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The influence of moisture content and chop length of forage maize on silage bulk density and the pressure on bunker silo walls
J. ugric. Engng Res. (1977) 22, 175-182
The Influence
of Moisture
Content
and Chop Length
of Forage Maize on Silage Bulk Density on Bunker
and the Pressure
Silo Walls
H. J. M. MESSER*;J. C. HAWKINS* The relationship between moisture content, mean bulk density, wall pressures, chop length and effluent production were determined in experimental silos holding about 10 tonnes of forage maize. The results were comparable with those already determined for grass silage under similar conditions in the same silos. 1.
Introduction
With the breeding of new varieties more suitable for northern Europe, the area of forage maize grown in southern Britain has increased from 990 ha in 1970 to 29 000 ha in 1976.’ Although information is available on the bulk densities and wall pressures of silage made from maize grown in north America, there is no such information for Britain. Accordingly, experiments were undertaken to determine the effect of the moisture content and chop length of forage maize on the bulk density of the resulting silage and on the horizontal pressure that it produced on the walls of experimental silos. 2. 2.1.
Apparatus and procedure The experimental silos
For results with experimental silos to be meaningful, it is important to establish a minimum size in which pressures measured on the centre line of one wall are not affected by friction on the two adjacent ones. Preliminary experiments were, therefore, carried out with 2.44 m high silos 0.61. l-22 and 1.83 m square. They were fitted with pressure panels both on the centre line of two sides and in the floor at different distances in from the edge. The results established that a 183 m square silo was the smallest that could safely be used and that pressure panels should be sited about 450 mm above the floor to avoid the effects of floor friction on their readings.* It was considered impracticable to produce more than about 10 tonnes of silage, equivalent to 20 m3 of fresh maize, to any pre-determined specification for any one treatment in the experiment. Therefore, six silos each 25 m square and 3.05 m high were built in two rows back-to-back from 230 mm thick brickwork reinforced in every third course with wire mesh. The walls were rendered with cement mortar and lined with 0.06 mm thick polythene film. The floors were of concrete with a slight fall from the back to the front of the silos, which had removable roofs made from a waterproof canvas sheet supported on a timber frame. The front walls were built of 25 mm thick tongued and grooved boarding, carried on a steel angle frame. Each one was removable and held on to the brickwork by 6 tie rods passing from the front to the back of the silos through the brickwork of the dividing walls. 2.2. Pressure measurement To minimize errors resulting from movement of the pressure sensing device, special apparatus was designed and has been described in detail elsewhere. 3 Briefly, two sensing panels, each 610 mm square, were fitted into the front wall of each silo with their centres 720 and 1330 mm above the *Farm Buildings Division, National Institute of Agricultural Engineering, Wrest Park, Silsoe, Bedford Received 4 October 1976; accepted in revised form 18 January 1977
175
176
MOISTURE
CONTENT
AND
CHOP
LENGTH
floor. The panels were suspended vertically from a rigid steel frame with a brass contact in each corner of their outside face supported by four similar contacts on the frame. To measure the load on the panels, a portable load measuring device, which was calibrated monthly, was attached to the frame, first near the top and then near the bottom edge of each panel. At each position, two projections on the device were screwed forward until the panel was moved forward sufficiently to separate the brass contacts so breaking a circuit including a light and extinguishing it. During this process, the panel had been moved forward by about 0.05 mm and the load on half of it transferred by a series of levers to a load cell. The loads measured in this way at the top and bottom edges were summed to give the total load on the panel. The pressure exerted on the panels was measured as soon as each silo was filled and thereafter every second day for about one month. The maximum pressure of the settled silage developed about day 18. 2.3. Efluent production The floor of each silo was covered with a plastic tray to collect the effluent produced during ensiling. Each tray extended 200 mm up the walls and had 50x 10 mm slats on the floor spaced 50 mm apart and running from the back to the front. A 30 mm bore tube at the front of each tray drained the effluent into a separate tank set in the ground and the amount of effluent contained was measured periodically. 2.4. Filling the silos In 1972 and 1973, the same variety of forage maize (K59A) was harvested into trailers with a metered chop forage harvester at the required moisture contents, which were determined in the field by a modified Brown Duval method. 4 The trailers and their contents were weighed and the contents were tipped into a dump box which fed a forage blower delivering into the centre of each silo. The maize was spread by hand by a man who weighed about 70 kg and provided a standard level of compaction by walking across the surface at 600 mm intervals of depth along parallel lines 300 mm apart, stepping every 300 mm. Each silo was filled until the maize was level with the top of the walls and covered with a butyl rubber sheet. The height of the contents as they settled was calculated from the measured distance between the surface of the silage and a point of fixed height above the floor. Some of the properties of the crops are given in Table I. 2.5. The density of the forage The density of the fresh maize when it was ensiled on day 0 and of the silage on day 18 was calculated by dividing the total weight ensiled by the volume it occupied. In 1972 effluent losses TABLE I Properties of forage maize at harvest
Silo A
Silo B
_22 September
Date of filling, 1972 Moisture content (% w.b.)
