73.75

76.68

75.30

72.27

305.90

299.26

295.28

291.64

292.30

294.31

38 cm

76 cm

114 cm

152 cm

190 cm

228 cm

Av. Moisture%

76.48

74.82

73.82

72.91

73.08

73.58

Fig. 8.2. Microsoft® Excel data sheet for moisture in silage from the Penglais third-cut, first sampling.

The next column converts the various data to dry matter of silage g kg-1. The formula to be entered in the first cell of this column is:

where E2 is the cell referring to the volume of aqueous distillate and I2 is the corrected volume of 0.05 M sodium hydroxide.

There are also other tables on the data sheet that allow the easier production of charts. They refer to DM and moisture at each depth and sampling position, also the average moisture for each depth.

Interpretation. The interpretation of the effects of moisture in silage is notoriously difficult. Moisture is essential for the proliferation of desirable microorganisms, but an excess will encourage the growth of undesirable types (Woolford, 1984). The DM range most favourable to the silage fermentation has been suggested as 200-250 g kg-1 and the optimum about 240 g kg-1.

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