M.A.D. fibre (%)
Nominal chop length (mm) Date of filling, 1973 Moisture content (% w.b.) M.A.D. fibre (%) Nominal chop length (mm) N.R. = Not recorded
31 October
Silo D
80.3 23.5
79.3 26.1
76.0 21.7
8
64
6.4
6.4
79.9 24.3 6
5 September
1 October
79.0 25.5
70.8 24.4
32
32
Silo E
Silo F
22 November
82.7 226
5 September
-
18 October
Silo C -___
1 October 71.3 22.0 6
30 October 62.4 N.R. 32
60.0 N.R.
-
6
H.
J.
M. MESSER;
J.
C.
177
HAWKINS
were ignored but in 1973 the weight of effluent produced by day 18, although very small in comparison with the weight ensiled, was deducted from that weight. 3. Results 3.1. Density On day 18, at all chop lengths, the mean bulk density of the silage in the experimental silos (Table II) at 576 kg me3 was lower than that measured in 12 commercial silos at 692 kg m- 3 (range 566 to 843, standard deviation&72).5 The two ranges, overlapped sufficiently, however, to make it likely that the experimental results would apply in practice. In all the silos, the mean bulk density of both the fresh maize and the resulting silage increased with increasing moisture content (Fig. 1).
100
200 Molsture
300
400
500
content (d.b.)(%)
Fig. I. Moisture content (d.b.) and mean bulk density TABLE II
Silage moisture content, density and silo wall pressures Moisture content, ‘?; (d.6.)
Day 0
Day 18
154 166 242 248 317 376 383 398 409 417
303 300 413 426 418 470 497 590 554 537
349 354 490 501 550 592 698 686 769 772
Mean bulk density, kgm” s Dry matter __.~ 138 133 143 144 132 124 145 138 152 134
Pressure/unit depth, kN m-2/m 1.0 1.1 1.4 1.5 1.6 I.9 1.9 1.9 1.9 2.1
Silo year
-
F E c D D B C A B A
73 73 73 73 72 73 72 73 72 72
178
MOISTURE CONTENT
AND CHOP LENGTH
The relationships for both fresh maize and the silage were linear and may be expressed by: d, = 0.83 m.c. +186*6 (r = 0.9254) . ..(l) L& = l-38 m.c.+-139-l (r = 0.9725), . ..(2) where d, = mean bulk density on the day of filling (day 0), kg rnm3, & = mean bulk density of silage on day 18, kg me3, and m.c. = moisture content (dry basis), “/,. Compared with grass harvested and ensiled in the same way in the same silos,6 the bulk density of maize was higher on day 0 and lower on day 18 (Table III). Further, as also shown in that table, TABLE 111 Mean bulk density of grass and maize
Moisture content, % (d.b.)
Mean bulk density, ___---
---__
kg m- 3 ~__
Day 0 _______ Maize
Grass
Day 18 ~~ ___~
100 200 300 400 500
194 292 355 395 420
t 16 : 61 t 81
210 353 436 519 601
-____
Grass
Diff.
I- 124 i-180
Maize
Difl.
__259 426 593 760 92-l
277 415 553 691 829
t-18 ~11 --40 69 ~~98
the increase in silage bulk density with moisture content was less with maize than with grass. Both of these results suggest that the physical structure and shape of particles of the maize were changed less by the ensiling process. The calculated mean dry matter bulk density on day 18 was 138 kg m- 3 (range 124 to 145, standard deviation h7.8) at moisture contents between 154 and 477 % (d.b.) and at the level of consolidation used in the experimental silos. This value was also less by about 2 kg rnm3, than that for grass ensiled in the same silos in the same way.6 In 1973, pairs of silos were filled at the same time with maize harvested at the two different nominal chop lengths of 6 and 32 mm. Except at the highest moisture contents, where the shorter material produced a higher mean bulk density, the size of the particles had very little effect on densities at days 0 and 18 (Table IV). 3.2. Wall pressures As with grass, the pressure on the silo walls rose during the first 10 days after filling and the silage settled more rapidly than later on. The pressure reached a plateau value between about the TABLE IV The effects of chop length on density and pressure
Silo
d’;&,
Nominal chop length, mm
__~ B A C
D E
F
376 398 242 248 166 154
32 6 32 6 32 6
Mean bulk density, kg mm3 __-__Fresh maize (day 0)
Silage (day 18)
410 590 413 426 300 303
592 686 490 501 354 349
Pressure/ unit height day 18, kNmw2/m
_ ____~ 1.9 1.9 1.4 1.5 1.1 1.0
H.
J.
M.
MESSER:
J.
C.
HAWKINS
179
15th and 25th days after filling and so, as the daily fluctuations in readings were very small, the silage pressures given in Tables II and IV are those recorded nearest to the 18th day. When grass was ensiled in the experimental silos a linear relationship between depth of silage and horizontal pressure was established from the two measurements of horizontal force together with zero pressure at the silage surface .6 This relationship was confirmed by similar calculations for the maize ensiled in the same silos (Table V and Fig. 2). TABLE V
Horizontal pressure/depth relationshipday
-
Silo no.
18
-
I
Horizontal pressrrre. kN 111~ 2
_~_
Panel I
Panel 2
I
Lineal correlatbn coefficient
____~
1972 A B C D
1.59 1.84 1.70 I.60
2.97 2.81 2.72 2.53
0.9994 0.9916 0.9984 0.9998
1913 A B C D E F
2.01 2.21 I.93 I.66 1.51 I.11
3.19 2.90 2.63 2.81 1.93 1.97
0.9999 0.9847 0.9974 0.9959 0.9920 0.9911
worst hnear correlation (siloF, 1973) -Best lmeor correlot1on (~110 A, 1973) -----
centre of top panel \ ‘\
\
\ ‘\.Centre
Horizontal
of bottom panel
pressure (kN m-z)
Fig. 2. Horizontal pressure versus
depth relationship
Calculations using the data from day 18 established that the average pressure acted 35 mm (standard deviation 533) below the centre of the panels and that a small change in the point at which the average pressure acted had little effect. For example, the best linear correlation (Silo A, 1973, Table V) was not changed and the worst (Silo F, 1973, Table V) was slightly improved to 0.9931, whilst the resulting changes in pressure per unit depth for these two silos were less than 2%. As the errors involved were so small, the centre of the panels was used to calculate both the horizontal pressure/depth relationship and the pressure per unit depth (Tables II and IV). Further since the length of the particles of the ensiled maize had very little effect on the horizontal pressure (Table IV) these results have been included with the others to establish the relationship between the horizontal pressure and depth. Again as with grass, the horizontal pressure of maize silage on the walls of the silos increased with increasing-mean bulk density and with increasing moisture content (d.b.) and the rate of
180
MOISTURE
CONTENT
AND
CHOP
LtNGl-H
increase in pressure was lower at the higher values of bulk density and moisture content (Figs 3 and 4). The line of best fit for the bulk density/pressure relationship must pass through the origin and the line for the moisture content/pressure relationship must have a similar shape because bulk density and moisture content are linearly related (Fig. I). Equations expressing these relationships are : p
_
4.25
p
=:
2.83__2.7&-0.003
(1 -e-O.0009
P = pressure
where
dL8).
. ..(3)
m.c,
. ..(4)
per unit depth, kN mm2/m.
1
100
I
I
200
x)0
I
4m
Mean bulk densty,
I
I
I
I
500
600
700
0oC
day I8 (kg mm31
Fig. 3. Mean bulk density and horizontal
Moisture
Fig. 4. Moisture
pressure
content (db.)(%)
content (d.6.) and horizontal
pressure
H.
J.
M.
MESSER;
J.
C.
181
HAWKINS
As the mean bulk density of maize silage was lower than that of grass ensiled at the same moisture content (Table III) it is not surprising that the horizontal pressure of maize was lower than that of grass both at the same mean bulk densities and at the same moisture contents (Table VI). 3.3. Efluent As is usual, the quantity of effluent increased with increasing moisture content, the amount being relatively small below 70% moisture content (w.b.) and most of it was produced within 18 days of ensiling. At similar moisture contents, the amount of effluent produced by long chopped silage (silos B and C) was less than for short chopped (silos A and D) the difference being greater *. with the wetter material (Table VII). TABLE VI The horizontal pressure of maize and grass
Density, kg m-S
filrage _-__Maize
-- _____-
kN m-“/m
--
Diflerence
m.c. (d.b.) 0, /0 ____
Pressure/unit depth kN me2/m
1.3 0.6
2.0
1.8
1.9
600
Grass
0.7
350
2.2
2.2 800
0.8 3.0
-
0.8 2.7
2.5
Maize
0.7
200
1.9
Maize
Diflerence
1.3
400
Grass
Grass
Pressure/unit depth
I.0
500 3.2
TABLE VII Production of effluent
EfPuent production m.c. %
Silo (d.b.)
(w.b.)
Total, litres
79.9 79.0 70.8 71.3 62.4 60.6
558 216 60 54 0 0
~A B C D E
F
398 376 242 248 166 154
Days O-18, % _____ 55 71 LOO 100 0 0
Litresltonne fresh maize 52 25 7 7 0 0
4. Conclusions 1. At any given moisture content, the mean bulk density of forage maize in experimental silos was greater than that of grass when it was ensiled. 2. Maize resisted compaction more than grass so that the mean bulk density of the resulting silage was lower than that of grass. 3. As with grass, the horizontal pressure of forage maize on silo walls increased with increasing mean bulk density and moisture content although the rate of increase became less at the higher values.
182
MOISTURE
CONTENT
AND
CHOP
LENGTH
4. The horizontal pressure of maize on silo walls was lower than that of grass with the same mean bulk density and moisture content. 5. Nominal chop lengths between 6 and 32 mm had no significant effect on wall pressures and increased density substantially only at the highest moisture content. 6. Effluent production from maize silage decreased with decreasing moisture content and with decreasing chop length, ceasing between 71 and 62% moisture content (w.b.).
Acknowledgements The experimental silos were built at the Grassland Research Institute, Hurley which supplied the maize. The co-operation of the Director and the assistance of his farm staff are gratefully acknowledged. REFERENCES
Agricultural Statistics. London: HMSO, 1976 2 Chaplin, R. V. Design and construction of bunker-type silos. Preliminary studies of the use of model bunker silos for lateralpressure investigation. DNjl l/FBD. Nat. Inst. Agric. Engng, Silsoe (unpubl.), ’
M.A.F.F.
1967 3 Chaplin, R. V. An improved method for the measurement of wall pressures in silos for forage crops. J. agric. Engng Res., 1976 21 9-14 4 Hughes, M.; Gale, G. E. A comparison of the infra-red and Brown Duval oil distillation methods of moisture determination. DN/S/HCD. Nat. Inst. Agric. Engng, Silsoe (unpubl.), 1966 5 Wheatcroft, W. P.; McLean, K.; Fairbairn, C. B. Maize silage study. Maize Bulletin, Jan. 1976 5-8 b Messer, H. J. M.; Hawkins, J. C. The influence of the properties of grass silage on bulk density and horizontalpressure. J. agric. Engng Res, 1977 22 55-64
The Influence
of Moisture
Content
and Chop Length
of Forage Maize on Silage Bulk Density on Bunker
and the Pressure
Silo Walls
H. J. M. MESSER*;J. C. HAWKINS* The relationship between moisture content, mean bulk density, wall pressures, chop length and effluent production were determined in experimental silos holding about 10 tonnes of forage maize. The results were comparable with those already determined for grass silage under similar conditions in the same silos. 1.
Introduction
With the breeding of new varieties more suitable for northern Europe, the area of forage maize grown in southern Britain has increased from 990 ha in 1970 to 29 000 ha in 1976.’ Although information is available on the bulk densities and wall pressures of silage made from maize grown in north America, there is no such information for Britain. Accordingly, experiments were undertaken to determine the effect of the moisture content and chop length of forage maize on the bulk density of the resulting silage and on the horizontal pressure that it produced on the walls of experimental silos. 2. 2.1.
Apparatus and procedure The experimental silos
For results with experimental silos to be meaningful, it is important to establish a minimum size in which pressures measured on the centre line of one wall are not affected by friction on the two adjacent ones. Preliminary experiments were, therefore, carried out with 2.44 m high silos 0.61. l-22 and 1.83 m square. They were fitted with pressure panels both on the centre line of two sides and in the floor at different distances in from the edge. The results established that a 183 m square silo was the smallest that could safely be used and that pressure panels should be sited about 450 mm above the floor to avoid the effects of floor friction on their readings.* It was considered impracticable to produce more than about 10 tonnes of silage, equivalent to 20 m3 of fresh maize, to any pre-determined specification for any one treatment in the experiment. Therefore, six silos each 25 m square and 3.05 m high were built in two rows back-to-back from 230 mm thick brickwork reinforced in every third course with wire mesh. The walls were rendered with cement mortar and lined with 0.06 mm thick polythene film. The floors were of concrete with a slight fall from the back to the front of the silos, which had removable roofs made from a waterproof canvas sheet supported on a timber frame. The front walls were built of 25 mm thick tongued and grooved boarding, carried on a steel angle frame. Each one was removable and held on to the brickwork by 6 tie rods passing from the front to the back of the silos through the brickwork of the dividing walls. 2.2. Pressure measurement To minimize errors resulting from movement of the pressure sensing device, special apparatus was designed and has been described in detail elsewhere. 3 Briefly, two sensing panels, each 610 mm square, were fitted into the front wall of each silo with their centres 720 and 1330 mm above the *Farm Buildings Division, National Institute of Agricultural Engineering, Wrest Park, Silsoe, Bedford Received 4 October 1976; accepted in revised form 18 January 1977
175
176
MOISTURE
CONTENT
AND
CHOP
LENGTH
floor. The panels were suspended vertically from a rigid steel frame with a brass contact in each corner of their outside face supported by four similar contacts on the frame. To measure the load on the panels, a portable load measuring device, which was calibrated monthly, was attached to the frame, first near the top and then near the bottom edge of each panel. At each position, two projections on the device were screwed forward until the panel was moved forward sufficiently to separate the brass contacts so breaking a circuit including a light and extinguishing it. During this process, the panel had been moved forward by about 0.05 mm and the load on half of it transferred by a series of levers to a load cell. The loads measured in this way at the top and bottom edges were summed to give the total load on the panel. The pressure exerted on the panels was measured as soon as each silo was filled and thereafter every second day for about one month. The maximum pressure of the settled silage developed about day 18. 2.3. Efluent production The floor of each silo was covered with a plastic tray to collect the effluent produced during ensiling. Each tray extended 200 mm up the walls and had 50x 10 mm slats on the floor spaced 50 mm apart and running from the back to the front. A 30 mm bore tube at the front of each tray drained the effluent into a separate tank set in the ground and the amount of effluent contained was measured periodically. 2.4. Filling the silos In 1972 and 1973, the same variety of forage maize (K59A) was harvested into trailers with a metered chop forage harvester at the required moisture contents, which were determined in the field by a modified Brown Duval method. 4 The trailers and their contents were weighed and the contents were tipped into a dump box which fed a forage blower delivering into the centre of each silo. The maize was spread by hand by a man who weighed about 70 kg and provided a standard level of compaction by walking across the surface at 600 mm intervals of depth along parallel lines 300 mm apart, stepping every 300 mm. Each silo was filled until the maize was level with the top of the walls and covered with a butyl rubber sheet. The height of the contents as they settled was calculated from the measured distance between the surface of the silage and a point of fixed height above the floor. Some of the properties of the crops are given in Table I. 2.5. The density of the forage The density of the fresh maize when it was ensiled on day 0 and of the silage on day 18 was calculated by dividing the total weight ensiled by the volume it occupied. In 1972 effluent losses TABLE I Properties of forage maize at harvest
Silo A
Silo B
_22 September
Date of filling, 1972 Moisture content (% w.b.)
M.A.D. fibre (%)
Nominal chop length (mm) Date of filling, 1973 Moisture content (% w.b.) M.A.D. fibre (%) Nominal chop length (mm) N.R. = Not recorded
31 October
Silo D
80.3 23.5
79.3 26.1
76.0 21.7
8
64
6.4
6.4
79.9 24.3 6
5 September
1 October
79.0 25.5
70.8 24.4
32
32
Silo E
Silo F
22 November
82.7 226
5 September
-
18 October
Silo C -___
1 October 71.3 22.0 6
30 October 62.4 N.R. 32
60.0 N.R.
-
6
H.
J.
M. MESSER;
J.
C.
177
HAWKINS
were ignored but in 1973 the weight of effluent produced by day 18, although very small in comparison with the weight ensiled, was deducted from that weight. 3. Results 3.1. Density On day 18, at all chop lengths, the mean bulk density of the silage in the experimental silos (Table II) at 576 kg me3 was lower than that measured in 12 commercial silos at 692 kg m- 3 (range 566 to 843, standard deviation&72).5 The two ranges, overlapped sufficiently, however, to make it likely that the experimental results would apply in practice. In all the silos, the mean bulk density of both the fresh maize and the resulting silage increased with increasing moisture content (Fig. 1).
100
200 Molsture
300
400
500
content (d.b.)(%)
Fig. I. Moisture content (d.b.) and mean bulk density TABLE II
Silage moisture content, density and silo wall pressures Moisture content, ‘?; (d.6.)
Day 0
Day 18
154 166 242 248 317 376 383 398 409 417
303 300 413 426 418 470 497 590 554 537
349 354 490 501 550 592 698 686 769 772
Mean bulk density, kgm” s Dry matter __.~ 138 133 143 144 132 124 145 138 152 134
Pressure/unit depth, kN m-2/m 1.0 1.1 1.4 1.5 1.6 I.9 1.9 1.9 1.9 2.1
Silo year
-
F E c D D B C A B A
73 73 73 73 72 73 72 73 72 72
178
MOISTURE CONTENT
AND CHOP LENGTH
The relationships for both fresh maize and the silage were linear and may be expressed by: d, = 0.83 m.c. +186*6 (r = 0.9254) . ..(l) L& = l-38 m.c.+-139-l (r = 0.9725), . ..(2) where d, = mean bulk density on the day of filling (day 0), kg rnm3, & = mean bulk density of silage on day 18, kg me3, and m.c. = moisture content (dry basis), “/,. Compared with grass harvested and ensiled in the same way in the same silos,6 the bulk density of maize was higher on day 0 and lower on day 18 (Table III). Further, as also shown in that table, TABLE 111 Mean bulk density of grass and maize
Moisture content, % (d.b.)
Mean bulk density, ___---
---__
kg m- 3 ~__
Day 0 _______ Maize
Grass
Day 18 ~~ ___~
100 200 300 400 500
194 292 355 395 420
t 16 : 61 t 81
210 353 436 519 601
-____
Grass
Diff.
I- 124 i-180
Maize
Difl.
__259 426 593 760 92-l
277 415 553 691 829
t-18 ~11 --40 69 ~~98
the increase in silage bulk density with moisture content was less with maize than with grass. Both of these results suggest that the physical structure and shape of particles of the maize were changed less by the ensiling process. The calculated mean dry matter bulk density on day 18 was 138 kg m- 3 (range 124 to 145, standard deviation h7.8) at moisture contents between 154 and 477 % (d.b.) and at the level of consolidation used in the experimental silos. This value was also less by about 2 kg rnm3, than that for grass ensiled in the same silos in the same way.6 In 1973, pairs of silos were filled at the same time with maize harvested at the two different nominal chop lengths of 6 and 32 mm. Except at the highest moisture contents, where the shorter material produced a higher mean bulk density, the size of the particles had very little effect on densities at days 0 and 18 (Table IV). 3.2. Wall pressures As with grass, the pressure on the silo walls rose during the first 10 days after filling and the silage settled more rapidly than later on. The pressure reached a plateau value between about the TABLE IV The effects of chop length on density and pressure
Silo
d’;&,
Nominal chop length, mm
__~ B A C
D E
F
376 398 242 248 166 154
32 6 32 6 32 6
Mean bulk density, kg mm3 __-__Fresh maize (day 0)
Silage (day 18)
410 590 413 426 300 303
592 686 490 501 354 349
Pressure/ unit height day 18, kNmw2/m
_ ____~ 1.9 1.9 1.4 1.5 1.1 1.0
H.
J.
M.
MESSER:
J.
C.
HAWKINS
179
15th and 25th days after filling and so, as the daily fluctuations in readings were very small, the silage pressures given in Tables II and IV are those recorded nearest to the 18th day. When grass was ensiled in the experimental silos a linear relationship between depth of silage and horizontal pressure was established from the two measurements of horizontal force together with zero pressure at the silage surface .6 This relationship was confirmed by similar calculations for the maize ensiled in the same silos (Table V and Fig. 2). TABLE V
Horizontal pressure/depth relationshipday
-
Silo no.
18
-
I
Horizontal pressrrre. kN 111~ 2
_~_
Panel I
Panel 2
I
Lineal correlatbn coefficient
____~
1972 A B C D
1.59 1.84 1.70 I.60
2.97 2.81 2.72 2.53
0.9994 0.9916 0.9984 0.9998
1913 A B C D E F
2.01 2.21 I.93 I.66 1.51 I.11
3.19 2.90 2.63 2.81 1.93 1.97
0.9999 0.9847 0.9974 0.9959 0.9920 0.9911
worst hnear correlation (siloF, 1973) -Best lmeor correlot1on (~110 A, 1973) -----
centre of top panel \ ‘\
\
\ ‘\.Centre
Horizontal
of bottom panel
pressure (kN m-z)
Fig. 2. Horizontal pressure versus
depth relationship
Calculations using the data from day 18 established that the average pressure acted 35 mm (standard deviation 533) below the centre of the panels and that a small change in the point at which the average pressure acted had little effect. For example, the best linear correlation (Silo A, 1973, Table V) was not changed and the worst (Silo F, 1973, Table V) was slightly improved to 0.9931, whilst the resulting changes in pressure per unit depth for these two silos were less than 2%. As the errors involved were so small, the centre of the panels was used to calculate both the horizontal pressure/depth relationship and the pressure per unit depth (Tables II and IV). Further since the length of the particles of the ensiled maize had very little effect on the horizontal pressure (Table IV) these results have been included with the others to establish the relationship between the horizontal pressure and depth. Again as with grass, the horizontal pressure of maize silage on the walls of the silos increased with increasing-mean bulk density and with increasing moisture content (d.b.) and the rate of
180
MOISTURE
CONTENT
AND
CHOP
LtNGl-H
increase in pressure was lower at the higher values of bulk density and moisture content (Figs 3 and 4). The line of best fit for the bulk density/pressure relationship must pass through the origin and the line for the moisture content/pressure relationship must have a similar shape because bulk density and moisture content are linearly related (Fig. I). Equations expressing these relationships are : p
_
4.25
p
=:
2.83__2.7&-0.003
(1 -e-O.0009
P = pressure
where
dL8).
. ..(3)
m.c,
. ..(4)
per unit depth, kN mm2/m.
1
100
I
I
200
x)0
I
4m
Mean bulk densty,
I
I
I
I
500
600
700
0oC
day I8 (kg mm31
Fig. 3. Mean bulk density and horizontal
Moisture
Fig. 4. Moisture
pressure
content (db.)(%)
content (d.6.) and horizontal
pressure
H.
J.
M.
MESSER;
J.
C.
181
HAWKINS
As the mean bulk density of maize silage was lower than that of grass ensiled at the same moisture content (Table III) it is not surprising that the horizontal pressure of maize was lower than that of grass both at the same mean bulk densities and at the same moisture contents (Table VI). 3.3. Efluent As is usual, the quantity of effluent increased with increasing moisture content, the amount being relatively small below 70% moisture content (w.b.) and most of it was produced within 18 days of ensiling. At similar moisture contents, the amount of effluent produced by long chopped silage (silos B and C) was less than for short chopped (silos A and D) the difference being greater *. with the wetter material (Table VII). TABLE VI The horizontal pressure of maize and grass
Density, kg m-S
filrage _-__Maize
-- _____-
kN m-“/m
--
Diflerence
m.c. (d.b.) 0, /0 ____
Pressure/unit depth kN me2/m
1.3 0.6
2.0
1.8
1.9
600
Grass
0.7
350
2.2
2.2 800
0.8 3.0
-
0.8 2.7
2.5
Maize
0.7
200
1.9
Maize
Diflerence
1.3
400
Grass
Grass
Pressure/unit depth
I.0
500 3.2
TABLE VII Production of effluent
EfPuent production m.c. %
Silo (d.b.)
(w.b.)
Total, litres
79.9 79.0 70.8 71.3 62.4 60.6
558 216 60 54 0 0
~A B C D E
F
398 376 242 248 166 154
Days O-18, % _____ 55 71 LOO 100 0 0
Litresltonne fresh maize 52 25 7 7 0 0
4. Conclusions 1. At any given moisture content, the mean bulk density of forage maize in experimental silos was greater than that of grass when it was ensiled. 2. Maize resisted compaction more than grass so that the mean bulk density of the resulting silage was lower than that of grass. 3. As with grass, the horizontal pressure of forage maize on silo walls increased with increasing mean bulk density and moisture content although the rate of increase became less at the higher values.
182
MOISTURE
CONTENT
AND
CHOP
LENGTH
4. The horizontal pressure of maize on silo walls was lower than that of grass with the same mean bulk density and moisture content. 5. Nominal chop lengths between 6 and 32 mm had no significant effect on wall pressures and increased density substantially only at the highest moisture content. 6. Effluent production from maize silage decreased with decreasing moisture content and with decreasing chop length, ceasing between 71 and 62% moisture content (w.b.).
Acknowledgements The experimental silos were built at the Grassland Research Institute, Hurley which supplied the maize. The co-operation of the Director and the assistance of his farm staff are gratefully acknowledged. REFERENCES
Agricultural Statistics. London: HMSO, 1976 2 Chaplin, R. V. Design and construction of bunker-type silos. Preliminary studies of the use of model bunker silos for lateralpressure investigation. DNjl l/FBD. Nat. Inst. Agric. Engng, Silsoe (unpubl.), ’
M.A.F.F.
1967 3 Chaplin, R. V. An improved method for the measurement of wall pressures in silos for forage crops. J. agric. Engng Res., 1976 21 9-14 4 Hughes, M.; Gale, G. E. A comparison of the infra-red and Brown Duval oil distillation methods of moisture determination. DN/S/HCD. Nat. Inst. Agric. Engng, Silsoe (unpubl.), 1966 5 Wheatcroft, W. P.; McLean, K.; Fairbairn, C. B. Maize silage study. Maize Bulletin, Jan. 1976 5-8 b Messer, H. J. M.; Hawkins, J. C. The influence of the properties of grass silage on bulk density and horizontalpressure. J. agric. Engng Res, 1977 22 55-64